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Vulnerabilities

Vulnerability · 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.

Published Apr 15, 2026 · catalog updated Jun 17, 2026 · Impact@fastify/express v4.0.4 and earlier

Check if you run it

No confirmed exploitation is on record for CVE-2026-33808. EPSS is 0.55%. Act only if a listed version of Impact@fastify/express v4.0.4 and earlier is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Impact@fastify/express v4.0.4 and earlier).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Critical9.1
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.55%
Remediation
Version ranges on file
Affected products
cpe fastify/fastify/express · stated fastify/express

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe fastify/fastify/expressversion unspecified
  • stated fastify/express<= 0.4

How to fix CVE-2026-33808

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-33808. EPSS is 0.55%. Act only if a listed version of Impact@fastify/express v4.0.4 and earlier is installed.

Which versions are affected?

Compare what you have installed with <= 0.4. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-436. EPSS 0.55%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Apr 15, 2026 · NIST NVD

    Detected

    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 normalize

  2. Jun 17, 2026 · NIST NVD

    Updated

    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 normalize

Sources

  • CVE.orgVulnerability recordCVE-2026-33808

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-33808

    30-day exploitation probability 0.6%

    first.org
  • NIST NVDVulnerability recordCVE-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 ignoreDuplicateSlashe

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.