CVE-2026-35209

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.

Affected Versions

<= 6.1.4

Credits

Reported by @BlackHatExploitation

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

CVE-2026-35209 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 (6.1.5); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2026-35209? CVE-2026-35209 is a high-severity security vulnerability in defu (npm), affecting versions <= 6.1.4. It is fixed in 6.1.5.
2. How severe is CVE-2026-35209? CVE-2026-35209 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 defu are affected by CVE-2026-35209? defu (npm) versions <= 6.1.4 is affected.
4. Is there a fix for CVE-2026-35209? Yes. CVE-2026-35209 is fixed in 6.1.5. Upgrade to this version or later.
5. Is CVE-2026-35209 exploitable, and should I be worried? Whether CVE-2026-35209 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-2026-35209 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-2026-35209? Upgrade defu to 6.1.5 or later.