CVE-2026-84428 is a high-severity (CVSS 7.5) CWE-178 vulnerability in fastify. A fix is available for fastify — see the affected versions and patch details below.
fastify vulnerable to header validation bypass via incomplete schema case normalization
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-84428.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-84428 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
fastifynpmDescription
Impact
Fastify lowercases header-schema property names before compiling the schema, because Node.js stores request header names in lowercase. That normalization was incomplete: it lowercased only top-level properties keys and the root required array, and did not lowercase the JSON Schema Draft 7 dependencies keyword (its trigger keys and dependent property names) or names in nested subschemas. As a result, a header schema that uses dependencies to require one header when another is present (for example X-Admin requiring X-Admin-Token) never matches the lowercased request headers, so the dependency assertion is silently skipped. An unauthenticated remote client can send the header that activates a privileged path while omitting the header the dependency was meant to require, bypassing a schema-enforced security control. The header schema is idiomatic, valid JSON Schema Draft 7, and no custom validator, malformed request, or misconfiguration is required.
Patches
Header-schema names are now normalized across all schema positions (properties, required, dependencies, dependentRequired, dependentSchemas, and nested subschemas). Patched in fastify 5.12.2. The fix is also included in the 6.0.0 release. Header schemas referenced through an external shared $ref (registered with addSchema) are not reached by this normalization and now emit an FSTSEC002 startup warning; inline the header schema to keep case-insensitive assertions in effect.
Workarounds
If upgrading is not immediately possible, write header-schema names in lowercase so the dependencies and other case-sensitive assertions match Node's lowercased request headers, or enforce the cross-header requirement in an onRequest or preValidation hook instead of the schema.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify | all versions | 5.12.2npm install fastify@5.12.2 |
Affected Products
fastifyfastifyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify to 5.12.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-84428 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw in fastify allows an unauthenticated remote attacker to bypass header validation checks. Due to incomplete case normalization of HTTP header names, an attacker can send specially crafted headers to activate privileged application branches, potentially leading to unauthorized access to sensitive functionality…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.Source: Red Hat security advisory for CVE-2026-84428 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat OpenShift AI 2.25 | rhoai/odh-dashboard-rhel9:1791853525 | RHSA-2026:87410 |
| Red Hat OpenShift AI 3.4 | rhoai/odh-dashboard-rhel9:1791861020 | RHSA-2026:84734 |
| Red Hat OpenShift AI 3.5 | rhoai/odh-core-bff-rhel9:1791945789 | RHSA-2026:86891 |
Frequently Asked Questions
Is CVE-2026-84428 in your dependencies?
Find it across npm, including transitive dependencies.