Broken Access Control in Loopback with Hmac Signatures

Broken Access Control in Loopback when using Hmac Signatures often arises when authorization checks are applied only after a request is authenticated by signature validity, rather than being enforced per endpoint and per subject. This combination can expose endpoints to authenticated-but-unauthorized access when signature validation does not include scope, role, or resource ownership checks.

Consider a Loopback application that uses Hmac Signatures to verify request integrity and authenticate calls. A common pattern is to validate the signature and attach a user identity to the request, then rely on older RBAC or ACL components to enforce permissions. If the authorization middleware is not configured to run for every route, or if it only checks whether a user is authenticated (and not whether they are permitted to access the specific resource), an attacker who obtains a valid signature can perform BOLA/IDOR by iterating over identifiers such as /users/123, /users/124, and so on. The signature remains valid because the secret key is correct, but the access control decisions are missing or misaligned with the data model.

Another scenario involves unsafe consumption of query or path parameters where permissions are evaluated using incomplete data. For example, an endpoint like GET /api/accounts/:accountId may verify the Hmac Signature and confirm the caller is a user, but if the authorization check does not assert that the user belongs to the requested accountId, a horizontally privileged user can access other users’ accounts. This is a classic Broken Access Control pattern, specifically BOLA (Broken Object Level Authorization), and it is exacerbated when signature-based authentication overshadows explicit per-request authorization logic.

In some configurations, developers might skip authorization for so-called read-only endpoints, assuming that Hmac Signatures prevent spoofing. However, read endpoints often return sensitive data objects (e.g., profile details, transaction records). If role or scope claims embedded in the signed payload are not validated on each request, or if the token’s claims are accepted without re-checking against the current resource, data exposure can occur. This intersects with the Data Exposure check when highly privileged data is returned without proper property-level authorization. Even with a valid Hmac Signature, an unauthenticated attacker is not required; a compromised or low-privilege account with a valid signature can abuse missing authorization checks to reach confidential endpoints.

These issues are detectable by middleBrick because it runs parallel checks including Authentication, Authorization, and Property Authorization alongside signature-based flows. When a scan reveals that endpoints accept valid Hmac Signatures but lack per-endpoint, per-subject authorization, the findings highlight BOLA/IDOR and Privilege Escalation risks. The scanner cross-references the OpenAPI spec definitions with runtime behavior, identifying routes where authorization is absent or conditional only on authentication rather than on resource ownership or role scopes. This helps surface misconfigurations where Hmac Signatures are treated as a sufficient access control mechanism rather than one component of a layered defense.

To remediate Broken Access Control when using Hmac Signatures in Loopback, enforce explicit authorization on every route and ensure that signature validation is one layer among many, not a replacement for object-level checks. Always couple authentication via Hmac with per-request authorization that validates subject-to-resource relationships, roles, and scopes.

Below are concrete code examples for a Loopback controller that uses Hmac Signatures and includes robust authorization. The first snippet shows a typical Hmac verification middleware added to the request pipeline. It computes the Hmac over selected parts of the request and attaches the verified principal to the context without assuming authorization is implicit.

// src/middleware/hmac-verify.ts
import { inject, LifeCycleBindings } from '@loopback/core';
import { Middleware, Next } from '@loopback/rest';
import crypto from 'crypto';

export class HmacVerifyMiddleware implements Middleware {
  constructor(
    @inject('services.secret', { optional: true })
    private secret: string = process.env.HMAC_SECRET || 'default-secret',
  ) {}

  async handle(context: RequestContext, next: Next) {
    const request = context.request;
    const signature = request.headers['x-hmac-signature'];
    if (!signature) {
      const err = new Error('Missing Hmac Signature');
      err['statusCode'] = 401;
      throw err;
    }
    const payload = [
      request.method,
      request.path,
      request.getQueryString(),
      request.getBodyString(),
    ].join('|');
    const expected = 'sha256=' + crypto.createHmac('sha256', this.secret).update(payload).digest('hex');
    if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
      const err = new Error('Invalid Hmac Signature');
      err['statusCode'] = 401;
      throw err;
    }
    // Attach a verified principal; authorization decisions should still happen per route
    const principal = { id: 'user-123', role: 'customer', scopes: ['accounts:read'] };
    (context as any).prismaUser = principal;
    await next();
  }
}

The second snippet shows a controller method where authorization is explicitly checked before accessing a specific account. This ensures that even with a valid Hmac Signature, the requesting user can only access resources they own.

// src/controllers/account.controller.ts
import { get, param, requestContext } from '@loopback/rest';
import { inject } from '@loopback/core';
import { AuthorizationBindings, Authorizer } from '@loopback/authorization';

export class AccountController {
  constructor(
    @inject(AuthorizationBindings.AUTHORIZER)
    private authorizer: Authorizer,
  ) {}

  @get('/accounts/{accountId}', {
    responses: {
      200: { description: 'Account details', content: { 'application/json': { schema: { type: 'object' } } } },
    },
  })
  async getAccount(
    @param.path.string('accountId') accountId: string,
    @requestContext() reqCtx: RequestContext,
  ) {
    const principal = (reqCtx as any).prismaUser;
    if (!principal) {
      const err = new Error('Unauthorized');
      err['statusCode'] = 401;
      throw err;
    }
    // Explicit object-level authorization: ensure principal owns the account
    const hasAccess = await this.authorizer.authorize(
      principal,
      { resource: 'Account', resourceId: accountId, permissions: ['read'] },
    );
    if (!hasAccess) {
      const err = new Error('Access denied');
      err['statusCode'] = 403;
      throw err;
    }
    // Fetch account for the userId, guaranteeing alignment between signature identity and resource ownership
    return fetchAccountById(accountId, principal.id);
  }
}

In addition to route-level checks, apply property-level authorization for sensitive fields and use scopes embedded in the Hmac claims to gate access. Configure the Loopback application to require scope validation on each authenticated request, and ensure that endpoints with sensitive operations (e.g., modifying account details) require both a valid Hmac Signature and explicit scope or role assertions. This layered approach mirrors the checks performed by middleBrick’s Authorization and Property Authorization tests, reducing the likelihood of Broken Access Control while preserving the integrity of Hmac-based authentication.