Prototype Pollution in Sveltejs Devalue
CVE-2026-123567
Sveltejs devalue before 5.6.4 contains a prototype injection vulnerability that allows attackers to inject __proto__ own properties into objects returned by devalue.parse and devalue.unflatten. Attackers can supply crafted serialized input that, when downstream code merges results via Object.assign, alters the target object's prototype.
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
EPSS: 0.002 (14.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 15th percentile — 14.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
The 30-day trend appears once there are two daily scores.
Affected products
Weakness classification (CWE)
References
- github.com/sveltejs/devalue/security/advisories/GHSA-mwv9-gp5h-frr4 (vendor-advisory)
- www.vulncheck.com/advisories/sveltejs-devalue-before-5.6.4-prototype-injection-… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-123567?
- CVE-2026-123567 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-2026-123567?
- Low severity. CVSS v3 base score is 3.7 out of 10.