Broken Access Control in Restify with Hmac Signatures

Broken Access Control in a Restify service that uses Hmac Signatures often arises when signature verification is limited to the presence of an Hmac header rather than validating scope, intended recipient, and replay characteristics. An attacker can exploit missing or weak authorization checks by reusing a captured, correctly signed request to access another user’s resource or elevate privileges. For example, if a Restify route relies only on Hmac-SHA256 to confirm request authenticity but does not enforce per-user or per-action authorization, an authenticated user can modify resource identifiers in the URL or body to interact with other users’ data.

Consider an endpoint /users/:userId/profile where the Hmac signature is computed over selected headers and the request body. If the server validates the Hmac but does not confirm that the authenticated principal matches the :userId in the path, a BOLA (Broken Object Level Authorization) / IDOR condition occurs. An attacker who knows another user’s numeric ID can send a validly signed request with that ID and gain unauthorized access. This is a classic case where Hmac Signatures ensure message integrity and authenticity between client and server, but do not replace proper access control decisions tied to identity and context.

In a microservice or API gateway setup, misconfigured routing or missing checks can amplify the risk. For instance, if a service trusts an Hmac signature but does not validate scopes, roles, or tenant boundaries, horizontal and vertical privilege escalation may occur. Attack patterns such as tampering with the Authorization header, replaying requests, or manipulating metadata can lead to unauthorized data exposure or operations. Because Restify allows fine-grained route and middleware control, it is essential to couple Hmac verification with explicit authorization checks at the resource and action level to prevent these access control bypasses.

Remediation focuses on ensuring that Hmac verification is combined with robust authorization logic and safe handling of identifiers. Always validate the Hmac signature before processing the request, then enforce that the identity derived from the signature matches the requested resource. Do not rely on obscurity of IDs or assume trust from a valid signature alone.

Below are concrete Restify code examples that demonstrate secure handling. The first example shows how to verify an Hmac signature and enforce ownership of the resource. The second example illustrates adding scope-based checks before allowing sensitive operations.

const restify = require('restify');
const crypto = require('crypto');

function verifyHmac(req, res, next) {
  const signature = req.headers['x-hmac-signature'];
  const timestamp = req.headers['x-timestamp'];
  const nonce = req.headers['x-nonce'];
  if (!signature || !timestamp || !nonce) {
    return res.send(401, { error: 'Missing authentication headers' });
  }
  const payload = timestamp + nonce + (req.body || '');
  const expected = crypto.createHmac('sha256', process.env.HMAC_SECRET)
                         .update(payload)
                         .digest('hex');
  if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
    return res.send(401, { error: 'Invalid signature' });
  }
  // attach verified principal from a trusted claim, e.g., a JWT or a mapping lookup
  req.principal = resolvePrincipal(req.headers['x-principal']);
  next();
}

function resolvePrincipal(principalId) {
  // map principalId to user object with roles/scopes; in practice, fetch from a data store
  return { id: principalId, roles: ['user'], scopes: ['profile:read', 'profile:write'] };
}

const server = restify.createServer();
server.pre(verifyHmac);

server.get('/profiles/:userId', (req, res, next) => {
  const profileOwnerId = req.params.userId;
  if (req.principal.id !== profileOwnerId) {
    return res.send(403, { error: 'Forbidden: cannot access another user profile' });
  }
  // fetch and return profile for req.principal.id
  res.send(200, { userId: profileOwnerId, data: 'profile data' });
  return next();
});

server.post('/profiles/:userId/email', (req, res, next) => {
  if (!req.principal.scopes.includes('email:write')) {
    return res.send(403, { error: 'Insufficient scope for email update' });
  }
  // perform email update for the profile owned by req.principal.id
  res.send(200, { message: 'email update initiated' });
  return next();
});

server.listen(8080, () => console.log('API listening on port 8080'));

In the second example, scope-based authorization ensures that even with a valid Hmac signature, operations are gated by explicit permissions. This prevents privilege escalation when endpoints are accidentally exposed or when role checks are missing. For production use, integrate these checks with your identity provider and ensure secrets are managed securely. Leverage middleware to keep authorization logic consistent across routes, and log suspicious mismatches for audit without exposing sensitive details to the client.

When integrating with CI/CD, tools such as the middleBrick GitHub Action can add API security checks to your pipeline, failing builds if risk scores drop below your chosen threshold. This complements runtime protections by catching regressions before deployment. For developers working locally, the middleBrick CLI allows on-demand scans from the terminal using middlebrick scan <url>, while the MCP Server enables scanning APIs directly from AI coding assistants within your IDE.