GHSA-737V-MQG7-C878

Vulnerability from github – Published: 2026-04-04 06:17 – Updated: 2026-04-06 23:42
VLAI
Summary
defu: Prototype pollution via `__proto__` key in defaults argument
Details

Impact

Applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to defu() are vulnerable to prototype pollution.

A crafted payload containing a __proto__ key can override intended default values in the merged result:

import { defu } from 'defu'

const userInput = JSON.parse('{"__proto__":{"isAdmin":true}}')
const config = defu(userInput, { isAdmin: false })

config.isAdmin // true — attacker overrides the server default

Root Cause

The internal _defu function used Object.assign({}, defaults) to copy the defaults object. Object.assign invokes the __proto__ setter, which replaces the resulting object's [[Prototype]] with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing __proto__ key guard in the for...in loop and land in the final result.

Fix

Replace Object.assign({}, defaults) with object spread ({ ...defaults }), which uses [[DefineOwnProperty]] and does not invoke the __proto__ setter.

Affected Versions

<= 6.1.4

Credits

Reported by @BlackHatExploitation

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 6.1.4"
      },
      "package": {
        "ecosystem": "npm",
        "name": "defu"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.1.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-35209"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-04T06:17:53Z",
    "nvd_published_at": "2026-04-06T18:16:44Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nApplications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution.\n\nA crafted payload containing a `__proto__` key can override intended default values in the merged result:\n\n```js\nimport { defu } from \u0027defu\u0027\n\nconst userInput = JSON.parse(\u0027{\"__proto__\":{\"isAdmin\":true}}\u0027)\nconst config = defu(userInput, { isAdmin: false })\n\nconfig.isAdmin // true \u2014 attacker overrides the server default\n```\n\n### Root Cause\n\nThe internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object\u0027s `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result.\n\n### Fix\n\nReplace `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.\n\n### Affected Versions\n\n\u003c= 6.1.4\n\n### Credits\n\nReported by [@BlackHatExploitation](https://github.com/BlackHatExploitation)",
  "id": "GHSA-737v-mqg7-c878",
  "modified": "2026-04-06T23:42:28Z",
  "published": "2026-04-04T06:17:53Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-35209"
    },
    {
      "type": "WEB",
      "url": "https://github.com/unjs/defu/pull/156"
    },
    {
      "type": "WEB",
      "url": "https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/unjs/defu"
    },
    {
      "type": "WEB",
      "url": "https://github.com/unjs/defu/releases/tag/v6.1.5"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "defu: Prototype pollution via `__proto__` key in defaults argument"
}



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