EUVD-2026-291386

fkie_cve-2026-33808

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.

GHSA-6hw5-45gm-fj88 — @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Affected: npm: @fastify/express

### Summary

`@fastify/express` v4.0.4 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 two vectors:

1. **Duplicate slashes** (`//admin/dashboard`) when `ignoreDuplicateSlashes: true` is configured 2. **Semicolon delimiters** (`/admin;bypass`) when `useSemicolonDelimiter: true` is configured

In both cases, Fastify's 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.

Note: This is distinct from GHSA-g6q3-96cp-5r5m (CVE-2026-22037), which addressed URL percent-encoding bypass and was patched in v4.0.3. These normalization gaps remain in v4.0.4. A similar class of normalization issue was addressed in `@fastify/middie` via GHSA-8p85-9qpw-fwgw (CVE-2026-2880), but `@fastify/express` does not include the equivalent fixes.

### Details

The vulnerability exists in `@fastify/express`'s `enhanceRequest` function (`index.js` lines 43-46):

```javascript const decodedUrl = decodeURI(url) req.raw.url = decodedUrl ```

The `decodeURI()` function only handles percent-encoding — it does not normalize duplicate slashes or strip semicolon-delimited parameters. When Fastify's router options are enabled, `find-my-way` applies these normalizations during route matching, but `@fastify/express` passes the original URL to Express middl…

Most recent entries from all

bdu:2026-05631

EUVD-2026-291386

fkie_cve-2026-33808

GHSA-6hw5-45gm-fj88 — @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)