CVE-2025-66456 CRITICAL

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

Vendor Elysiajs
Product elysia
Weakness CWE-1321
Published December 9, 2025
Last update December 9, 2025
View on NVD All CVEs

CVSS base score

9.1/10
Attack vector Network
Attack complexity Low
Privileges required None
User interaction None
Confidentiality —
Integrity —

CVSS 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

What the vulnerability does

01Description

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.

Key dates

02Disclosure timeline

December 9, 2025 CVE published
December 9, 2025 Record updated