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Vulnerability record · CVE-2026-84504 · published 4 September 2026

CVE-2026-84504: Fastify improper input validation vulnerability

Fastify · Fastify

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.

8.1 CVSS 3.1 High EPSS 0.43% · top 65.4% CWE-20 · Improper input validation
8.1CVSS 3.1 base score
0.43%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
2References
15 Sep 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://cna.openjsf.org/security-advisories.html Third Party Advisory
https://github.com/fastify/fastify/security/advisories/GHSA-667r-xxjv-c9mm MitigationVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-84504), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.