Vulnerability · Published Mar 18, 2026 · Assigned by GitHub_M

CVE-2026-31865Elysia Cookie Value Prototype Pollution

Medium 6.5

In plain English

A medium-severity prototype pollution in elysia from elysiajs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 16% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.2%top 84.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	elysia	1.4.27	GHSA-8hq9-phh3-p2wp

Affected products (as reported)

Vendor	Product	Versions
elysiajs	elysia	< 1.4.27

Weakness: CWE-1321 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

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