Fastify up to 5.12.0 input validation

Summaryinfo

A vulnerability was found in Fastify up to 5.12.0. It has been rated as critical. This affects an unknown function. This manipulation causes input validation. This vulnerability appears as CVE-2026-18504. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability was found in Fastify up to 5.12.0. It has been declared as critical. Affected by this vulnerability is an unknown function. The manipulation with an unknown input leads to a input validation vulnerability. The CWE definition for the vulnerability is 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. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

fastify is a fast and low overhead web framework for Node.js. Versions of fastify before 5.12.1 are affected by a schema validation bypass when a request body schema targets a root primitive value. When the schema validates a top-level primitive such as an integer, Ajv can coerce a JSON string into the expected type during validation, but Fastify does not replace the root request body with the coerced value, so the route handler receives the original unvalidated string. As a result, a request that should have failed validation can reach application logic with a value that does not satisfy the schema, which can undermine integrity and access-control checks that rely on the validated type. Users should upgrade to fastify 5.12.1, which fixes the mismatch. No known workarounds are available.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-18504 since 07/31/2026. The exploitation appears to be easy. The attack can be launched remotely. Neither technical details nor an exploit are publicly available.

Upgrading to version 5.12.1 eliminates this vulnerability.

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Productinfo

Name

• Fastify

Version

• 5.0
• 5.1
• 5.2
• 5.3
• 5.4
• 5.5
• 5.6
• 5.7
• 5.8
• 5.9
• 5.10
• 5.11
• 5.12.0

Website

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

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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