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Fastify (< 5.12.2, < 6.0.0)

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high advisory

Fastify Request Body Replacement Vulnerability via Async Validation Result

A vulnerability in Fastify's request validation logic (CVE-2026-84504) allows attackers to perform request body replacement when using $async JSON schema validators, leading to potential unauthorized state changes or data disclosure.

fastify +1 web-application vulnerability cve authentication-bypass security-bypass cve-2026-84428
1t 1c