What’s Vulnerable › Vulnerabilities

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

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2025-12-09 · updated 2026-10-14
• Weakness: CWE-1321

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 products

• elysiajs elysia: from 1.4.0 before 1.4.17 — fixed in 1.4.17

How to fix

Upgrade to a fixed version: elysiajs elysia 1.4.17. Check the vendor advisory for mitigations.

References

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

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