CVE-2026-84469: fastify Improper Input Validation

Vulnerability summary

CVE-2026-84469 is a high (CVSS 7.5) Improper Input Validation vulnerability in fastify. EPSS estimates a 0.49% chance of exploitation in the next 30 days, higher than 40.9% of scored vulnerabilities.

Timeline

1. 04-09-2026
   PublishedVulnerability first disclosed.
2. 04-09-2026
   Last modifiedVulnerability information updated.

Description

fastify versions before 5.12.2 decide whether to compile a request schema based on JavaScript truthiness, but JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance. When an application assigns false to a route's body, querystring, params, or headers schema to deny all input, fastify treats it as a missing schema, compiles no validator, and runs the route handler on any request. An unauthenticated remote client can therefore reach a handler that a valid deny-all schema was intended to make unreachable, a complete validation bypass that can lead to unauthorized state changes or execution of disabled operations. Users should upgrade to fastify 5.12.2 or later.

• CWE-20Improper Input ValidationThe 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.