Bug 2528450 (CVE-2026-84504)

Summary: CVE-2026-84504 fastify: fastify: Unauthorized state changes and data disclosure via request body replacement
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: bbrownin, sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in fastify. An authenticated low-privilege caller can exploit a vulnerability where the framework incorrectly processes the result of an Ajv asynchronous (async) validator. If a request body contains a root-level property named 'value', fastify replaces the entire request body with this property's content, bypassing the intended schema validation. This can lead to unauthorized state changes and the disclosure of sensitive data, as the application handler receives an unvalidated object.
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Description OSIDB Bzimport 2026-09-04 09:52:01 UTC 
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.