Phishing Api Keys with Hmac Signatures

Fixing HMAC signature vulnerabilities requires architectural changes to how signatures are constructed and validated. Here are HMAC-specific remediation patterns:

Secure Signature Construction

Remove URLs from HMAC signatures and use relative paths instead:

const createSecureHmacSignature = (path, method, body, apiKey, secret) => {
  const timestamp = Date.now();
  const nonce = crypto.randomBytes(16).toString('hex');
  
  // Use relative path, not full URL
  const stringToSign = `${method.toUpperCase()}
${path}
${timestamp}
${nonce}
${body}`;
  
  const hmac = crypto.createHmac('sha256', secret);
  const signature = hmac.update(stringToSign).digest('hex');
  
  return { signature, timestamp, nonce };
};

Tenant-Aware Signatures

For multi-tenant systems, include tenant identifiers:

const createTenantAwareSignature = (tenantId, path, method, body, apiKey, secret) => {
  const timestamp = Date.now();
  const nonce = crypto.randomBytes(16).toString('hex');
  
  const stringToSign = `${tenantId}
${method.toUpperCase()}
${path}
${timestamp}
${nonce}
${body}`;
  
  const hmac = crypto.createHmac('sha256', secret);
  const signature = hmac.update(stringToSign).digest('hex');
  
  return { signature, timestamp, nonce, tenantId };
};

Strict Timestamp Validation

Validate timestamps with minimal windows:

const validateHmacRequest = (request, secret) => {
  const { signature, timestamp, nonce, tenantId } = request.headers;
  const currentTimestamp = Date.now();
  
  // Reject requests older than 300 seconds (5 minutes)
  if (currentTimestamp - parseInt(timestamp) > 300000) {
    throw new Error('Signature expired');
  }
  
  // Check for clock skew (allow 30 seconds)
  if (Math.abs(currentTimestamp - parseInt(timestamp)) > 30000) {
    throw new Error('Invalid timestamp');
  }
  
  // Verify nonce hasn't been used recently
  if (nonceCache.has(nonce)) {
    throw new Error('Nonce already used');
  }
  nonceCache.set(nonce, currentTimestamp);
  
  // Validate signature
  const expectedSignature = createSecureHmacSignature(
    request.path,
    request.method,
    request.body,
    tenantId,
    secret
  );
  
  if (expectedSignature !== signature) {
    throw new Error('Invalid signature');
  }
  
  return true;
};

API Key Rotation Security

Implement secure key rotation without exposing current keys:

// Never expose current key during rotation
const rotateApiKey = async (userId) => {
  const newSecret = crypto.randomBytes(32).toString('hex');
  const newKey = generateApiKey();
  
  // Create new key in disabled state
  await db.keys.create({
    userId,
    key: newKey,
    secret: newSecret,
    status: 'pending'
  });
  
  // Send secure notification (not the key itself)
  await sendRotationNotification(userId, newKey);
  
  return { success: true };
};

GitHub Action Integration

Add HMAC security checks to your CI/CD pipeline:

name: API Security Scan
on: [push, pull_request]

jobs:
  hmac-security:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Run middleBrick HMAC Scan
        run: |
          npm install -g middlebrick
          middlebrick scan https://api.example.com --test=hmac-signature-construction --fail-on-high-severity
        env:
          MIDDLEBRICK_API_KEY: ${{ secrets.MIDDLEBRICK_API_KEY }}

Continuous Monitoring

Pro customers can set up continuous HMAC signature monitoring:

middlebrick monitor --target=https://api.example.com \
  --test=hmac-signature-construction \
  --alert=slack \
  --schedule=daily \
  --threshold=high

This monitors your HMAC implementation daily and alerts your team if new vulnerabilities are detected, helping prevent phishing attacks before they impact production systems.