CVE-2026-84469

fastify vulnerable to request validation bypass via skipped boolean false schemas

Workarounds

If upgrading is not immediately possible, express a deny-all request schema with an always-failing object schema instead of the boolean false (for example { "not": {} }), or reject the request in an onRequest hook.

Fastify decided whether to validate a request part by checking its schema for JavaScript truthiness. JSON Schema Draft 7 defines the boolean false as a valid schema that rejects every instance, but because false is falsy, a route that set body, querystring, params, or headers to false had that part left uncompiled: no validator was attached and the request reached the handler. An application that used false as a deny-all schema to make a route unreachable was therefore fully bypassed, and an unauthenticated remote client could reach the handler with any input. The same applied to the documented query alias for querystring. This is a complete bypass rather than a weak-schema issue, since false is the strongest JSON Schema assertion and must always fail.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-84469 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.12.2); upgrading removes the vulnerable code path.

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Request-part schemas are now selected by an explicit presence check rather than truthiness, so a boolean false (or true) schema is compiled and enforced, including through the query alias. Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release.

1. What is CVE-2026-84469? CVE-2026-84469 is a high-severity improper input validation vulnerability in fastify (npm), affecting versions < 5.12.2. It is fixed in 5.12.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-84469? CVE-2026-84469 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fastify are affected by CVE-2026-84469? fastify (npm) versions < 5.12.2 is affected.
4. Is there a fix for CVE-2026-84469? Yes. CVE-2026-84469 is fixed in 5.12.2. Upgrade to this version or later.
5. Is CVE-2026-84469 exploitable, and should I be worried? Whether CVE-2026-84469 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-84469 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-84469? Upgrade fastify to 5.12.2 or later.

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