CVE-2026-84428: Fastify Fastify Vulnerable To Header Validation Vulnerability

HIGH
Advisory Identifier: GHSA-9q9j-q6p8-xq58
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CVSS 7.5
CWE-178
Published
Advisory Summary

CVE-2026-84428 is a HIGH (CVSS 7.5) fastify vulnerable to header validation vulnerability (CWE-178: Improper Handling of Case Sensitivity) affecting Fastify libraries in the npm ecosystem. The issue involves fastify vulnerable to header validation bypass via incomplete schema case normalization. Remediation is available by upgrading affected packages to version 5.12.2.

CVE-2026-84428 at a Glance

CVE Identifier CVE-2026-84428
GHSA Identifier GHSA-9q9j-q6p8-xq58
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-178: Improper Handling of Case Sensitivity
Affected Product Fastify
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2026-84428

To remediate this vulnerability, upgrade all affected Fastify dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fastify fixed: 5.12.2 
npm install fastify@5.12.2
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls fastify

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-84428, inspect your project dependencies using npm:

npm ls fastify

Analyze the terminal output to locate any version of Fastify in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 5.12.2), your application is affected.

Note: The npm ls fastify command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-178: Improper Handling of Case Sensitivity, where the product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results. In Fastify, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-84428 Affected Fastify Versions

1 component(s)
Fixed Versions:
5.12.2
Product / Package Ecosystem Affected Versions Fixed Version
fastify
npm/fastify
npm
< 5.12.2
5.12.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58
NVD — CVE-2026-84428 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-84428
Security Fix Commit FIX COMMIT
https://github.com/fastify/fastify/commit/179619c8e99d54924309b9933402151ef9af37e0
Fastify Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
Fastify Source Repository REPOSITORY
https://github.com/fastify/fastify
Fastify Release RELEASE
https://github.com/fastify/fastify/releases/tag/v5.12.2

Frequently Asked Questions

What is CVE-2026-84428?
CVE-2026-84428 is a security vulnerability in Fastify involving fastify vulnerable to header validation bypass via incomplete schema case normalization under CWE-178: Improper Handling of Case Sensitivity.
Which Fastify versions are affected?
Affected versions include releases prior to 5.12.2 across affected packages: fastify.
What version fixes CVE-2026-84428?
Upgrading to 5.12.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fastify affected?
Yes, fastify is affected in releases prior to 5.12.2. Upgrade your dependency to 5.12.2 or later.
What is the CVSS score of CVE-2026-84428?
CVE-2026-84428 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
What CWE is associated with CVE-2026-84428?
CVE-2026-84428 is classified under CWE-178: Improper Handling of Case Sensitivity (The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.).
How can I check npm dependencies for CVE-2026-84428?
Run the dependency tree command: 'npm ls fastify'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9q9j-q6p8-xq58",
  "modified" : "2026-09-30T23:44:27Z",
  "published" : "2026-09-30T23:44:27Z",
  "aliases" : [ "CVE-2026-84428" ],
  "summary" : "fastify vulnerable to header validation bypass via incomplete schema case normalization",
  "details" : "### Impact\n\nFastify 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.\n\n### Patches\n\nHeader-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.\n\n### Workarounds\n\nIf 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fastify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.12.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/security/advisories/GHSA-9q9j-q6p8-xq58"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-84428"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/commit/179619c8e99d54924309b9933402151ef9af37e0"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fastify/fastify"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/releases/tag/v5.12.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-178" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T23:44:27Z",
    "nvd_published_at" : "2026-09-04T11:17:19Z"
  }
}