CVE-2026-33804: @fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ign

Routine

HIGH 7.4REMOTEUPGRADE

CVE-2026-33804 is a high-severity flaw (CVSS 7.4) affecting fastify\/middie. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

@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

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 80%)
CISA exploited list	No
Weakness	CWE-436
Published	2026-04-16
Last modified	2026-06-17

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-16 15:17 UTCPublished CVE record published
2026-06-17 10:38 UTCUpdated Record last modified

fastify fastify\/middie UPGRADE

Affected: < 9.3.2

Fixed in: 9.3.2

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls fastifymiddie

2. UPGRADE to 9.3.2

npm install fastifymiddie@">=9.3.2"

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-2026-33804?

fastify\/middie: upgrade to 9.3.2. Exact commands for each OS are on this page.

Is CVE-2026-33804 being exploited?

CVE-2026-33804 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-2026-33804 affect?

CVE-2026-33804 affects fastify\/middie. Severity is high (CVSS 7.4). @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 perfo

How urgent is CVE-2026-33804?

Priority 36/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-85184 CRITICAL 9.1 UPGRADE @fastify/middie versions >= 9.1.0 and before 9.3.4 decide whether to run path-scoped middleware by matching ag
CVE-2026-14181 HIGH 7.5 UPGRADE @fastify/middie versions 9.1.0 through 9.3.2 fail to guard the URL normalization step used by the standalone e
CVE-2026-14198 CRITICAL 9.1 UPGRADE @fastify/middie versions 9.1.0 through 9.3.2 decode the encoded slash %2F inside path parameter values before
CVE-2026-6270 CRITICAL 9.1 UPGRADE @fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engin

References

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