CVE-2026-107832

fastify is vulnerable to header validation bypass via case-sensitive propertyNames

Workarounds

Write the propertyNames header-name constraint in lowercase (for example enum: ['x-debug-override']), or enforce the header check in an onRequest or preValidation hook instead of the schema.

Fastify lowercases header names in a headers schema before compilation, because Node.js stores request header names in lowercase. This normalization does not reach the header names held in a propertyNames subschema's enum and const. A header-name deny-list written in the canonical header case, such as propertyNames: { not: { enum: ['X-Debug-Override'] } }, therefore never matches the lowercased request headers, and the forbidden header reaches the handler. Any application that uses a propertyNames deny-list or allow-list to constrain request header names does not get the constraint it expects. The same rule written in lowercase works, and the not: { required: [...] } form is unaffected, so this is a gap in the built-in normalization rather than a wrong schema. This is an incomplete fix of CVE-2026-84428 (GHSA-9q9j-q6p8-xq58), whose normalization covered header names used as property keys but not those used as values inside propertyNames.

CVE-2026-107832 has a CVSS score of 3.7 (Low). 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 (5.12.6); upgrading removes the vulnerable code path.

Kodem intelligence

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Users should upgrade to fastify 5.12.6 or later, which lowercases header names held in enum and const inside a propertyNames subschema so they match the lowercased request headers. Header-name constraints that cannot be normalized automatically, regular expressions (propertyNames pattern and patternProperties keys) and a $ref used inside propertyNames, are documented and, for the $ref case, warned at startup (FSTSEC003). Author those to match the lowercased request headers.

1. What is CVE-2026-107832? CVE-2026-107832 is a low-severity security vulnerability in fastify (npm), affecting versions >= 5.12.2, < 5.12.6. It is fixed in 5.12.6.
2. How severe is CVE-2026-107832? CVE-2026-107832 has a CVSS score of 3.7 (Low). 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-2026-107832? fastify (npm) versions >= 5.12.2, < 5.12.6 is affected.
4. Is there a fix for CVE-2026-107832? Yes. CVE-2026-107832 is fixed in 5.12.6. Upgrade to this version or later.
5. Is CVE-2026-107832 exploitable, and should I be worried? Whether CVE-2026-107832 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-2026-107832 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-2026-107832? Upgrade fastify to 5.12.6 or later.

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