CVE-2025-66456 is a prototype pollution vulnerability identified in the elysia Typescript framework, specifically affecting versions 1.4.0 through 1.4.16. The vulnerability exists in the mergeDeep function, which merges the results of two standard schema validations that share the same key. Due to the order in which merging occurs and the presence of an 'any' type used as a standalone guard, the __proto__ property can be maliciously injected and merged into the object prototype. Prototype pollution allows attackers to manipulate the prototype chain of JavaScript objects, potentially altering application behavior or enabling further attacks. When this vulnerability is chained with GHSA-8vch-m3f4-q8jf, it enables an attacker to achieve full remote code execution (RCE) on the affected system without requiring authentication or user interaction. The vulnerability is remotely exploitable over the network (AV:N), requires low attack complexity (AC:L), and does not require privileges (PR:N) or user interaction (UI:N). The vulnerability impacts confidentiality and integrity highly, with no impact on availability. The issue was publicly disclosed on December 9, 2025, and fixed in elysia version 1.4.17. Until patched, a workaround involves sanitizing incoming request bodies to remove the __proto__ key to prevent prototype pollution. No known exploits are currently in the wild, but the critical CVSS score and potential for RCE make this a high-priority issue.

For European organizations, this vulnerability poses a significant risk, especially those developing or deploying web applications using the elysia framework for request validation, type inference, and API communication. Successful exploitation can lead to full remote code execution, allowing attackers to execute arbitrary code, potentially leading to data breaches, system compromise, lateral movement, and disruption of services. Organizations handling sensitive data, such as financial institutions, healthcare providers, and government agencies, face heightened risks of confidentiality and integrity breaches. The vulnerability's ease of exploitation without authentication or user interaction increases the threat landscape. Additionally, supply chain risks exist if third-party services or libraries incorporate vulnerable elysia versions. The potential for widespread impact is amplified by the framework's use in modern Typescript-based applications common across Europe. Failure to patch or mitigate promptly could result in significant operational and reputational damage.

1. Immediate upgrade to elysia version 1.4.17 or later, which contains the fix for this vulnerability. 2. Implement input validation and sanitization to explicitly remove or reject any __proto__ keys in incoming JSON or request bodies to prevent prototype pollution. 3. Employ runtime application self-protection (RASP) or Web Application Firewalls (WAF) with custom rules to detect and block payloads attempting to exploit prototype pollution. 4. Conduct code audits and dependency scans to identify usage of vulnerable elysia versions across all projects and third-party dependencies. 5. Monitor application logs and network traffic for unusual behavior indicative of exploitation attempts, especially those involving prototype manipulation patterns. 6. Educate development teams on secure coding practices related to object merging and prototype pollution risks. 7. Establish incident response plans specifically addressing potential RCE scenarios stemming from this vulnerability. 8. Coordinate with supply chain partners to ensure they have remediated the vulnerability in their software stacks.