9.1

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-33808

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

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.

(GitHub Advisory)

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CVE-2026-33808

CVE-2026-33808:

9.1

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation pocAutomatable YesTech Impact Total

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→SSVCScheduled60 days

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
@fastify/express	npm	<= 4.0.4	4.0.5

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability described is an authentication bypass in @fastify/express due to improper URL normalization. The advisory explicitly points to the enhanceRequest function in index.js as the source of the vulnerability. By analyzing the patch that fixes the issue, I confirmed that the changes are centered within this function. I first identified the repository fastify/fastify-express and the patched version 4.0.5. By comparing the git tags of the vulnerable version 4.0.4 and the patched version 4.0.5, I located the fixing commit 674020f27ddc1d1709e4369cb40158d4c958d42b. The diff of this commit clearly shows that the enhanceRequest function was modified to add proper URL normalization before passing the request to Express middleware. The original code simply used decodeURI, while the patched code introduces new functions (normalizeUrl, splitPathAndQuery, removeDuplicateSlashes) to correctly handle duplicate slashes and semicolon delimiters, thus closing the bypass vulnerability. Therefore, the enhanceRequest function is the exact location of the vulnerable code.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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