GHSA-737v-mqg7-c878

Source

https://github.com/advisories/GHSA-737v-mqg7-c878

Import Source

https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-737v-mqg7-c878/GHSA-737v-mqg7-c878.json

JSON Data

https://api.test.osv.dev/v1/vulns/GHSA-737v-mqg7-c878

Aliases

CVE-2026-35209

Published

2026-04-04T06:17:53Z

Modified

2026-04-04T06:31:29.180552Z

Severity

7.5 (High) CVSS_V3 - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N CVSS Calculator

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

Database specific

{
    "cwe_ids": [
        "CWE-1321"
    ],
    "severity": "HIGH",
    "nvd_published_at": null,
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-04T06:17:53Z"
}

References

Affected packages

npm / defu

Package

Name: defu; View open source insights on deps.dev
Purl: pkg:npm/defu

Affected ranges

Type: SEMVER
Events: Introduced

0Unknown introduced version / All previous versions are affected

Fixed

6.1.5

Database specific

last_known_affected_version_range

"<= 6.1.4"

source

"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-737v-mqg7-c878/GHSA-737v-mqg7-c878.json"