GHSA-7P8R-X3MC-P8W7

Vulnerability from github – Published: 2026-08-03 19:16 – Updated: 2026-08-03 19:16
VLAI
Summary
fast-uri vulnerable to host confusion via backslash authority introducer
Details

Impact

fast-uri v4.1.1 and earlier require a literal // to recognize a URI authority, so a reference that uses \\, /\, or \/ as the authority introducer (in place of //, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) instead treats \ as interchangeable with / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input.

For example, fast-uri resolves \\evil.com/path against base https://allowed.com/ to https://allowed.com/%5C%5Cevil.com/path (confined to the trusted host), while Node's WHATWG URL resolves the same reference to https://evil.com/path.

Applications that use fast-uri to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination.

Patches

Upgrade to fast-uri v4.1.2, v3.1.5, v2.4.4.

Workarounds

None. Upgrade to the patched version.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.4.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0"
            },
            {
              "fixed": "3.1.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.1.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-18446"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-436"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-03T19:16:43Z",
    "nvd_published_at": "2026-07-31T15:16:27Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\n`fast-uri` v4.1.1 and earlier require a literal `//` to recognize a URI authority, so a reference that uses `\\\\`, `/\\`, or `\\/` as the authority introducer (in place of `//`, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node\u0027s native WHATWG `URL` (used by `fetch()`, `undici`, and Node\u0027s `http`/`https` clients) instead treats `\\` as interchangeable with `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input.\n\nFor example, `fast-uri` resolves `\\\\evil.com/path` against base `https://allowed.com/` to `https://allowed.com/%5C%5Cevil.com/path` (confined to the trusted host), while Node\u0027s WHATWG URL resolves the same reference to `https://evil.com/path`.\n\nApplications that use `fast-uri` to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node\u0027s URL or `fetch()` consumers see a policy/use desync and can be steered to an unintended destination.\n\n### Patches\n\nUpgrade to `fast-uri` v4.1.2, v3.1.5, v2.4.4.\n\n### Workarounds\n\nNone. Upgrade to the patched version.",
  "id": "GHSA-7p8r-x3mc-p8w7",
  "modified": "2026-08-03T19:16:43Z",
  "published": "2026-08-03T19:16:43Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/security/advisories/GHSA-7p8r-x3mc-p8w7"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18446"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/commit/f3c6c905f47831007490f466c5945012e905cc52"
    },
    {
      "type": "WEB",
      "url": "https://cna.openjsf.org/security-advisories.html"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fastify/fast-uri"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/releases/tag/v4.1.2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-uri vulnerable to host confusion via backslash authority introducer"
}



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