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Vulnerabilities

Vulnerability · CVE-2026-33807

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins.

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

Check if you run it

No confirmed exploitation is on record for CVE-2026-33807. EPSS is 0.53%. Act only if a listed version of @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 (@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
0.53%
Remediation
Version ranges on file
Affected products
cpe fastify/fastify/express · stated fastify/express

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required. Upgrade 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-2026-33807

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-2026-33807. EPSS is 0.53%. Act only if a listed version of @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 0.53%. 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

    @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware securi

  2. Jun 17, 2026 · NIST NVD

    Updated

    @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware securi

Sources

  • CVE.orgVulnerability recordCVE-2026-33807

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-33807

    30-day exploitation probability 0.5%

    first.org
  • NIST NVDVulnerability recordCVE-2026-33807

    @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed

    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.