What is CVE-2025-66456?

CVE-2025-66456 is a critical-severity vulnerability in Elysiajs Elysia, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 9.8/10. Published 2025-12-09.

How severe is CVE-2025-66456?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Prototype Pollution in Elysiajs Elysia

CVE-2025-66456

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.

Vulnerability class: Prototype Pollution

Published 2025-12-09 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.006 (44.4th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 44th percentile — 44.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.006 · change: +0.000

Affected products

Elysiajs Elysia — versions >= 1.4.0, < 1.4.17

Weakness classification (CWE)

CWE-1321 — Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution)

References

github.com/elysiajs/elysia/security/advisories/GHSA-hxj9-33pp-j2cc (Exploit, Patch, Vendor Advisory)
github.com/elysiajs/elysia/security/advisories/GHSA-8vch-m3f4-q8jf (Not Applicable)
github.com/elysiajs/elysia/pull/1564 (Patch, Issue Tracking)
github.com/elysiajs/elysia/commit/26935bf76ebc43b4a43d48b173fc853de43bb51e (Patch)
github.com/elysiajs/elysia/commit/3af978663e437dccc6c1a2a3aff4b74e1574849e (Patch)

Frequently asked questions

What is CVE-2025-66456?: CVE-2025-66456 is a critical-severity vulnerability in Elysiajs Elysia, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 9.8/10. Published 2025-12-09.
How severe is CVE-2025-66456?: Critical severity. CVSS v3 base score is 9.8 out of 10.