CVE-2026-84504fastify vulnerable to request body replacement via an async validation result collision
Updated under an hour ago ()
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No exploitation reported and no EPSS estimate yet.
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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
- 8.1 HIGH (v3.1, CNA)CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
- Weakness (CWE)
- CWE-20
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- openjs
- NVD analysis
- Analyzed
- Record last updated
- Sep 4, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
fastify fastify fastify:fastify (from the name) | before 5.12.2; 5.12.2 not affected | CNA record |
fastify fastify fastify:fastify | before 5.12.2 | NVD |
References
- https://github.com/fastify/fastify/security/advisories/GHSA-667r-xxjv-c9mmmitigation, vendor-advisory
- https://cna.openjsf.org/security-advisories.htmlthird-party-advisory
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.