GHSA-737v-mqg7-c878 · Severity: high · Ecosystem: npm — defu: Prototype pollution via `__proto__` key in defaults argument
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.
Conclusion & alert: CVE-2026-35209 is rated Low Risk (33.1/100): CVSS High severity, with low exploitation likelihood (EPSS 0.03%). Mandatory action: Monitor for updates and reassess as exploit intelligence or EPSS changes.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-04-07 | — | 0.03% | — |
Full EPSS history (1 record total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 7.5 | 3.1 | HIGH |
|
3.9 | 3.6 | [email protected] |
GHSA-737v-mqg7-c878 · Severity: high · Ecosystem: npm — defu: Prototype pollution via `__proto__` key in defaults argument
| URL | Tags |
|---|---|
| https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29 | Patch |
| https://github.com/unjs/defu/pull/156 | Issue Tracking Patch |
| https://github.com/unjs/defu/releases/tag/v6.1.5 | Release Notes |
| https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878 | Mitigation Vendor Advisory |