CVE-2026-84428 fastify vulnerable to header validation bypass via incomplete schema case normalization
An unauthenticated attacker can bypass conditional header validation checks in Fastify by exploiting incomplete case normalization in JSON Schema dependencies.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84428?
CVE-2026-84428 is a high-severity authentication bypass vulnerability in fastify, published on 4 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-178
Which versions are affected by CVE-2026-84428?
2 open source packages ship an affected version, including fastify. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fastify npm | All versions | 5.12.2 |
fastify npm | All versions | 5.12.2 |
Products include Fastify Fastify.
How do I fix CVE-2026-84428?
Upgrade fastify to 5.12.2 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-84428 being exploited?
There is no known exploitation of CVE-2026-84428 in the wild and no public exploit code.
Other vulnerabilities in fastify
- CVE-2026-84504 fastify vulnerable to request body replacement via an async validation result collision
- CVE-2026-76169 fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
- CVE-2026-84469 fastify vulnerable to request validation bypass via skipped boolean false schemas
- CVE-2026-33806 fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header
- CVE-2026-25223 Fastify's Content-Type header tab character allows body validation bypass
Sources
Also known as CLSA-2026-1791634874, CLSA-2026-1792242589, GHSA-9q9j-q6p8-xq58, MINI-chf9-9mrc-3rhf.