---
title: "CVE-2025-66456: Elysia vulnerable to prototype pollution with multiple standalone schema validation"
url: https://erroneo.us/security/cve/CVE-2025-66456/
description: "CVE-2025-66456 (elysia (npm)): Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation and client-server…"
type: vulnerability-record
section: "Security Digest"
date_published: 2025-12-09
date_modified: 2025-12-09
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-66456"
kev: false
cvss: 9.1
epss: 0.0056
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.6% , percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes elysia (npm) >= 1.4.0, < 1.4.17, fixed in 1.4.17 . Check your lockfile, including transitive dependencies.

## What it is

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

- npm `elysia` >= 1.4.0, < 1.4.17 → fixed: 1.4.17

## Affected products (CVE record)

- elysiajs elysia: >= 1.4.0, < 1.4.17

## Severity

- GitHub_M CVSS 4.0: 9.1 `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`
- GitHub CVSS 4.0: 9.1 `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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.56% probability of exploitation activity in the next 30 days; percentile 46.

## Sources

- [GitHub Advisory GHSA-hxj9-33pp-j2cc](https://github.com/advisories/GHSA-hxj9-33pp-j2cc)
- [CVE.org record CVE-2025-66456](https://www.cve.org/CVERecord?id=CVE-2025-66456)
- [NVD entry CVE-2025-66456](https://nvd.nist.gov/vuln/detail/CVE-2025-66456)
- [github.com](https://github.com/elysiajs/elysia/security/advisories/GHSA-hxj9-33pp-j2cc)
- [github.com](https://github.com/elysiajs/elysia/security/advisories/GHSA-8vch-m3f4-q8jf)
- [github.com](https://github.com/elysiajs/elysia/pull/1564)
- [github.com](https://github.com/elysiajs/elysia/commit/26935bf76ebc43b4a43d48b173fc853de43bb51e)
- [github.com](https://github.com/elysiajs/elysia/commit/3af978663e437dccc6c1a2a3aff4b74e1574849e)
- [github.com](https://github.com/sportshead/elysia-poc)

---
Source page: https://erroneo.us/security/cve/CVE-2025-66456/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt