Prototype Pollution in Sveltejs Devalue
CVE-2026-123561
devalue before 5.9.3 allows malformed null-prototype object keys to bypass parse's __proto__ rejection through property-key coercion, an incomplete fix for GHSA-mwv9-gp5h-frr4. Attackers can supply crafted payloads that yield objects with a __proto__ own property, which alone does not cause prototype pollution.
Vulnerability class: Prototype Pollution
Published .
CVSS v3 metric
CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
Weakness classification (CWE)
References
- github.com/sveltejs/devalue/security/advisories/GHSA-4q55-j62x-fr9h (vendor-advisory)
- www.vulncheck.com/advisories/sveltejs-devalue-before-5.9.3-proto-rejection-bypa… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-123561?
- CVE-2026-123561 is a low-severity vulnerability in Sveltejs Devalue, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 3.7/10. Published 2026-11-08.
- How severe is CVE-2026-123561?
- Low severity. CVSS v3 base score is 3.7 out of 10.