---
id: CVE-2026-35209
title: defu is software that allows uers to assign default properties recursively
summary: >-
  defu is software that allows uers to assign default properties recursively.
  Prior to version 6.1.5, applications that pass unsanitized user input (e.g.
  parsed JSON request bodies, database records, or config files from untrusted
  sources)…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N'
cwe:
  - CWE-1321
vendor: unjs
product: defu
affected:
  - defu < 6.1.5
patched:
  - defu 6.1.5
published: '2026-04-06'
updated: '2026-07-21'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-35209'
references:
  - url: >-
      https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29
    label: security-advisories@github.com
  - url: 'https://github.com/unjs/defu/pull/156'
    label: security-advisories@github.com
  - url: 'https://github.com/unjs/defu/releases/tag/v6.1.5'
    label: security-advisories@github.com
  - url: 'https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878'
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00398
epssPercentile: 0.33883
ingestedAt: '2026-07-22T01:56:36.541Z'
---

## Overview

defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, 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 resul. 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. Version 6.1.5 replaces `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.

## Affected

- `defu < 6.1.5`

## Remediation

Upgrade past the affected range:

- `defu 6.1.5`