CVE-2026-16732: Fastify Fastify Vulnerable To X-Forwarded-* Spoofing Vulnerability

MODERATE
Advisory Identifier: GHSA-3m5p-2c4r-xxw2
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CVSS 6.1
CWE-348
Published
Advisory Summary

CVE-2026-16732 is a MODERATE (CVSS 6.1) fastify vulnerable to x-forwarded-* spoofing vulnerability (CWE-348: Use of Less Trusted Source) affecting Fastify libraries in the npm ecosystem. The issue involves fastify vulnerable to x-forwarded-* spoofing under trustproxy hop-count. Remediation is available by upgrading affected packages to version 5.12.1.

CVE-2026-16732 at a Glance

CVE Identifier CVE-2026-16732
GHSA Identifier GHSA-3m5p-2c4r-xxw2
Severity MODERATE
CVSS v3.1 Base Score 6.1
CWE Classification CWE-348: Use of Less Trusted Source
Affected Product Fastify
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 6.1

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

Attack Vector Adjacent (AV:A)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-16732

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: 5.12.1 
npm install fastify@5.12.1
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-2026-16732, 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 5.12.1), 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-348: Use of Less Trusted Source, where the product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack. 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 The fix for [CVE-2026-3635](https://www.cve.org/CVERecord?id=CVE-2026-3635) ([GHSA-444r-cwp2-x5xf](https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf)) added a `proxyFn(socket.remoteAddress, 0)` guard on the `X-Forwarded-*` reads in `request.host`, `request.protocol`, `request.hostname`, `request.ip`, and `request.ips`. That guard closes the IP, CIDR, and custom-function forms of `trustProxy` correctly because those forms compile to predicates that inspect the connecting address. The hop-count form (`trustProxy: <number>`) compiles to a predicate that structurally ignores the address argument, so the guard reduces to `0 < tp`, always true for any `tp >= 1`. Applications configured with `trustProxy: <number>` (documented as "behind N reverse proxies", `trustProxy: 1` being the canonical single-proxy setting) remain vulnerable. An attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the request fields exactly as in the unpatched version. Impact class matches the parent CVE-2026-3635: host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie / CSRF-origin bypass, host-based routing and cache poisoning. ## Patches Patched in fastify 5.12.1. The numeric form of `trustProxy` is now disabled at runtime and removed from the TypeScript type union. ## Workarounds - Migrate to an IP / CIDR / custom-function `trustProxy` value that validates the connecting address. Custom functions must inspect the `address` argument, not only the hop index. - Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).

CVE-2026-16732 Affected Fastify Versions

1 component(s)
Fixed Versions:
5.12.1
Product / Package Ecosystem Affected Versions Fixed Version
fastify
npm/fastify
npm
≥ 5.8.3
< 5.12.1
5.12.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2
NVD — CVE-2026-16732 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-16732
Security Fix Commit FIX COMMIT
https://github.com/fastify/fastify/commit/8acfea7eada05383a7357e3eb21c2628416df280
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/v5.12.1

Frequently Asked Questions

What is CVE-2026-16732?
CVE-2026-16732 is a security vulnerability in Fastify involving fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count under CWE-348: Use of Less Trusted Source.
Which Fastify versions are affected?
Affected versions include releases prior to 5.12.1 across affected packages: fastify.
What version fixes CVE-2026-16732?
Upgrading to 5.12.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fastify affected?
Yes, fastify is affected in releases prior to 5.12.1. Upgrade your dependency to 5.12.1 or later.
What is the CVSS score of CVE-2026-16732?
CVE-2026-16732 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N.
What CWE is associated with CVE-2026-16732?
CVE-2026-16732 is classified under CWE-348: Use of Less Trusted Source (The product has two different sources of the same data or information, but it uses the source that has less support for verification, is less trusted, or is less resistant to attack.).
How can I check npm dependencies for CVE-2026-16732?
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-3m5p-2c4r-xxw2",
  "modified" : "2026-09-02T15:14:06Z",
  "published" : "2026-09-02T15:14:06Z",
  "aliases" : [ "CVE-2026-16732" ],
  "summary" : "fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count",
  "details" : "## Impact\n\nThe fix for [CVE-2026-3635](https://www.cve.org/CVERecord?id=CVE-2026-3635) ([GHSA-444r-cwp2-x5xf](https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf)) added a `proxyFn(socket.remoteAddress, 0)` guard on the `X-Forwarded-*` reads in `request.host`, `request.protocol`, `request.hostname`, `request.ip`, and `request.ips`. That guard closes the IP, CIDR, and custom-function forms of `trustProxy` correctly because those forms compile to predicates that inspect the connecting address. The hop-count form (`trustProxy: <number>`) compiles to a predicate that structurally ignores the address argument, so the guard reduces to `0 < tp`, always true for any `tp >= 1`.\n\nApplications configured with `trustProxy: <number>` (documented as \"behind N reverse proxies\", `trustProxy: 1` being the canonical single-proxy setting) remain vulnerable. An attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the request fields exactly as in the unpatched version. Impact class matches the parent CVE-2026-3635: host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie / CSRF-origin bypass, host-based routing and cache poisoning.\n\n## Patches\n\nPatched in fastify 5.12.1. The numeric form of `trustProxy` is now disabled at runtime and removed from the TypeScript type union.\n\n## Workarounds\n\n- Migrate to an IP / CIDR / custom-function `trustProxy` value that validates the connecting address. Custom functions must inspect the `address` argument, not only the hop index.\n- Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fastify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.8.3"
      }, {
        "fixed" : "5.12.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-16732"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fastify/commit/8acfea7eada05383a7357e3eb21c2628416df280"
  }, {
    "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/v5.12.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-348" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:14:06Z",
    "nvd_published_at" : "2026-08-18T21:16:34Z"
  }
}