Description

axios Vulnerable to Credential Theft and Response Hijacking via Prototype Pollution Gadget in Config Merge ## Summary Axios versions before the fixed releases contain prototype-pollution gadgets in request config processing. If another vulnerability in the same JavaScript process has already polluted Object.prototype.transformResponse, affected Axios versions may treat that inherited value as request configuration or as an option validator. Axios does not itself create the prototype pollution. Exploitability requires a separate prototype-pollution vulnerability or equivalent attacker control over Object.prototype before Axios creates a request. ## Impact For ordinary prototype-pollution primitives that can only assign JSON-like values, this issue primarily results in request failures or denial-of-service attacks. If the attacker can pollute Object.prototype.transformResponse with a function, affected versions of Axios may execute it. In fully affected versions, the function can observe response data and request config, including URL, headers, and auth, and can change the response data returned to application code. This function-valued condition is important. Most query-string or JSON parser prototype-pollution bugs cannot create JavaScript functions on their own, so credential exposure and response tampering are conditional rather than automatic consequences of such bugs. ## Affected Functionality The affected functionality is Axios request config processing and response transformation. Affected use requires all of the following: - An affected Axios version. - A polluted Object.prototype in the same process or browser context. - Pollution before Axios merges or validates the request config. - A polluted key relevant to Axios config, especially transformResponse. This is not specific to the Node HTTP adapter. Browser and Node usage can both pass through the shared config/transform pipeline, though real-world exploitability depends on the surrounding application and any helper vulnerabilities. ## Technical Details In affected versions, mergeConfig() reads config values through normal property access. For config keys present in Axios defaults, including transformResponse, a missing own property on the request config can fall through to Object.prototype. In the fully affected path, this means Object.prototype.transformResponse can replace Axios's default response transform. The selected transform is later executed by transformData() with the request config as this. Some later affected v1 releases guarded the merge path but still used inherited properties while looking up validators in validator.assertOptions(). In that narrower case, a polluted function can still run during config validation and inspect the config argument, but it does not replace the response transform. Fixed versions use own-property checks and null-prototype config objects, so inherited Object.prototype values are not treated as Axios config or validator schema entries. ## Proof of Concept of Attack js import http from 'http'; import axios from 'axios'; const seen = []; const server = http.createServer((req, res) => { res.setHeader('Content-Type', 'application/json'); res.end(JSON.stringify({ secret: 'response-secret' })); }); await new Promise(resolve => server.listen(0, '127.0.0.1', resolve)); Object.prototype.transformResponse = function pollutedTransform(data, headers, status) { if (headers && typeof status === 'number') { seen.push({ url: this.url, username: this.auth && this.auth.username, password: this.auth && this.auth.password, responseData: data }); return { hijacked: true }; } return true; }; try { const { port } = server.address(); const response = await axios.get(`http://127.0.0.1:${port}/users`, { auth: { username: 'svc-account', password: 'prod-secret-key-123' } }); console.log(response.data); // { hijacked: true } console.log(seen[0]); // request config plus original response body } finally { delete Object.prototype.transformResponse; server.close(); } Expected result on fully affected versions: the polluted transform runs, captures request config and response data, and replaces the response returned to the caller. Expected result on fixed versions: the polluted transform is ignored, and the original response is returned.

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