CVE-2026-33807: @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middle

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CRITICAL 9.1REMOTEUPGRADE

CVE-2026-33807 is a critical-severity flaw (CVSS 9.1) affecting fastify\/express. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@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, authorizati

Why this priority

Critical severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 63%)
CISA exploited list	No
Weakness	CWE-436
Published	2026-04-15
Last modified	2026-10-26

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-15 10:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

fastify fastify\/express UPGRADE

Affected: < 4.0.5

Fixed in: 4.0.5

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls fastifyexpress

2. UPGRADE to 4.0.5

npm install fastifyexpress@">=4.0.5"

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-33807?

fastify\/express: upgrade to 4.0.5. Exact commands for each OS are on this page.

Is CVE-2026-33807 being exploited?

CVE-2026-33807 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-33807 affect?

CVE-2026-33807 affects fastify\/express. Severity is critical (CVSS 9.1). @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 middlewar

How urgent is CVE-2026-33807?

Priority 51/100. Critical severity; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-33808 CRITICAL 9.1 UPGRADE Impact@fastify/express v4.0.4 and earlier fails to normalize URLs before passing them to Express middleware wh
CVE-2026-6556 CRITICAL 9.1 UPGRADE @fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the

References

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