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Vulnerabilities

Vulnerability · CVE-2026-33804

@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled.

Published Apr 16, 2026 · catalog updated Jun 17, 2026 · @fastify/middie versions 9.3.1 and

Check if you run it

No confirmed exploitation is on record for CVE-2026-33804. EPSS is 0.49%. Act only if a listed version of @fastify/middie versions 9.3.1 and is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (@fastify/middie versions 9.3.1 and).
  • 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
0.49%
Remediation
Version ranges on file
Affected products
cpe fastify/fastify/middie · stated fastify/middie

@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.

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/middieversion unspecified
  • stated fastify/middie<= 9.3.1

How to fix CVE-2026-33804

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-2026-33804. EPSS is 0.49%. Act only if a listed version of @fastify/middie versions 9.3.1 and is installed.

Which versions are affected?

Compare what you have installed with <= 9.3.1. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-436. EPSS 0.49%. 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.

Timeline

Detection, updates, and resolution for this record.

  1. Apr 16, 2026 · NIST NVD

    Detected

    @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications

  2. Jun 17, 2026 · NIST NVD

    Updated

    @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications

Sources

  • CVE.orgVulnerability recordCVE-2026-33804

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-33804

    30-day exploitation probability 0.5%

    first.org
  • NIST NVDVulnerability recordCVE-2026-33804

    @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing reque

    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.