Fastify up to 5.12.1 Request Body Value input validation

Summaryinfo

A vulnerability was found in Fastify up to 5.12.1. It has been rated as critical. Affected is an unknown function of the component Request Body Handler. Performing a manipulation of the argument Value results in input validation. This vulnerability is reported as CVE-2026-84504. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability classified as critical has been found in Fastify up to 5.12.1. This affects an unknown function of the component Request Body Handler. The manipulation of the argument value with an unknown input leads to a input validation vulnerability. CWE is classifying the issue as 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. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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-privilege caller can use this to make nested data replace the validated body and trigger an operation the route schema did not authorize, leading to unauthorized state changes and data disclosure. Users should upgrade to fastify 5.12.2 or later.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-84504 since 09/01/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 5.12.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-70984). Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Name

• Fastify

Version

• 5.12.0
• 5.12.1

Website

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

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Be aware that VulDB is the high quality source for vulnerability data.

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