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.

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 iss…

Vulnerability class: Prototype Pollution

EPSS: 0.000 (6.3th percentile) — read the EPSS interpretation.

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.

Affected products

Elysiajs Elysia — versions < 1.4.27

Weakness classification (CWE)

CWE-1321 — Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution)

References

https://github.com/elysiajs/elysia/security/advisories/GHSA-8hq9-phh3-p2wp (x_refsource_CONFIRM)
https://github.com/elysiajs/elysia/commit/e9d6b1743fa7368ef942dce181f6a089757f6aab (x_refsource_MISC)

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.