Bola Idor in Restify with Hmac Signatures

Broken Object Level Authorization (BOLA) in a Restify service that uses Hmac Signatures can occur when the signature is tied to a resource identifier that an attacker can manipulate or guess. In this setup, the client computes an Hmac over a canonical representation of the request (method, path, query, selected headers, and body) using a shared secret. The server recomputes the Hmac and, only if verification passes, proceeds to handle the request. The vulnerability arises when authorization checks are missing or incomplete after successful signature validation, for example when the signed path includes a user-controlled identifier such as /users/{id}/settings and the server uses the same signature verification logic for any ID without confirming that the authenticated subject owns that specific resource.

An attacker who can enumerate or guess valid resource IDs (e.g., sequential numeric IDs or predictable UUIDs) can craft requests with valid Hmac Signatures but unauthorized targets. Because the signature is computed over the raw path and parameters, simply rotating the ID changes the signed input; if the server does not bind the signature to an authorization context (e.g., the authenticated user’s permissions), the verification can pass while the server operates on a different object. Common root causes include missing ownership checks, incorrect scoping of keys, or treating the Hmac as a global authentication token rather than a request-specific integrity proof combined with proper authorization. In such configurations, the signature ensures data integrity and authenticity of the request format, but it does not by itself prevent horizontal privilege escalation where one user accesses another user’s resources.

Consider an API endpoint GET /v1/users/123/profile verified with Hmac Signatures in Restify. The client signs the canonical request for user 123. If an attacker changes the path to /v1/users/124/profile and the server only verifies the Hmac without checking that the authenticated principal corresponds to user 124, a BOLA flaw exists. The signature verification may still succeed if the attacker knows or can brute-force the next valid ID and if the shared secret is static, because the server does not enforce per-request authorization tied to the resource identifier. This combination demonstrates how cryptographic integrity controls can coexist with authorization gaps, making BOLA harder to detect through traffic inspection alone.

To discover such issues, scanners perform black-box testing by sending requests with different resource identifiers while keeping the Hmac valid through replay or reconstruction when the secret is exposed or predictable. They also check whether signature scope inadvertently includes query parameters that should not affect authorization and whether the server fails to enforce tenant or ownership constraints after verification. The risk is particularly acute when the signed representation does not include a user or tenant context, enabling cross-object access without triggering alerts.

Remediation focuses on ensuring that Hmac Signatures are used for integrity and that authorization is evaluated per request with explicit ownership checks. Do not rely on signature verification alone to enforce access control. In Restify, structure your route handlers so that after signature validation, you resolve the subject and the target resource and confirm that the subject is allowed to operate on that resource.

Below are concrete code examples for Restify that demonstrate secure handling of Hmac Signatures with proper authorization. The first example shows a helper to compute and verify Hmac over a canonical request representation, and the second shows a route that combines verification with per-request ownership checks.

// hmac-utils.js
const crypto = require('crypto');
const algorithm = 'sha256';

function buildCanonicalString(req) {
  // Method, path without query, sorted query keys, selected headers, and body
  const method = req.method.toUpperCase();
  const path = req.path.split('?')[0];
  const query = req.query ? Object.keys(req.query).sort().map(k => `${k}=${req.query[k]}`).join('&') : '';
  const headers = ['x-date', 'x-request-id']
    .map(k => `${k}:${req.get(k) || ''}`)
    .filter(Boolean)
    .join('\n');
  const body = req.body ? JSON.stringify(req.body) : '';
  return [method, path, query, headers, body].filter(Boolean).join('\n');
}

function computeHmac(secret, req) {
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(buildCanonicalString(req));
  return hmac.digest('hex');
}

function verifyHmac(secret, req) {
  const received = req.get('x-signature');
  if (!received) return false;
  const expected = computeHmac(secret, req);
  // timing-safe compare
  return crypto.timingSafeEqual(Buffer.from(received), Buffer.from(expected));
}

module.exports = { buildCanonicalString, computeHmac, verifyHmac };

Now use this in a Restify route with explicit ownership checks. Assume you have a lightweight user context resolved from a session or JWT and a function canAccess(userId, resourceId) that encodes your policy.

// server.js
const restify = require('restify');
const { verifyHmac } = require('./hmac-utils');

const server = restify.createServer();
server.use(restify.plugins.bodyParser());

// Example in-memory store; replace with your authorization service
const userResources = {
  'user-a': ['123', '124'],
  'user-b': ['125']
};

function canAccess(userId, resourceId) {
  return Array.isArray(userResources[userId]) && userResources[userId].includes(String(resourceId));
}

server.get('/v1/users/:id/profile', (req, res, next) => {
  if (!verifyHmac(SHARED_SECRET, req)) {
    return next(new restify.UnauthorizedError('Invalid signature'));
  }
  const userId = resolveUserIdFromSession(req); // implement your auth context resolution
  const resourceId = req.params.id;
  if (!canAccess(userId, resourceId)) {
    return next(new restify.ForbiddenError('Not authorized for this resource'));
  }
  // fetch and return the profile for resourceId owned by userId
  return res.send({ id: resourceId, name: 'Profile Data' });
});

function resolveUserIdFromSession(req) {
  // Example: extract from a session cookie or JWT claim
  return req.headers['x-user-id'] || 'unknown';
}

Key points in this remediation:

• Signature verification is performed before any business logic, rejecting tampered or replayed requests early.
• Ownership is checked explicitly using the authenticated user’s identity and the resource identifier from the path.
• The canonical string excludes query parameters that should not affect authorization, avoiding scope creep in the signature.
• Use timing-safe comparisons to prevent side-channel attacks on the Hmac verification.

For production, store secrets securely, rotate keys as part of a key management strategy, and ensure your authorization function reflects your tenant or ownership model. You can integrate these checks into your API tests and, if using the middleBrick platform, rely on its scans to surface missing authorization findings alongside signature-related risks; the CLI and GitHub Action integrations help enforce thresholds in CI/CD pipelines, while the Dashboard lets you track changes over time.