CVE-2026-33808 @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)
An unauthenticated attacker can bypass path-scoped authentication middleware in @fastify/express by exploiting URL normalization gaps when specific router options are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33808?
CVE-2026-33808 is a critical authentication bypass vulnerability in @fastify/express, published on 15 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via duplicate slashes when ignoreDuplicateSlashes is enabled, or via semicolon delimiters when useSemicolonDelimiter is enabled. In both cases, Fastify router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped. An unauthenticated attacker can access protected routes by manipulating the URL path.
PatchesUpgrade to @fastify/express v4.0.5 or later.
Weakness: CWE-436
Which versions are affected by CVE-2026-33808?
2 open source packages ship an affected version, including @fastify/express. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
@fastify/express npm | All versions | 4.0.5 |
@fastify/express npm | All versions | 4.0.5 |
Products include Fastify Fastify\/Express.
How do I fix CVE-2026-33808?
Upgrade @fastify/express to 4.0.5 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-33808 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33808 exists.
Other vulnerabilities in @fastify/express
Sources
Also known as GHSA-6hw5-45gm-fj88.