CVE-2026-6556: @fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the

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CRITICAL 9.1REMOTEUPGRADE

CVE-2026-6556 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 versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed plugin scopes, so middleware registered with those forms does not match the actual prefixed request path. Applications that use path-scoped middleware for authentication, authorization, rate limiting, or auditin

Why this priority

Critical severity
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	0.5% (top 56%)
CISA exploited list	No
Weakness	CWE-285
Published	2026-06-30
Last modified	2026-07-01

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-07-01 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-30 13:19 UTCPublished CVE record published
2026-07-01 15:57 UTCUpdated Record last modified

fastify fastify\/express UPGRADE

Affected: < 4.0.7

Fixed in: 4.0.7

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls fastifyexpress

2. UPGRADE to 4.0.7

npm install fastifyexpress@">=4.0.7"

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-2026-6556?

fastify\/express: upgrade to 4.0.7. Exact commands for each OS are on this page.

Is CVE-2026-6556 being exploited?

CVE-2026-6556 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-2026-6556 affect?

CVE-2026-6556 affects fastify\/express. Severity is critical (CVSS 9.1). @fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed p

How urgent is CVE-2026-6556?

Priority 48/100. Critical severity; 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-2026-33807 CRITICAL 9.1 UPGRADE @fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middle

References

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