CVE-2026-84428: Fastify header validation bypass via incomplete schema case normalization

CVE-2026-84428 · Severity: high · CVSS 7.5 · Published 2026-09-04

Fastify is a web framework used to build Node.js-based REST APIs and web services. A vulnerability in header validation allows an unauthenticated attacker to bypass security checks that enforce conditional header requirements (such as requiring an authentication token when accessing privileged endpoints). An attacker can send privileged requests while omitting required authentication headers, potentially gaining unauthorized access to protected functionality.

Technologies: fastify (npm). Vendors: npm.

Quick answers

Is CVE-2026-84428 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-84428 being exploited in the wild.
How severe is CVE-2026-84428?
High severity, with a CVSS score of 7.5, published 4 September 2026.
Which products and versions are affected?
fastify (npm) < 5.12.2, SEMVER:introduced=0,fixed=5.12.2.
Is there a fix for CVE-2026-84428?
Yes. fastify (npm): upgrade to 5.12.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-84428 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-84428?
Check whether you run fastify (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • fastify (npm): upgrade to 5.12.2 or later
    npm install fastify@5.12.2

Technical details

The vulnerability exists in Fastify's header schema normalization logic. Fastify lowercases header property names to match Node.js's lowercase storage of request headers, but the normalization was incomplete: it only lowercased top-level properties and the root required array, but failed to lowercase the JSON Schema Draft 7 dependencies keyword (trigger keys and dependent property names) and nested subschema names. As a result, dependency assertions that require one header when another is present never match the lowercased request headers, silently skipping the validation. An unauthenticated remote attacker can send a request with a privileged header while omitting a required dependent header, bypassing the schema-enforced security control. The attack requires no privileges, user interaction, or malformed requests. The fix normalizes header-schema names across all schema positions and is available in Fastify 5.12.2 and later.

Affected products

  • OpenJS Foundation Fastify < 5.12.2

Identifiers

  • CVE-2026-84428
  • GHSA-9q9j-q6p8-xq58

Timeline

  • 2026-09-04: disclosed: CVE-2026-84428 published
  • 2026-09-04: patched: Fastify 5.12.2 and 6.0.0 released with fix

References

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