Default-value override via Object.assign prototype mutation
Published Apr 6, 2026 · Updated Apr 6, 2026
Prototype pollution in UnJS defu before 6.1.5 allows attackers to override merged defaults through an untrusted proto key. The internal _defu function copies defaults with Object.assign, invoking the proto setter and replacing the result prototype before its for-in guard runs. Exposure requires a consuming application to pass attacker-controlled data as the first defu argument; inherited properties can then replace security-sensitive defaults.
Summary
What happened
Prototype pollution in UnJS defu before 6.1.5 allows attackers to override merged defaults through an untrusted proto key. The internal _defu function copies defaults with Object.assign, invoking the proto setter and replacing the result prototype before its for-in guard runs. Exposure requires a consuming application to pass attacker-controlled data as the first defu argument; inherited properties can then replace security-sensitive defaults.
The record
- CVE
- CVE-2026-35209
- Published
- Apr 6, 2026
- Updated
- Apr 6, 2026
- Vendor
- UnJS
- Product
- defu
- Classifications
- CWE-1321, T1565
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 6, 2026CVE publishedPublication date reported by the CVE source.
- Apr 6, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 6.1.5
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- GHSA-737v-mqg7-c878 demonstrative payloadproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo