CVE-2026-22037 @fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)
Attackers can bypass middleware security controls on protected endpoints by using URL-encoded characters in request paths.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-22037?
CVE-2026-22037 is a high-severity authentication bypass vulnerability in fastify-express, published on 19 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-22037?
2 open source packages ship an affected version, including @fastify/express.
| Package | Affected | Fixed in |
|---|---|---|
@fastify/express npm | All versions | 4.0.3 |
@fastify/express npm | All versions | 4.0.3 |
How do I fix CVE-2026-22037?
Upgrade @fastify/express to 4.0.3 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-22037 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-22037 exists.
Sources
Also known as GHSA-g6q3-96cp-5r5m.