Vulnerability-Lookup

GHSA-3M5P-2C4R-XXW2

Vulnerability from github – Published: 2026-09-02 15:14 – Updated: 2026-09-02 15:14

Summary

fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count

Details

Impact

The fix for CVE-2026-3635 (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).

Severity

6.1 (Medium)


                  
                    CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fastify"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "5.8.3"
            },
            {
              "fixed": "5.12.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-16732"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-348"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-02T15:14:06Z",
    "nvd_published_at": "2026-08-18T21:16:34Z",
    "severity": "MODERATE"
  },
  "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: \u003cnumber\u003e`) compiles to a predicate that structurally ignores the address argument, so the guard reduces to `0 \u003c tp`, always true for any `tp \u003e= 1`.\n\nApplications configured with `trustProxy: \u003cnumber\u003e` (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).",
  "id": "GHSA-3m5p-2c4r-xxw2",
  "modified": "2026-09-02T15:14:06Z",
  "published": "2026-09-02T15:14:06Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count"
}

CVE-2026-16732 (GCVE-0-2026-16732)

Vulnerability from cvelistv5 – Published: 2026-08-18 20:30 – Updated: 2026-08-19 13:16

Title

fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count

Summary

fastify is a fast and low overhead web framework for Node.js. Impact: the fix for CVE-2026-3635 added a guard on the forwarded-header reads used to derive the request host, protocol, hostname, ip, and ips values, checking the connecting address. 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, where trustProxy is set to a number, compiles to a predicate that structurally ignores the address, so the guard is always satisfied for any hop count of one or more. Applications configured with a numeric trustProxy value, such as trustProxy set to 1 for a single reverse proxy, remain vulnerable: an attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the forwarded request fields exactly as in the unpatched version. The impact class matches the parent CVE-2026-3635, including host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, and host-based routing and cache poisoning. Affected versions are fastify from 5.8.3 up to but not including 5.12.1. Patches: patched in fastify 5.12.1, where the numeric form of trustProxy is disabled at runtime and removed from the TypeScript type union. Workarounds: migrate to an IP, CIDR, or custom-function trustProxy value that validates the connecting address, and ensure the Fastify origin is only reachable through the trusted proxy chain.

Severity

6.1 (Medium)


                        
                          CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-19 13:15 UTC

CWE

CWE-348 - Use of Less Trusted Source

Assigner

openjs redhat root

References

2 references

URL	Tags
https://github.com/fastify/fastify/security/advis…
https://cna.openjsf.org/security-advisories.html

Impacted products

1 product

Vendor	Product	Version
fastify	fastify	Affected: 5.8.3 , < 5.12.1 (semver) Unaffected: 5.12.1 (semver)

Credits

alimony mcollina climba03003 UlisesGascon

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JSON

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Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.