CVE-2026-18504

Name of the Vulnerable Software and Affected Versions fastify versions prior to 5.12.1

Description A schema validation bypass occurs when a request body schema targets a root primitive value. When validating a top-level primitive, such as an integer, Ajv (a JSON schema validator) may coerce a JSON string into the expected type. However, the framework fails to replace the root request body with this coerced value, causing the route handler to receive the original unvalidated string. This allows requests that should have failed validation to reach application logic with values that do not satisfy the schema, potentially undermining integrity and access-control checks that rely on the validated type.

Recommendations Update to version 5.12.1.