Fastify up to 5.12.1 Header Schema Compilation input validation

Summaryinfo

A vulnerability was found in Fastify up to 5.12.1. It has been rated as problematic. This affects an unknown part of the component Header Schema Compilation. The manipulation leads to input validation. This vulnerability is documented as CVE-2026-84428. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in Fastify up to 5.12.1 and classified as problematic. This issue affects an unknown function of the component Header Schema Compilation. The manipulation with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is integrity. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-84428 since 09/01/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Neither technical details nor an exploit are publicly available.

Upgrading to version 5.12.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-70998). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Name

• Fastify

Version

• 5.12.0
• 5.12.1

Website

• Product: https://github.com/fastify/fastify/

CPE 2.3info

• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Discussion