CVE-2025-66456: Elysiajs Elysia

Critical severity, CVSS 9.8. EPSS: 0.6% chance of exploitation in the next 30 days.

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 products

• Elysiajs Elysia: from 1.4.0, before 1.4.17 (fixed in 1.4.17)

Published 2025-12-09. Last modified 2026-06-17.