CVE-2026-18504

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Workarounds

Until you can upgrade, avoid relying on the validated type of a root primitive body. Wrap the value in an object schema (object properties are coerced in place), for example accept { "value": 10 } and read request.body.value, or re-check the type in the handler.

fastify before 5.12.1, when a route uses a root-level primitive body schema (for example an integer with a minimum and maximum) and the default type coercion, validates the coerced value but exposes the original, uncoerced value to the route handler. For example, a JSON body "10" is coerced to the number 10 and passes an integer 1 to 10 schema, but request.body stays the string "10". An application that trusts the validated type is handed a value that did not satisfy the schema, which can bypass limits the application enforces on that typed value. Object and array body schemas are not affected, they coerce their members in place.

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-2026-18504 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low 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 (5.12.1); upgrading removes the vulnerable code path.

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1. What is CVE-2026-18504? CVE-2026-18504 is a medium-severity improper input validation vulnerability in fastify (npm), affecting versions < 5.12.1. It is fixed in 5.12.1. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-18504? CVE-2026-18504 has a CVSS score of 5.4 (Medium). 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-2026-18504? fastify (npm) versions < 5.12.1 is affected.
4. Is there a fix for CVE-2026-18504? Yes. CVE-2026-18504 is fixed in 5.12.1. Upgrade to this version or later.
5. Is CVE-2026-18504 exploitable, and should I be worried? Whether CVE-2026-18504 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-2026-18504 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-2026-18504? Upgrade fastify to 5.12.1 or later.