CVE-2026-31865 in elysia
Summary
by MITRE • 03/18/2026
Elysia is a Typescript framework for request validation, type inference, OpenAPI documentation, and client-server communication. Prior to version 1.4.27, an Elysia cookie can be overridden by prototype pollution , eg. `__proto__`. This issue is patched in 1.4.27. As a workaround, use t.Cookie validation to enforce validation value and/or prevent iterable over cookie if possible.
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Analysis
by VulDB Data Team • 03/22/2026
CVE-2026-31865 represents a prototype pollution vulnerability affecting the Elysia Typescript framework, which is designed for request validation, type inference, OpenAPI documentation, and client-server communication. This vulnerability stems from insufficient input validation within the cookie handling mechanism, specifically allowing malicious actors to manipulate prototype properties through cookie values containing prototype pollution vectors such as _proto_. The flaw exists in versions prior to 1.4.27, making all earlier releases susceptible to this type of attack that can potentially compromise application security and data integrity.
The technical implementation of this vulnerability exploits the inherent weaknesses in how Elysia processes cookie data, particularly when handling user-provided inputs that may contain prototype pollution indicators. When a cookie value contains malicious content like _proto_, the framework fails to properly sanitize or validate these inputs before incorporating them into the application's prototype chain. This allows attackers to inject arbitrary properties into the Object.prototype, which can then be leveraged to manipulate the behavior of the application or potentially execute unintended code. The vulnerability aligns with CWE-471, which specifically addresses the issue of prototype pollution in programming languages that support prototype-based inheritance.
The operational impact of this vulnerability extends beyond simple data manipulation, as prototype pollution can lead to various security consequences including but not limited to denial of service conditions, information disclosure, and potential remote code execution depending on the application's architecture and how it utilizes the polluted prototype properties. Attackers can exploit this weakness to modify core object behaviors, inject malicious code into the application's execution flow, or manipulate application logic in ways that may go undetected. The vulnerability affects the framework's core functionality, particularly its request validation and type inference capabilities, which are fundamental to maintaining application security and data integrity.
The recommended mitigation strategy involves upgrading to Elysia version 1.4.27 or later, which includes proper input validation and sanitization measures to prevent prototype pollution attacks. Additionally, developers should implement explicit cookie validation using t.Cookie validation to enforce proper value validation and prevent iterable over cookie operations that could introduce prototype pollution vectors. Organizations should also consider implementing additional security controls such as Content Security Policy headers, input sanitization at multiple layers, and regular security assessments to detect and prevent similar vulnerabilities in their application frameworks. This vulnerability demonstrates the importance of proper input validation and the potential risks associated with prototype-based inheritance mechanisms in modern web applications, aligning with ATT&CK technique T1190 for exploiting vulnerabilities in web applications and T1059 for executing malicious code through application flaws.