CVE-2026-33808: @fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

(updated )

@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.

References

Code Behaviors & Features

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