defu: Prototype pollution via __proto__ key in defaults argument

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-35209: 0.40% estimated exploitation activity in the next 30 days; percentile 34.77 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: 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

Published by the source

4 Apr 2026, 06:17 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-737v-mqg7-c878

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-737v-mqg7-c878

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# Patch Next advisory brief: CVE-2026-35209

Advisory: defu: Prototype pollution via `__proto__` key in defaults argument
IDs: CVE-2026-35209, GHSA-737v-mqg7-c878
Record: https://patchnext.com/advisories/cve-2026-35209 (revision 2, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: defu 6.1.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm defu: affected <= 6.1.4; fixed in 6.1.5

## Suggested inspection (read-only)
- `npm ls defu --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on defu.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-737v-mqg7-c878 (OSV record GHSA-737v-mqg7-c878 lists affected ranges: npm defu >=0 <6.1.5 (fixed))
- Verified: https://github.com/advisories/GHSA-737v-mqg7-c878 (GitHub-reviewed advisory GHSA-737v-mqg7-c878 lists affected ranges: npm defu >=0 <=6.1.4)

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