CVE-2026-76169: Fastify Authentication Bypass Vulnerability

HIGH
Advisory Identifier: GHSA-p68q-wchp-6fh7
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CVSS 7.5
CWE-288
Published
Advisory Summary

CVE-2026-76169 is a HIGH (CVSS 7.5) authentication bypass vulnerability (CWE-288: Authentication Bypass Using an Alternate Path or Channel) affecting Fastify libraries in the npm ecosystem. The issue involves fastify vulnerable to authentication bypass via malformed urls reaching encapsulated not-found handlers. Remediation is available by upgrading affected packages to version 5.12.2.

CVE-2026-76169 at a Glance

CVE Identifier CVE-2026-76169
GHSA Identifier GHSA-p68q-wchp-6fh7
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-288: Authentication Bypass Using an Alternate Path or Channel
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:H/I:N/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 High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-76169

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-2026-76169, 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-288: Authentication Bypass Using an Alternate Path or Channel, where the product requires authentication, but the product has an alternate path or channel that does not require authentication. 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 routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the `preHandler` declared in its `setNotFoundHandler()`. When the request method has no route in the main router, a malformed request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected. ### Patches Patched in fastify 5.12.2. Malformed URLs are now routed through the configured `onBadUrl` and `onMaxParamLength` handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed. ### Workarounds Reject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global `onRequest` authentication hook does not mitigate this, because the malformed-URL path skips it.

CVE-2026-76169 Affected Fastify Versions

1 component(s)
Fixed Versions:
5.12.2
Product / Package Ecosystem Affected Versions Fixed Version
fastify
npm/fastify
npm
≥ 4.0.0
< 5.12.2
5.12.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fastify/security/advisories/GHSA-p68q-wchp-6fh7
NVD — CVE-2026-76169 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-76169
Security Fix Commit FIX COMMIT
https://github.com/fastify/fastify/commit/93c239a380f3f2778bb7565fcdf777e91cfe1fbc
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-2026-76169?
CVE-2026-76169 is a security vulnerability in Fastify involving fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers under CWE-288: Authentication Bypass Using an Alternate Path or Channel.
Which Fastify versions are affected?
Affected versions include releases prior to 5.12.2 across affected packages: fastify.
What version fixes CVE-2026-76169?
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-2026-76169?
CVE-2026-76169 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:H/I:N/A:N.
What CWE is associated with CVE-2026-76169?
CVE-2026-76169 is classified under CWE-288: Authentication Bypass Using an Alternate Path or Channel (The product requires authentication, but the product has an alternate path or channel that does not require authentication.).
How can I check npm dependencies for CVE-2026-76169?
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-p68q-wchp-6fh7",
  "modified" : "2026-09-30T23:45:34Z",
  "published" : "2026-09-30T23:45:34Z",
  "aliases" : [ "CVE-2026-76169" ],
  "summary" : "fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers",
  "details" : "### Impact\n\nFastify routes a malformed URL under one plugin prefix to the custom not-found handler of a different sibling plugin, invoking the handler registered last and skipping the `preHandler` declared in its `setNotFoundHandler()`. When the request method has no route in the main router, a malformed request target reaches Fastify's internal not-found router before URL decoding and is dispatched through a single shared handler pointer, regardless of prefix and without the normal request lifecycle. An unauthenticated request to a public prefix can therefore reach an authentication-protected not-found handler registered under a different prefix and receive its full response, breaking prefix encapsulation and bypassing the authentication hook. Applications whose private or tenant fallbacks return protected data from a not-found handler are affected.\n\n### Patches\n\nPatched in fastify 5.12.2. Malformed URLs are now routed through the configured `onBadUrl` and `onMaxParamLength` handlers so they fail closed before any application not-found handler runs, and the shared not-found handler pointer has been removed.\n\n### Workarounds\n\nReject malformed request targets before they reach the application, for example at an upstream proxy or gateway, and do not rely on a not-found handler to serve protected data. A global `onRequest` authentication hook does not mitigate this, because the malformed-URL path skips it.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fastify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "5.12.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/security/advisories/GHSA-p68q-wchp-6fh7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-76169"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/commit/93c239a380f3f2778bb7565fcdf777e91cfe1fbc"
  }, {
    "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-288" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T23:45:34Z",
    "nvd_published_at" : "2026-09-04T10:17:12Z"
  }
}