CVE-2026-31865 is a medium-severity prototype pollution vulnerability affecting the Elysia Typescript framework, a tool used for request validation, type inference, OpenAPI documentation, and client-server communication. The vulnerability arises from improper control over modification of object prototype attributes (CWE-1321), specifically allowing an attacker to override cookie objects by injecting or manipulating the __proto__ property. This can lead to unexpected behavior in the application, such as altering cookie values or application state, potentially compromising confidentiality and integrity. The flaw exists in all Elysia versions prior to 1.4.27 and is exploitable remotely without authentication or user interaction, increasing its risk profile. Although no known exploits are currently reported in the wild, the vulnerability is significant due to the widespread use of Elysia in Typescript-based web applications. The recommended fix is upgrading to version 1.4.27 or later, where the issue is patched. As a temporary mitigation, developers can enforce strict cookie validation using t.Cookie validation to prevent iterable access or prototype pollution attacks on cookies. This vulnerability highlights the risks of prototype pollution in JavaScript frameworks, which can lead to subtle but impactful security issues.

The primary impact of this vulnerability is on the confidentiality and integrity of application data, specifically cookie values managed by the Elysia framework. An attacker exploiting this flaw can manipulate prototype attributes to override cookie objects, potentially leading to unauthorized access, session manipulation, or bypass of security controls relying on cookie data. While availability is not directly affected, the integrity compromise can facilitate further attacks or data leakage. Since exploitation requires no authentication or user interaction and can be performed remotely, the vulnerability poses a moderate risk to any organization using vulnerable Elysia versions. Applications relying heavily on cookies for authentication or state management are particularly at risk. The scope includes all web applications using Elysia versions before 1.4.27, which may be significant given the framework's adoption in Typescript web development. Although no active exploits are reported, the ease of exploitation and potential for chaining with other vulnerabilities make timely remediation important.

1. Upgrade all Elysia framework instances to version 1.4.27 or later immediately to apply the official patch addressing prototype pollution. 2. Implement strict validation on cookies using t.Cookie validation or equivalent mechanisms to enforce allowed values and prevent iterable or prototype property manipulation. 3. Review application code for any direct or indirect use of __proto__ or other prototype properties in cookie handling or request processing and refactor to avoid unsafe prototype modifications. 4. Employ runtime protections such as object freezing or sealing where feasible to prevent prototype pollution attacks. 5. Conduct thorough security testing, including fuzzing and static analysis, focused on prototype pollution vectors in the application. 6. Monitor application logs for unusual cookie or prototype-related activity that may indicate attempted exploitation. 7. Educate developers on the risks of prototype pollution and secure coding practices in JavaScript/Typescript frameworks. 8. Consider implementing Content Security Policy (CSP) and other web security headers to reduce attack surface. These steps combined will reduce the risk of exploitation and limit the impact if an attack occurs.