CVE-2025-66456
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk45th percentile - higher than 45% of all known CVEs
Summary
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.
Risk Assessment
An attacker can exploit the vulnerability to achieve remote code execution, leading to full server compromise and data breach.
Recommendation
Update Elysia to version 1.4.17 or later. As a temporary workaround, remove the __proto__ key from request bodies.
Other vulnerabilities in Elysia
See all- CVE-2026-56669High
A vulnerability in Elysia prior to version 1.4.29 causes quadratic CPU workload growth when processing multipart/form-data with many unique key-value pairs. The use of getAll in data normalization leads to CPU exhaustion.
- CVE-2025-66457High
Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation and client-server communication. Versions 1.4.17 and below are subject to arbitrary code execution from cookie config; when dynamic cookies are enabled, the cookie config is injected into the compiled route without first being sanitised. This issue is fixed in version 1.4.18.
Original NVD description (English source)
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 data from NVD (NIST) · CISA KEV · EPSS