CVE-2026-84504
fastify vulnerable to request body replacement via an async validation result collision
No known exploitation. EPSS puts it in the 34th percentile. Only a workaround so far.
What to do
No fixed build is published yet. The vendor describes a workaround.
What it is
From the CVE record
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.
In plain language
No plain-language summary for this CVE yet.
Exploitation
Where each signal puts this CVE on the scale from published to confirmed exploited.
- CISA KEV
Not in the catalog. CISA has not confirmed exploitation.
- Public exploits
No public exploit or proof of concept found in the sources we track.
- EPSS
0.4% chance of exploitation activity in the next 30 days, which ranks it in the 34th percentile of scored CVEs.
Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.
Lifecycle
8 events over 11 days, from the signal feeds we watch.
- Workaround availablerecord updated
- Record updated
- Publishedweakness classified, att&ck mapped, record updated, record updated
Affected products
Technical detail
CVSS 3.1 vector
Open in the CVSS calculatorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Scored 8.1 by NVD.
How it is reached
- Attack Vector NetworkExploitable remotely over the network without any special conditions
- Attack Complexity LowNo special conditions — the attack can be reliably reproduced
- Privileges Required LowRequires basic user-level privileges
- User Interaction NoneNo user interaction needed — fully automated exploitation
Scope
- Scope UnchangedImpact is limited to the vulnerable component itself
Impact if exploited
- Confidentiality HighTotal information disclosure — all data in the component is compromised
- Integrity HighTotal loss of integrity — attacker can modify any data in the component
- Availability NoneNo availability impact
Weaknesses
ATT&CK techniques
Mapped from the weaknesses above (CWE to ATT&CK), not observed in attacks.
- Initial AccessT1190Exploit Public-Facing Applicationlow confidence
Sources
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