CVE-2026-25639

Denial of Service via proto Key in mergeConfig

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:

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

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:

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}}.

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.

CVE-2026-25639 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.13.5, 0.30.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25639? CVE-2026-25639 is a high-severity security vulnerability in axios (npm), affecting versions >= 1.0.0, <= 1.13.4. It is fixed in 1.13.5, 0.30.3.
2. How severe is CVE-2026-25639? CVE-2026-25639 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of axios are affected by CVE-2026-25639? axios (npm) versions >= 1.0.0, <= 1.13.4 is affected.
4. Is there a fix for CVE-2026-25639? Yes. CVE-2026-25639 is fixed in 1.13.5, 0.30.3. Upgrade to this version or later.
5. Is CVE-2026-25639 exploitable, and should I be worried? Whether CVE-2026-25639 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25639 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25639?
   • Upgrade axios to 1.13.5 or later
   • Upgrade axios to 0.30.3 or later