Prototype Pollution in Elysiajs Elysia
CVE-2026-31865
Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation, and client-server communication. Prior to version 1.4.27, an Elysia cookie can be overridden by prototype pollution , eg. `__proto__`. This issue is patched in 1.4.27. As a workaround, use t.Cookie validation to enforce validation value and/or prevent iterable over cookie if possible.
Vulnerability class: Prototype Pollution
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (21.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 21.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001
Affected products
- Elysiajs Elysia — versions < 1.4.27
Weakness classification (CWE)
References
- github.com/elysiajs/elysia/security/advisories/GHSA-8hq9-phh3-p2wp (Patch, Mitigation, Vendor Advisory)
- github.com/elysiajs/elysia/commit/e9d6b1743fa7368ef942dce181f6a089757f6aab (Patch)
Frequently asked questions
- What is CVE-2026-31865?
- CVE-2026-31865 is a medium-severity vulnerability in Elysiajs Elysia, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 6.5/10. Published 2026-03-18.
- How severe is CVE-2026-31865?
- Medium severity. CVSS v3 base score is 6.5 out of 10.