CVE-2026-76169: fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the cust

Q: How do I fix CVE-2026-76169?

fastify: upgrade to 5.12.2. Exact commands for each OS are on this page.

Q: Is CVE-2026-76169 being exploited?

CVE-2026-76169 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-76169 affect?

CVE-2026-76169 affects fastify. Severity is high (CVSS 7.5). fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The inter

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-76169 is a high-severity flaw (CVSS 7.5) affecting fastify. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The internal not-found router for encapsulated handlers dispatches malformed paths through a single shared handler pointer before URL decoding, ignoring the prefix and skipping the selected handler's normal lifecycle. A

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 55%)
CISA exploited list	No
Weakness	CWE-288
Published	2026-09-04
Last modified	2026-09-15

Publisher (CNA): openjs · Vendor: fastify

Last updated 2026-09-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-04 10:17 UTCPublished CVE record published
2026-09-15 20:07 UTCUpdated Record last modified

fastify fastify UPGRADE

Affected: >= 4.0.0 and < 5.12.2

Fixed in: 5.12.2

Safe to downgrade to: a release before 4.0.0

1. Check installed version

npm ls fastify

2. UPGRADE to 5.12.2

npm install fastify@">=5.12.2"

3. If no update is available yet, DOWNGRADE to a release before 4.0.0

npm view fastify versions npm install fastify@"<4.0.0"

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-76169?

fastify: upgrade to 5.12.2. Exact commands for each OS are on this page.

Is CVE-2026-76169 being exploited?

CVE-2026-76169 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-2026-76169 affect?

CVE-2026-76169 affects fastify. Severity is high (CVSS 7.5). fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the custom not-found handler of a different sibling plugin, and invoke it without the preHandler hook declared for that handler. The inter

How urgent is CVE-2026-76169?

Priority 42/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-84469 HIGH 7.5 UPGRADE fastify versions before 5.12.2 decide whether to compile a request schema based on JavaScript truthiness, but
CVE-2026-84504 HIGH 8.1 UPGRADE fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value resu
CVE-2026-84428 HIGH 7.5 UPGRADE fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names

References

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