CVE-2026-84504
HIGHCVE-2026-84504 is a high-severity (CVSS 8.1) Improper Input Validation vulnerability. O3 Security confirms whether CVE-2026-84504 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
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…
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-84504.
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.
Detection & mitigation playbook
VulnerabilityDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for the affected component. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Remediation status
No patched version of the affected component has shipped for CVE-2026-84504 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-84504 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-84504. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-84504 in your dependencies?
O3 detects CVE-2026-84504 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.