GHSA-w9j2-pvgh-6h63 Axios: Authentication Bypass via Prototype Pollution Gadget in `validateStatus` Merge Strategy
Vulnerability Disclosure: Authentication Bypass via Prototype Pollution Gadget in `validateStatus` Merge StrategySummaryThe 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 AuthenticationCVSS 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" VulnerabilitiesThis 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 VulnerableAll other config properties use `defaultToConfig2`, which reads `config2[prop]` (traverses prototype). But `validateStatus` uses `mergeDirectKeys`, which uses the `in` operator: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 Concept1. The Setup (Simulated Pollution)2. The Gadget Trigger (Safe Code)3. The Execution4. The ImpactVerified PoC OutputImpact 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 FixReplace the `in` operator with `hasOwnProperty` in `mergeDirectKeys`: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 |
CVSS v3.1 score: 4.8