Vulnerability · Published Dec 9, 2025 · Assigned by GitHub_M

CVE-2025-66456Elysia vulnerable to prototype pollution with multiple standalone schema validation

Critical 9.1

In plain English

A critical-severity prototype pollution in elysia from elysiajs. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)9.1Critical

Exploit odds (30 days)0.6%top 53.2% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation and client-server communication. Versions 1.4.0 through 1.4.16 contain a prototype pollution vulnerability in `mergeDeep` after merging results of two standard schema validations with the same key. Due to the ordering of merging, there must be an any type that is set as a standalone guard, to allow for the `__proto__ prop` to be merged. When combined with GHSA-8vch-m3f4-q8jf this allows for a full RCE by an attacker. This issue is fixed in version 1.4.17. To workaround, remove the `__proto__ key` from body.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	elysia	1.4.17	GHSA-hxj9-33pp-j2cc

Affected products (as reported)

Vendor	Product	Versions
elysiajs	elysia	>= 1.4.0, < 1.4.17

Weakness: CWE-1321 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

References

https://github.com/elysiajs/elysia/security/advisories/GHSA-hxj9-33pp-j2cc x_refsource_CONFIRM
https://github.com/elysiajs/elysia/security/advisories/GHSA-8vch-m3f4-q8jf x_refsource_MISC
https://github.com/elysiajs/elysia/pull/1564 x_refsource_MISC
https://github.com/elysiajs/elysia/commit/26935bf76ebc43b4a43d48b173fc853de43bb51e x_refsource_MISC
https://github.com/elysiajs/elysia/commit/3af978663e437dccc6c1a2a3aff4b74e1574849e x_refsource_MISC