GHSA-W74X-V6F9-J3XG

Vulnerability from github – Published: 2026-10-23 21:41 – Updated: 2026-10-23 21:41
VLAI
Summary
fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding
Details

Impact

fast-uri percent-decodes the scheme component with the legacy global unescape() before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but fast-uri does.

For example, normalize('%6Aavascript:alert(1)') returns javascript:alert(1), resolve('https://a.example/', '%6Aavascript:alert(1)') returns javascript:alert(1), and parse('%6Aavascript:alert(1)').scheme returns javascript. The %uXXXX form (%u006Aavascript:) produces the same result, and equal('%68ttp://a/', 'http://a/') returns true.

An application that screens dangerous schemes on the raw input and then stores or renders normalize(input) or resolve(base, input) turns an inert string into an executable javascript:, data:, or vbscript: URL, a stored or reflected cross-site scripting vector in the consuming application. equal() also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists.

Patches

Upgrade to fast-uri 2.4.8, 3.1.9, or 4.2.3.

Workarounds

Reject input whose scheme part (before the first :, and before any /, ?, or #) contains %. Alternatively, check parse(x).scheme against an allowlist after normalization and use that same parsed result rather than the raw string.

Severity
6.1 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.4.8"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0"
            },
            {
              "fixed": "3.1.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "npm",
        "name": "fast-uri"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.2.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-110306"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-172",
      "CWE-180"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-23T21:41:36Z",
    "nvd_published_at": "2026-10-20T07:17:08Z",
    "severity": "MODERATE"
  },
  "details": "### Impact\n\n`fast-uri` percent-decodes the scheme component with the legacy global `unescape()` before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but `fast-uri` does.\n\nFor example, `normalize(\u0027%6Aavascript:alert(1)\u0027)` returns `javascript:alert(1)`, `resolve(\u0027https://a.example/\u0027, \u0027%6Aavascript:alert(1)\u0027)` returns `javascript:alert(1)`, and `parse(\u0027%6Aavascript:alert(1)\u0027).scheme` returns `javascript`. The `%uXXXX` form (`%u006Aavascript:`) produces the same result, and `equal(\u0027%68ttp://a/\u0027, \u0027http://a/\u0027)` returns `true`.\n\nAn application that screens dangerous schemes on the raw input and then stores or renders `normalize(input)` or `resolve(base, input)` turns an inert string into an executable `javascript:`, `data:`, or `vbscript:` URL, a stored or reflected cross-site scripting vector in the consuming application. `equal()` also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists.\n\n### Patches\n\nUpgrade to `fast-uri` `2.4.8`, `3.1.9`, or `4.2.3`.\n\n### Workarounds\n\nReject input whose scheme part (before the first `:`, and before any `/`, `?`, or `#`) contains `%`. Alternatively, check `parse(x).scheme` against an allowlist after normalization and use that same parsed result rather than the raw string.",
  "id": "GHSA-w74x-v6f9-j3xg",
  "modified": "2026-10-23T21:41:36Z",
  "published": "2026-10-23T21:41:36Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/security/advisories/GHSA-w74x-v6f9-j3xg"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-110306"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/commit/39933a1f1422986f14e2d844f552238feb40f5d4"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/commit/597f254c86d151568511eeb9a1df761fd12520b3"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/commit/e60e716f41d1d73717e2d803d20b68d2c6d20a0f"
    },
    {
      "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/v2.4.8"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/releases/tag/v3.1.9"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fast-uri/releases/tag/v4.2.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding"
}



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