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CVE-2026-84428

HIGHCVSS 7.5/10EPSS 0.28%

Last modified

CVE-2026-84428 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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 dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. EPSS estimates a 0.28% chance of exploitation in the next 30 days.

Description

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 dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.

Metrics

CVSS 3.1
7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS Probability
0.28%

20.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

VendorProductVersions
FastifyFastify< 5.12.2

References

Timeline

Published
Last Modified
Status
Analyzed

Frequently Asked Questions

What is CVE-2026-84428?
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 dependency such as requiring an authentication header whenever a privileged-mode header is present never matches, and the presence assertion is silently skipped. An unauthenticated remote client can therefore send the header that activates a privileged branch while omitting the header the dependency was meant to require, bypassing the conditional check. Users should upgrade to fastify 5.12.2 or later.
How severe is CVE-2026-84428?
CVE-2026-84428 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 0.28% probability of exploitation in the next 30 days.
How do I fix CVE-2026-84428?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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Source: NVD / NIST