CVE-2026-35209
defu is software that allows uers to assign default properties recursively.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.40%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
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 3.1
- 7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- EPSS
- 0.40% probability · 33th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-1321
- Affected
- unjs/defu
- Source
- security-advisories@github.com
References
- https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29Patch
- https://github.com/unjs/defu/pull/156Issue Tracking, Patch
- https://github.com/unjs/defu/releases/tag/v6.1.5Release Notes
- https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878Mitigation, Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.