What’s Vulnerable › Vulnerabilities

CVE-2026-35209: defu: Prototype pollution via `__proto__` key in defaults argument

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-06 · updated 2026-07-21
• Weakness: CWE-1321

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.

Affected products

• unjs defu: before 6.1.5 — fixed in 6.1.5

How to fix

Upgrade to a fixed version: unjs defu 6.1.5. Check the vendor advisory for mitigations.

References

• https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29
• https://github.com/unjs/defu/pull/156
• https://github.com/unjs/defu/releases/tag/v6.1.5
• https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878

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