unjs defu up to 6.1.4 prototype pollution

Summaryinfo

A vulnerability labeled as problematic has been found in unjs defu up to 6.1.4. Impacted is the function defu. Executing a manipulation can lead to prototype pollution. The identification of this vulnerability is CVE-2026-35209. The attack may be launched remotely. There is no exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in unjs defu up to 6.1.4. It has been declared as problematic. This vulnerability affects the function defu. The manipulation with an unknown input leads to a prototype pollution vulnerability. The CWE definition for the vulnerability is CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. As an impact it is known to affect integrity. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-35209 since 04/01/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1059.

Upgrading to version 6.1.5 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 3942bfbbcaa72084bd4284846c83bd61ed7c8b29 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• unjs

Name

• defu

Version

• 6.1.0
• 6.1.1
• 6.1.2
• 6.1.3
• 6.1.4

License

• open-source

Website

• Product: https://github.com/unjs/defu/

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