CVE-2026-35209 Unjs CVE debrief

Who should care

Developers and administrators using Unjs Defu in their applications should be aware of this vulnerability and take steps to mitigate it. This includes updating to version 6.1.5 or later, and being cautious when passing unsanitized user input to the defu function. Affected operators, platforms, and security teams should review the vulnerability and implement necessary controls.

Technical summary

The vulnerability in Unjs Defu arises from the use of Object.assign to copy default values. This method invokes the __proto__ setter, allowing an attacker to override intended default values with malicious ones. By including a __proto__ key in a crafted payload, an attacker can pollute the prototype of objects, leading to potential security issues. The fix in version 6.1.5 replaces Object.assign with object spread ({ ...defaults }), which uses [[DefineOwnProperty]] and does not invoke the __proto__ setter.

Defensive priority

High

Recommended defensive actions

• Update to version 6.1.5 or later
• Be cautious when passing unsanitized user input to the defu function
• Monitor for suspicious activity
• Implement compensating controls
• Perform inventory checks
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-04-06T18:16:44.157Z and was last modified on 2026-07-21T19:10:00.107Z. The NVD entry is currently Analyzed. The vulnerability affects Unjs Defu, a software that allows users to assign default properties recursively. The internal _defu function used Object.assign({}, defaults) to copy the defaults object, which invokes the __proto__ setter, allowing an attacker to override intended default values with malicious ones.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.