CVE-2025-66456 is a prototype pollution vulnerability identified in the Elysia Typescript framework, specifically in versions 1.4.0 through 1.4.16. Elysia is used for request validation, type inference, OpenAPI documentation, and client-server communication in web applications. The vulnerability exists in the mergeDeep function, which merges the results of two standard schema validations sharing the same key. Due to the merging order and the presence of an 'any' type used as a standalone guard, an attacker can inject or manipulate the __proto__ property of JavaScript objects. This prototype pollution can alter the behavior of the application by modifying object prototypes, leading to unexpected code execution paths. When combined with another vulnerability identified as GHSA-8vch-m3f4-q8jf, this prototype pollution can be escalated to full remote code execution (RCE) without requiring authentication or user interaction. The vulnerability has been assigned a CVSS 4.0 score of 9.1, reflecting its critical nature with network attack vector, low attack complexity, no privileges required, and no user interaction needed. The issue was publicly disclosed on December 9, 2025, and fixed in Elysia version 1.4.17. Until patching, a workaround involves sanitizing incoming request bodies to remove the __proto__ key to prevent pollution. No known exploits have been reported in the wild yet, but the vulnerability poses a significant risk to applications relying on affected Elysia versions, especially those exposed to untrusted input.

For European organizations, the impact of this vulnerability can be severe. Applications using the affected Elysia versions for API validation or client-server communication are at risk of prototype pollution leading to remote code execution. This could result in full system compromise, data breaches, unauthorized access, and disruption of services. Given the critical CVSS score and the ability to exploit the vulnerability remotely without authentication or user interaction, attackers can leverage this flaw to execute arbitrary code on servers, potentially leading to lateral movement within networks and exfiltration of sensitive data. Organizations in sectors such as finance, healthcare, government, and critical infrastructure, which often rely on modern web frameworks and have high regulatory requirements, could face significant operational and compliance consequences. Additionally, the ability to manipulate prototype attributes can undermine application integrity and confidentiality, increasing the risk of persistent threats and advanced attacks.

1. Immediately upgrade all instances of the Elysia framework to version 1.4.17 or later, where the vulnerability is patched. 2. Implement input validation and sanitization to explicitly remove or reject any __proto__ keys in incoming JSON or request bodies before processing. 3. Employ runtime application self-protection (RASP) or web application firewalls (WAF) with custom rules to detect and block payloads attempting prototype pollution. 4. Conduct thorough code reviews and dependency audits to identify and remediate any other instances of unsafe object merging or prototype manipulation. 5. Monitor application logs and network traffic for suspicious activities indicative of exploitation attempts, especially unusual object property modifications. 6. Educate development teams on secure coding practices related to object merging and prototype pollution risks. 7. In environments where immediate patching is not feasible, consider isolating vulnerable services behind additional security layers and restricting access to trusted networks only.