CVE-2026-25223

Workarounds

If upgrading is not immediately possible, user can implement a custom onRequest hook to reject requests containing tab characters in the Content-Type header:

fastify.addHook('onRequest', async (request, reply) => {
  const contentType = request.headers['content-type']
  if (contentType && contentType.includes('\t')) {
    reply.code(400).send({ error: 'Invalid Content-Type header' })
  }
})

Resources

• https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/validation.js#L272
• https://github.com/fastify/fastify/blob/759e9787b5669abf953068e42a17bffba7521348/lib/content-type-parser.js#L125
• Fastify Validation and Serialization Documentation
• https://hackerone.com/reports/3464114

A validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t) followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type.

For example, a request with Content-Type: application/json\ta will bypass JSON schema validation but still be parsed as JSON.

This vulnerability affects all Fastify users who rely on Content-Type-based body validation schemas to enforce data integrity or security constraints. The concrete impact depends on the handler implementation and the level of trust placed in the validated request body, but at the library level, this allows complete bypass of body validation for any handler using Content-Type-discriminated schemas.

This issue is a regression or missed edge case from the fix for a previously reported vulnerability.

CVE-2026-25223 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 (5.7.2); upgrading removes the vulnerable code path.

Kodem intelligence

This vulnerability has been patched in Fastify v5.7.2. All users should upgrade to this version or later immediately.

1. What is CVE-2026-25223? CVE-2026-25223 is a high-severity security vulnerability in fastify (npm), affecting versions < 5.7.2. It is fixed in 5.7.2.
2. How severe is CVE-2026-25223? CVE-2026-25223 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-2026-25223? fastify (npm) versions < 5.7.2 is affected.
4. Is there a fix for CVE-2026-25223? Yes. CVE-2026-25223 is fixed in 5.7.2. Upgrade to this version or later.
5. Is CVE-2026-25223 exploitable, and should I be worried? Whether CVE-2026-25223 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-2026-25223 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-2026-25223? Upgrade fastify to 5.7.2 or later.