What’s Vulnerable › Vulnerabilities

CVE-2026-31865: Elysia Cookie Value Prototype Pollution

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-18 · updated 2026-06-17
• Weakness: CWE-1321

Description

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.

Affected products

• elysiajs elysia: before 1.4.27 — fixed in 1.4.27

How to fix

Upgrade to a fixed version: elysiajs elysia 1.4.27. Check the vendor advisory for mitigations.

References

• https://github.com/elysiajs/elysia/commit/e9d6b1743fa7368ef942dce181f6a089757f6aab
• https://github.com/elysiajs/elysia/security/advisories/GHSA-8hq9-phh3-p2wp

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