Arp Spoofing in Loopback

Arp Spoofing in Loopback environments presents unique challenges due to the framework's dynamic model and relation handling. In Loopback applications, this vulnerability often surfaces through the way the framework automatically generates REST endpoints and handles relation-based queries.

The most common manifestation occurs in relation endpoints where Loopback's default behavior exposes internal data structures. Consider a typical User-Account relationship:

// Typical Loopback model with relations
const User = app.model('User', {
  properties: {
    id: { type: 'number', id: true },
    username: { type: 'string' },
    email: { type: 'string' }
  }
});

const Account = app.model('Account', {
  properties: {
    id: { type: 'number', id: true },
    userId: { type: 'number' },
    accountNumber: { type: 'string' },
    balance: { type: 'number' }
  }
});

// Relation definition
User.hasMany(Account, { foreignKey: 'userId' });
Account.belongsTo(User, { foreignKey: 'userId' });

The vulnerability emerges when Loopback automatically generates endpoints like /users/{id}/accounts. Without proper authorization checks, an attacker can enumerate accounts by simply changing the user ID in the URL:

GET /users/1/accounts
GET /users/2/accounts
GET /users/3/accounts

Each request returns all accounts belonging to the specified user, regardless of who is authenticated. This is classic Arp Spoofing where the API trusts the URL parameter without verifying the requester's rights to access that specific resource.

Loopback's dynamic discovery mechanism for relations exacerbates this issue. The framework's RelationDefinitionBuilder automatically creates endpoints based on model definitions, but these endpoints inherit no security context from their parent operations. A user authenticated as 'user-1' can access /users/2/accounts if no explicit authorization logic is implemented.

Another common pattern appears in Loopback's beforeRemote hooks. Developers often implement partial authorization but miss edge cases:

User.beforeRemote('**', async (ctx, unused, next) => {
  const userId = ctx.args.filter.id;
  const authenticatedUserId = ctx.state.user.id;
  
  // BUG: Only checks top-level resource, not nested relations
  if (userId !== authenticatedUserId) {
    return next(new Error('Unauthorized'));
  }
  next();
});

This hook prevents direct access to /users/{id} but fails to protect relation endpoints like /users/{id}/accounts, allowing attackers to enumerate related resources through the nested API structure.

Detecting Arp Spoofing in Loopback applications requires understanding the framework's auto-generated endpoint patterns. The vulnerability typically appears in relation endpoints and dynamic route handlers that accept resource identifiers.

middleBrick's scanner identifies Loopback-specific Arp Spoofing patterns through several mechanisms:

1. Relation endpoint analysis - scanning for /{model}/{id}/{relation} patterns without proper authorization
2. Dynamic model discovery - detecting auto-generated endpoints that expose internal relations
3. Parameter manipulation testing - systematically changing ID parameters to test access controls
4. Response analysis - checking if different user IDs return different data sets

For manual testing, you can use curl to probe potential vulnerabilities:

# Test if user can access another user's accounts
curl -H "Authorization: Bearer $TOKEN" \
  http://localhost:3000/users/1/accounts

If this returns accounts belonging to user ID 1 without proper authorization, you have an Arp Spoofing vulnerability. The scanner automates this process across all relation endpoints and parameter combinations.

middleBrick specifically tests Loopback applications by:

• Analyzing OpenAPI specs generated by Loopback's REST API
• Testing all relation endpoints for parameter-based access control
• Checking for missing authorization in beforeRemote hooks
• Verifying that nested relation calls properly validate parent resource ownership

The scanner's LLM/AI security module also checks for prompt injection vulnerabilities in Loopback applications that use AI features, as these can sometimes bypass traditional authorization checks through crafted inputs.

Fixing Arp Spoofing in Loopback requires implementing proper authorization at the relation level. The framework provides several mechanisms to secure these endpoints.

The most effective approach uses Loopback's beforeRemote hooks with proper context checking:

// Secure relation endpoints
User.beforeRemote('**', async (ctx, unused, next) => {
  const method = ctx.methodString;
  const userIdFromURL = ctx.args.filter?.where?.id;
  const authenticatedUserId = ctx.state.user?.id;
  
  // Check if this is a relation endpoint
  const isRelationCall = method.includes('prototype.__get__') || 
                        method.includes('prototype.__findById__');
  
  if (isRelationCall) {
    // For relation calls, check the parent resource ownership
    const parentId = ctx.instance?.id || ctx.args.id;
    if (parentId !== authenticatedUserId) {
      return next(new Error('Unauthorized access to related resources'));
    }
  } else if (userIdFromURL) {
    // For direct resource access
    if (userIdFromURL !== authenticatedUserId) {
      return next(new Error('Unauthorized'));
    }
  }
  next();
});

For more granular control, implement operation-level permissions:

module.exports = function(User) {
  User.beforeRemote('prototype.__get__accounts', async (ctx, unused, next) => {
    const userInstance = ctx.instance;
    const authenticatedUserId = ctx.state.user.id;
    
    // Verify the requesting user owns the parent resource
    if (userInstance.id !== authenticatedUserId) {
      return next(new Error('Unauthorized: cannot view other users accounts'));
    }
    next();
  });

  // Similarly secure other relation operations
  User.beforeRemote('prototype.__create__accounts', async (ctx, unused, next) => {
    const userInstance = ctx.instance;
    const authenticatedUserId = ctx.state.user.id;
    
    if (userInstance.id !== authenticatedUserId) {
      return next(new Error('Unauthorized: cannot create accounts for other users'));
    }
    next();
  });
};

Loopback 4 and later versions provide built-in authorization decorators that simplify this process:

import {inject} from '@loopback/core';
import {repository} from '@loopback/repository';
import {AccountRepository} from '../repositories';
import {UserProfile} from 'loopback4-authentication';

export class AccountController {
  constructor(
    @repository(AccountRepository)
    public accountRepo: AccountRepository,
  ) {}

  @authenticate('jwt')
  @authorize({allow: ['owner', 'admin']})
  async findAccounts(
    @param.path.number('userId') userId: number,
    @inject(AuthenticationBindings.CURRENT_USER)
    currentUser: UserProfile,
  ): Promise {
    // Authorization is handled by the decorator
    return this.accountRepo.find({
      where: { userId },
    });
  }
}

For comprehensive security, combine these approaches with Loopback's access control lists (ACLs):

{
  "accessType": "READ",
  "principalType": "ROLE",
  "principalId": "$authenticated",
  "permission": "ALLOW",
  "property": "findAccounts"
},
{
  "accessType": "READ",
  "principalType": "ROLE",
  "principalId": "admin",
  "permission": "ALLOW",
  "property": "findAccounts"
}

Finally, use middleBrick's continuous monitoring to verify that your fixes work. The scanner will test your endpoints after each deployment to ensure Arp Spoofing vulnerabilities don't reappear through code changes or new relation definitions.