CVE-2026-84428: fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names

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HIGH 7.5REMOTEUPGRADE

CVE-2026-84428 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 before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the root-level required array, and does not lowercase the trigger and dependent names inside the JSON Schema Draft 7 dependencies keyword. Because Node stores request header names in lowercase, a canonical-case depe

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 56%)
CISA exploited list	No
Weakness	CWE-178
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 11:17 UTCPublished CVE record published
2026-09-15 19:54 UTCUpdated Record last modified

fastify fastify UPGRADE

Affected: < 5.12.2

Fixed in: 5.12.2

Safe to downgrade to: no unaffected older release

1. Check installed version

npm ls fastify

2. UPGRADE to 5.12.2

npm install fastify@">=5.12.2"

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-2026-84428?

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

Is CVE-2026-84428 being exploited?

CVE-2026-84428 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-2026-84428 affect?

CVE-2026-84428 affects fastify. Severity is high (CVSS 7.5). fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header names by lowercasing names in a route's header schema before compiling it, but the transformation is incomplete: it lowercases the properties keys and the r

How urgent is CVE-2026-84428?

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

Related CVEs

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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
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References

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