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.
Advisories

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Fixes

Solution

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Workaround

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History

Fri, 04 Sep 2026 17:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}


Fri, 04 Sep 2026 12:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-179
References
Metrics threat_severity

None

threat_severity

Important


Fri, 04 Sep 2026 10:45:00 +0000

Type Values Removed Values Added
First Time appeared Fastify
Fastify fastify
Vendors & Products Fastify
Fastify fastify

Fri, 04 Sep 2026 09:45:00 +0000

Type Values Removed Values Added
Description 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.
Title fastify vulnerable to request body replacement via an async validation result collision
Weaknesses CWE-20
References
Metrics cvssV3_1

{'score': 8.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N'}


Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-09-04T16:34:58.664Z

Reserved: 2026-09-01T21:10:05.765Z

Link: CVE-2026-84504

cve-icon Vulnrichment

Updated: 2026-09-04T16:34:53.741Z

cve-icon NVD

Status : Received

Published: 2026-09-04T10:17:13.790

Modified: 2026-09-04T17:17:01.740

Link: CVE-2026-84504

cve-icon Redhat

Severity : Important

Publid Date: 2026-09-04T09:40:18Z

Links: CVE-2026-84504 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-09-04T13:30:05Z

Weaknesses