# CVE-2025-66456: Elysia prototype pollution in schema validation

Source: https://junglewise.ai/threats/cve-2025-66456-elysia-prototype-pollution-in-schema-validation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-12-09. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2025-66456 · Severity: critical · CVSS 9.8 · Published 2025-12-09

Elysia is a TypeScript web framework used to build HTTP APIs and web services. A prototype pollution vulnerability in the framework's schema validation allows attackers to inject malicious properties into JavaScript objects through specially crafted HTTP requests. When combined with another code injection flaw, this can lead to complete compromise of the application running the framework.

Technologies: [ElysiaJS Elysia](https://junglewise.ai/threats/technologies/elysia), [elysia (npm)](https://junglewise.ai/threats/technologies/npm-elysia). Vendors: [ElysiaJS](https://junglewise.ai/threats/vendors/elysiajs), [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2025-66456 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-66456 being exploited in the wild.

**How severe is CVE-2025-66456?** Critical severity, with a CVSS score of 9.8, published 9 December 2025.

**Which products and versions are affected?** elysia (npm) SEMVER:introduced=1.4.0,fixed=1.4.17.

**Is there a fix for CVE-2025-66456?** Yes. elysia (npm): upgrade to 1.4.17 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2025-66456 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2025-66456?** Check whether you run ElysiaJS Elysia or elysia (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- elysia (npm): upgrade to 1.4.17 or later

  ```sh
  npm install elysia@1.4.17
  ```


## Technical details

The vulnerability exists in the mergeDeep function used when processing multiple standalone schema validations with the same key. When merging validation results, the code fails to sanitize the __proto__ property, allowing an attacker to pollute the prototype chain and inject arbitrary properties into all objects. The attack requires routes with more than 2 standalone schema validators and relies on an "any" type in the schema guard. The vulnerability is network-reachable with no authentication required. When chained with GHSA-8vch-m3f4-q8jf (arbitrary code injection through cookie config), it enables remote code execution. Patched in version 1.4.17.

## Affected products

- Elysia Elysia 1.4.0 to 1.4.16

## Identifiers

- CVE-2025-66456
- GHSA-hxj9-33pp-j2cc

## Timeline

- 2025-12-09: disclosed
- 2025-12-09: patched: Fixed in version 1.4.17

## References

- [https://github.com/elysiajs/elysia/security/advisories/GHSA-8vch-m3f4-q8jf](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-hxj9-33pp-j2cc)

## Related threats

- [Elysia denial of service via quadratic complexity in form data normalization](https://junglewise.ai/threats/cve-2026-56669-elysia-denial-of-service-via-quadratic-complexity-in-form-data)
- [Elysia cookie value prototype pollution](https://junglewise.ai/threats/cve-2026-31865-elysia-cookie-value-prototype-pollution)
- [Elysia string URL format ReDoS](https://junglewise.ai/threats/cve-2026-30837-elysia-string-url-format-redos)
- [Elysia arbitrary code injection through cookie config](https://junglewise.ai/threats/cve-2025-66457-elysia-arbitrary-code-injection-through-cookie-config)