CVE-2026-84504: fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value resu

Routine

HIGH 8.1UPGRADE

CVE-2026-84504 is a high-severity flaw (CVSS 8.1) 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 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the root, fastify replaces the entire request body with that property's value before the handler runs, so the handler receives a different object than the one that satisfied the schema. An authenticated low-privileg

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	8.1 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.4% (top 63%)
CISA exploited list	No
Weakness	CWE-20
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 20:05 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-2026-84504?

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

Is CVE-2026-84504 being exploited?

CVE-2026-84504 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-2026-84504 affect?

CVE-2026-84504 affects fastify. Severity is high (CVSS 8.1). fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the r

How urgent is CVE-2026-84504?

Priority 38/100. High severity

Related CVEs

CVE-2026-76169 HIGH 7.5 UPGRADE fastify versions >= 4.0.0 and before 5.12.2 can route a malformed URL sent under one plugin prefix to the cust
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-2026-84428 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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