axios has DoS & Header Injection via Prototype Pollution Read-Side Gadgets in axios merge functions

Description

## Summary axios `1.15.2` exposes two read-side prototype-pollution gadgets. When `Object.prototype` is polluted by an upstream dependency in the same process (e.g. lodash `_.merge` / [CVE-2018-16487](https://nvd.nist.gov/vuln/detail/CVE-2018-16487)), axios silently picks up the polluted values: 1. **Header injection** - `lib/utils.js` line 406 builds `merge()`'s accumulator as `result = {}`, so `result[targetKey]` (line 414) walks `Object.prototype` and the polluted bucket's own keys are copied into the merged headers and ride out on the wire. 2. **Crash DoS** - `lib/core/mergeConfig.js` line 26 builds the `hasOwnProperty` descriptor as a plain-object literal. `Object.defineProperty` reads `descriptor.get`/`descriptor.set` via the prototype chain, so a polluted `Object.prototype.get` or `Object.prototype.set` makes the call throw `TypeError` synchronously on every axios request. ## Affected Properties | Polluted slot | Effect | |---|---| | `Object.prototype.common` | injects headers on every method | | `Object.prototype.delete` / `.head` / `.post` / `.put` / `.patch` / `.query` | injects headers on the matching method | | `Object.prototype.get` | every axios request throws `TypeError: Getter must be a function` from `mergeConfig.js:26` | | `Object.prototype.set` | every axios request throws `TypeError: Setter must be a function` from `mergeConfig.js:26` | Per-request headers (`axios.request(url, { headers: {...} })`) overwrite polluted entries. Polluting `Object.prototype.get` triggers the crash before any header is built. ## Proof of Concept ```javascript const axios = require('axios'); // Finding A - header injection Object.prototype.common = { 'X-Poisoned': 'yes' }; await axios.get('http://api.example.com/users'); // Wire request carries `X-Poisoned: yes`. // Finding B - crash DoS Object.prototype.get = { something: 'anything' }; await axios.get('http://api.example.com/users'); // TypeError: Getter must be a function: #<Object> // at Function.defineProperty (<anonymous>) // at mergeConfig (lib/core/mergeConfig.js:26:10) ``` ## Impact - **Server hang** (`Content-Length: 99999`): receiver waits for a body that never arrives. Affects requests with a body. - **CL+TE conflict** (`Transfer-Encoding: chunked` rides alongside axios's auto `Content-Length`): receiver rejects with `400 Bad Request`. Affects requests with a body. - **Response suppression** (`If-None-Match: *`): receiver returns empty `304 Not Modified`. Affects GET / HEAD. - **Crash DoS** (`Object.prototype.get` / `.set`): every axios request fails synchronously with `TypeError`, not `AxiosError`, so handlers filtering on `error.isAxiosError` mishandle the failure. ## Attack Flow ```mermaid flowchart TD ROOT["Polluted Object.prototype via upstream gadget (e.g. lodash <= 4.17.10 _.merge / CVE-2018-16487) axios <= 1.15.2"] ROOT --> CLASS_A["A. Arbitrary HTTP Header Injection Polluted defaults.headers slot rides along on every outbound axios request"] ROOT --> CLASS_B["B. Crash DoS via Object.prototype.get / .set Polluted descriptor breaks Object.defineProperty in mergeConfig"] CLASS_A --> PRE_A["Precondition: header not set per-request by the app Injected via defaults.headers slot (common, delete, head, post, put, patch, query)"] PRE_A --> PA1["Response Suppression Trigger: common = {If-None-Match: *} Affects GET / HEAD"] PA1 --> SA1["DoS 304 Not Modified empty"] PRE_A --> PA2["Server Hang Trigger: common = {Content-Length: 99999} Affects requests with body"] PA2 --> SA2["DoS connection hang"] PRE_A --> PA3["CL+TE Conflict Trigger: common = {Transfer-Encoding: chunked} Affects requests with body"] PA3 --> SA3["DoS 400 Bad Request"] CLASS_B --> SB1["DoS TypeError: Getter / Setter must be a function Crashes every axios request, not only GET"] %% Styles style ROOT fill:#f87171,stroke:#991b1b,color:#fff style CLASS_A fill:#fb923c,stroke:#9a3412,color:#fff style CLASS_B fill:#fb923c,stroke:#9a3412,color:#fff style PRE_A fill:#e2e8f0,stroke:#64748b,color:#1e293b style PA1 fill:#fbbf24,stroke:#92400e,color:#000 style PA2 fill:#fbbf24,stroke:#92400e,color:#000 style PA3 fill:#fbbf24,stroke:#92400e,color:#000 style SA1 fill:#ef4444,stroke:#991b1b,color:#fff style SA2 fill:#ef4444,stroke:#991b1b,color:#fff style SA3 fill:#ef4444,stroke:#991b1b,color:#fff style SB1 fill:#ef4444,stroke:#991b1b,color:#fff ``` ## Root Cause **Finding A.** `lib/utils.js:404-429`'s `merge()` creates `result = {}` at line 406. The dangerous-keys filter on lines 408-411 blocks the write side, but the read at line 414 (`isPlainObject(result[targetKey])`) still walks the prototype chain. When `targetKey` matches a polluted slot, `result[targetKey]` returns the polluted nested object, and the recursive `merge(result[targetKey], val)` on line 415 iterates that object's own keys via `forEach` and copies them as own properties into the new accumulator. Those keys flow through `mergeConfig.js:35` → `Axios.js:148` (`utils.merge(headers.common, headers[config.method])`) → `Axios.js:155` (`AxiosHeaders.concat(...)`) → onto the wire via `http.js:677` (`headers: headers.toJSON()`) → `http.js:767` (`transport.request(options, ...)`). **Finding B.** `lib/core/mergeConfig.js:25` correctly makes `config = Object.create(null)`, but the descriptor passed on line 26 is a plain-object literal - its `get`/`set` lookups walk `Object.prototype`. A polluted non-function `Object.prototype.get` or `.set` makes `Object.defineProperty` throw `TypeError: Getter must be a function` (or `Setter must be a function`) before the call returns. The descriptor is built unconditionally on every `mergeConfig` invocation, so every axios request throws - POST, PUT, DELETE, PATCH, HEAD, QUERY, not only GET. ## Suggested Fix Use null-prototype objects in place of the plain-object literals at `lib/utils.js:406` and `lib/core/mergeConfig.js:26-31`. The same descriptor pattern recurs at `lib/core/AxiosError.js:37`, `lib/core/AxiosHeaders.js:100`, `lib/utils.js:447/454/492/498`, and `lib/adapters/adapters.js:28/32`. ## Resources - [CVE-2018-16487](https://nvd.nist.gov/vuln/detail/CVE-2018-16487) - `lodash.merge` prototype pollution in `lodash <= 4.17.10` - [CWE-1321](https://cwe.mitre.org/data/definitions/1321.html) - Improperly Controlled Modification of Object Prototype Attributes