Vulnerability Description
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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Unjs | Defu | < 6.1.5 |
Related Weaknesses (CWE)
References
- https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29Patch
- https://github.com/unjs/defu/pull/156Issue TrackingPatch
- https://github.com/unjs/defu/releases/tag/v6.1.5Release Notes
- https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878MitigationVendor Advisory
FAQ
What is CVE-2026-35209?
CVE-2026-35209 is a vulnerability with a CVSS score of 7.5 (HIGH). 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...
How severe is CVE-2026-35209?
CVE-2026-35209 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-35209?
Check the references section above for vendor advisories and patch information. Affected products include: Unjs Defu.