Broken Access Control in Loopback with Mutual Tls

Broken Access Control (BAC) in Loopback applications using Mutual Transport Layer Security (mTLS) can occur when authorization checks are missing or bypassed, even though the transport layer verifies client certificates. mTLS ensures that both client and server present valid certificates, but it does not enforce business-level permissions. Developers may mistakenly assume that mTLS alone is sufficient for access control, leading to BOLA/IDOR and BFLA/Privilege Escalation when sensitive endpoints rely solely on certificate validation rather than explicit authorization logic.

For example, an authenticated client with a valid certificate might invoke a User Profile endpoint like /users/{id}. If the server does not verify that the requesting user’s certificate subject or mapped identity matches the requested resource id, an attacker can enumerate numeric IDs and access other users’ data. This is a classic BOLA/IDOR scenario. The BAC manifests because the authorization check — ensuring the requesting principal owns or is permitted to access the target resource — is absent or incorrectly implemented atop mTLS.

Additionally, Privilege Escalation can arise when an endpoint performs role-based checks in application code but misconfigures the mapping between mTLS client certificates and roles. An attacker could present a certificate that the server accepts but then manipulate requests to access admin-only endpoints if role assertions are not independently validated. In Loopback, this can happen if the app relies on implicit trust from mTLS without applying model-level ACLs or context-based authorization. Data Exposure and Property Authorization checks are often implicated here, as improperly filtered model properties may be returned to a client that should only see a subset of fields.

Input Validation weaknesses can compound BAC in mTLS setups. If authorization logic parses certificate fields (e.g., Distinguished Name or SAN) and uses them directly in queries without validation, attackers might inject crafted certificate attributes to elevate access. SSRF and Unsafe Consumption can also interact with BAC when endpoints that verify mTLS calls external services without proper scoping, potentially exposing internal resources.

middleBrick’s 12 security checks, including BOLA/IDOR, BFLA/Privilege Escalation, and Property Authorization, are designed to detect these issues in unauthenticated scans. The scanner maps findings to frameworks such as OWASP API Top 10 and provides remediation guidance, helping teams identify missing authorization boundaries even when mTLS is in place.

To remediate Broken Access Control while retaining mTLS, enforce explicit authorization on each endpoint and bind certificate identity to business permissions. Below are concrete Loopback examples that combine mTLS with robust access checks.

1) Configure mTLS in Loopback to extract certificate details and map them to a principal:

// server/middleware/mtls-auth.js
const fs = require('fs');
const path = require('path');
const sslCert = require('ssl-cert');

module.exports = function configureMTLS(app) {
  const server = app.loopback.get('server');
  const options = {
    cert: fs.readFileSync(path.join(__dirname, '../../certs/ca.pem')),
    requestCert: true,
    rejectUnauthorized: true,
  };
  server.set('sslOptions', options);

  // Middleware to map client certificate to user context
  server.use((req, res, next) => {
    const cert = req.connection.getPeerCertificate();
    if (!cert || Object.keys(cert).length === 0) {
      return res.status(401).send('Client certificate required');
    }
    const subject = cert.subject;
    const commonName = subject.CN;
    // Map CN to a user identity in your app (e.g., via a lookup service)
    req.remotingContext = req.remotingContext || {};
    req.remotingContext.principal = {
      id: commonName,
      roles: resolveRolesForCert(commonName),
    };
    next();
  });
};

function resolveRolesForCert(commonName) {
  // Implement a lookup based on your directory or database
  if (commonName.startsWith('admin-')) return ['admin'];
  if (commonName.startsWith('user-')) return ['user'];
  return ['guest'];
}

2) Apply model-level ACLs in Loopback models to enforce ownership and role checks:

// common/models/user.json
{
  "name": "User",
  "base": "User",
  "acls": [
    {
      "accessType": "*",
      "principalType": "ROLE",
      "principalId": "$everyone",
      "permission": "DENY"
    },
    {
      "accessType": "READ",
      "principalType": "ROLE",
      "principalId": "user",
      "permission": "ALLOW",
      "property": "findById",
      "condition": { "where": { "id": { "inq": { "_loopback_context.principal.id" } } } }
    },
    {
      "accessType": "WRITE",
      "principalType": "ROLE",
      "principalId": "admin",
      "permission": "ALLOW"
    }
  ]
}

3) In your controller or repository, validate that the requesting principal matches the target resource explicitly:

// common/controllers/user.controller.js
module.exports = function(User) {
  User.prototype.getProfile = async function(context) {
    const userId = context.req.params.id;
    const principal = context.remotingContext.principal;
    if (!principal) throw new Error('Unauthorized');
    if (principal.id !== userId && !principal.roles.includes('admin')) {
      throw new Error('Access denied: insufficient permissions');
    }
    return User.findById(userId);
  };

  User.remoteMethod('getProfile', {
    accepts: { arg: 'id', type: 'string', required: true, http: { source: 'path' } },
    returns: { arg: 'user', type: 'object' },
    http: { verb: 'get', path: '/users/:id' },
  });
};

4) Ensure role mapping is kept up to date and avoid relying on certificate fields alone for permissions. Use middleware to enrich the request context with roles and ownership metadata, and validate this context in datasources or repository methods. Combine mTLS with application-level checks so that even if a certificate is accepted, each request is verified against explicit authorization rules.