Auth Bypass on Fly Io

Auth bypass vulnerabilities in Fly Io applications typically emerge through misconfigured authentication middleware, exposed endpoints, and improper token validation. The stateless nature of Fly Io's deployment model means authentication must be handled explicitly in application code, creating multiple failure points.

A common manifestation occurs in Fly Io applications using JWT tokens without proper signature verification. Consider this vulnerable pattern:

const token = req.headers.authorization?.split(' ')[1];
const payload = jwt.decode(token); // ❌ Missing verification
if (payload.role === 'admin') {
  // Admin functionality accessible without proper auth
}

This code decodes the token but never verifies the signature, allowing attackers to forge tokens with arbitrary payloads. The issue is particularly prevalent in Fly Io apps because developers often rely on the platform's security reputation without implementing proper safeguards.

Another Fly Io-specific pattern involves misconfigured Fly Edge middleware. When authentication is handled in Edge middleware but application routes bypass it:

// fly.toml middleware configuration
[[middleware]]
  [middleware.functions]
  handler = "auth-middleware"

[[middleware]]
  [middleware.functions]
  handler = "rate-limit"

# Routes that bypass auth
[[http_service]]
  [http_service.functions]
  handler = "api-handler"
  include = ["/api/public/*"]
  
  [http_service.upstream]
  domain = "api.internal"

The problem intensifies when developers add public routes without considering that sensitive endpoints might be accessible through path traversal or when middleware ordering creates gaps in protection.

Fly Io's built-in SQLite database access through Deno's standard library can also introduce auth bypass when query parameters are concatenated without proper validation:

// Vulnerable to auth bypass via SQL injection
const userId = req.query.userId || '1';
const user = await db.query(`
  SELECT * FROM users WHERE id = ${userId}
`);

An attacker could manipulate the userId parameter to access other users' data, effectively bypassing authentication by exploiting the query logic rather than breaking authentication mechanisms directly.

Detecting auth bypass in Fly Io applications requires examining both the application code and deployment configuration. The first step is scanning exposed endpoints without authentication to identify publicly accessible sensitive routes.

Using middleBrick's CLI for Fly Io applications:

npx middlebrick scan https://your-fly-app.fly.dev/api

The scanner tests unauthenticated access to all endpoints, identifying those that should require authentication but don't. For Fly Io apps specifically, it examines:

• Edge middleware configuration in fly.toml for authentication gaps
• API routes that lack authentication decorators or middleware
• Database queries that don't validate user permissions
• JWT handling without signature verification
• CORS misconfigurations that might expose admin endpoints

middleBrick's OpenAPI analysis is particularly valuable for Fly Io apps since many use OpenAPI specs for API documentation. The scanner cross-references your spec with actual runtime behavior:

middlebrick scan --spec openapi.json https://your-fly-app.fly.dev

This identifies discrepancies between documented security requirements and actual implementation. For example, if your OpenAPI spec marks an endpoint as requiring authentication but middleBrick finds it accessible without credentials, you've discovered an auth bypass.

Manual detection techniques for Fly Io apps include:

# Test public access to admin endpoints
curl -i https://your-fly-app.fly.dev/admin/users

# Test token manipulation
curl -H "Authorization: Bearer invalid.token" https://your-fly-app.fly.dev/api/data

# Test parameter manipulation
curl https://your-fly-app.fly.dev/api/data?userId=9999

Look for responses that should require authentication but return data anyway. Pay special attention to endpoints that handle user IDs, resource IDs, or other identifiers that could be manipulated to access unauthorized data.

Remediating auth bypass in Fly Io applications requires a defense-in-depth approach using Fly's native features and proper authentication patterns. Start with proper JWT verification using Fly Io's built-in libraries:

import { create, verify } from "https://deno.land/x/[email protected]/mod.ts";

// Secure middleware for Fly Io applications
const authMiddleware = async (req: Request) => {
  const authHeader = req.headers.get('authorization');
  if (!authHeader || !authHeader.startsWith('Bearer ')) {
    return new Response('Missing token', { status: 401 });
  }
  
  const token = authHeader.substring(7);
  try {
    const payload = await verify(token, 
      Deno.env.get('JWT_SECRET')!, 
      "HS256"
    );
    
    // Attach user to request context
    req.user = payload;
    return await handleRequest(req);
  } catch (err) {
    return new Response('Invalid token', { status: 401 });
  }
};

For Fly Io's Edge middleware, implement authentication at the edge layer to prevent unauthorized requests from reaching your application:

// edge-auth-middleware.js
export default {
  async fetch(req, env) {
    const authHeader = req.headers.get('authorization');
    if (!authHeader || !authHeader.startsWith('Bearer ')) {
      return new Response('Unauthorized', { status: 401 });
    }
    
    const token = authHeader.substring(7);
    try {
      const payload = await verifyToken(token);
      req.user = payload;
      return env.API.fetch(req);
    } catch {
      return new Response('Unauthorized', { status: 401 });
    }
  }
};

Configure fly.toml to apply this middleware to protected routes:

[[middleware]]
  [middleware.functions]
  handler = "edge-auth-middleware"

[[http_service]]
  [http_service.functions]
  handler = "api-handler"
  include = ["/api/*"]
  exclude = ["/api/public/*"]

For database-level authorization in Fly Io apps using SQLite:

// Secure data access with proper authorization
const getUserData = async (userId: string, requestingUserId: string) => {
  const db = await openDB();
  
  // Verify user owns the data or has admin role
  const canAccess = await db.query(`
    SELECT role FROM users 
    WHERE id = ? AND (id = ? OR role = 'admin')
  `, [requestingUserId, userId]);
  
  if (!canAccess.length) {
    throw new Error('Access denied');
  }
  
  return await db.query(`
    SELECT * FROM user_data WHERE user_id = ?
  `, [userId]);
};

Implement role-based access control (RBAC) using Fly Io's environment variables for configuration:

const checkPermission = (userRole: string, requiredRole: string) => {
  const roleHierarchy = {
    'user': 1,
    'moderator': 2,
    'admin': 3
  };
  
  return roleHierarchy[userRole] >= roleHierarchy[requiredRole];
};

// Usage
if (!checkPermission(req.user.role, 'admin')) {
  return new Response('Insufficient permissions', { status: 403 });
}

Finally, use Fly Io's built-in rate limiting to prevent brute force attacks on authentication endpoints:

// fly.toml rate limiting
[[middleware]]
  [middleware.functions]
  handler = "rate-limit"
  [middleware.functions.arguments]
    max_requests = 100
    window = "1m"
    identifier = "ip"