Vulnerability Intelligence Report
@fastify/express vulnerable to middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)
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.
No Active Exploit Signals
CVSS Base Score
9.1
CRITICAL
EPSS Probability:0.48%
Executive Threat Verdict
Evaluating...
Evaluating Threat Landscape...
Assessing known weaponization, exploitation telemetry, and federal advisories.
Attack Surface
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Authentication
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Weaponization
—
SSVC Action
—
Weaknesses (CWE)
CWE-436 ↗CWE-436: Interpretation Conflict
Affected Products & Versions
| Vendor | Product | Affected Versions |
|---|---|---|
| fastify | @fastify/express | 0 < 4.0.5 (affected), 4.0.5 (unaffected) |
References & Technical Advisories
No reference links found.
Threat Intelligence Signals
EPSS Score
0.483%
GitHub Advisory
Identity & Timeline
| Status | PUBLISHED |
| Assigning Authority | OpenJS Foundation · Open Source · USA |
| Reserved | 2026-03-23T19:48:48 |
| Published | 2026-04-15T09:29:46 |
| Last Updated | 2026-04-15T13:10:24 |
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