Information Disclosure with Hmac Signatures

Information disclosure in HMAC signatures occurs when cryptographic secrets or implementation details leak through various channels, enabling attackers to bypass authentication or forge valid signatures. The most common manifestation involves timing attacks on comparison operations.

Consider this vulnerable HMAC verification pattern:

function verifyHmac(receivedHmac, message, key) {
  const expectedHmac = crypto.createHmac('sha256', key)
    .update(message)
    .digest('hex');
  
  return receivedHmac === expectedHmac; // Vulnerable to timing attacks
}

The strict equality operator (===) performs character-by-character comparison and exits on the first mismatch. This creates a timing side-channel where an attacker can measure response times to determine how many characters of the HMAC match. For a 256-bit HMAC, this reduces the attack complexity from 2^256 to approximately 2^16 operations.

Another disclosure vector involves error messages that reveal implementation details:

try {
  const hmac = crypto.createHmac('sha256', key)
    .update(message)
    .digest('hex');
  
  return hmac;
} catch (err) {
  throw new Error('HMAC generation failed: ' + err.message);
  // Reveals: algorithm support, key format, internal errors
}

Stack traces from HMAC failures can expose the exact library version, key derivation paths, and cryptographic parameters being used. An attacker receiving this information gains valuable intelligence for crafting targeted attacks.

Key leakage through logging represents another critical disclosure vector:

function logHmacRequest(req, res, next) {
  const hmacHeader = req.get('X-HMAC-Signature');
  console.log(`Received HMAC: ${hmacHeader}`); // Logs sensitive header
  next();
}

Production logs containing HMAC signatures, keys, or intermediate values become treasure troves for attackers who compromise log storage systems. Even redacted logs can be exploited if the redaction pattern is predictable.

Environment variable exposure poses risks when HMAC keys are stored in process.env but inadvertently logged:

console.log(`Starting service with config: ${JSON.stringify(process.env)}`);
// May expose: HMAC_KEY, SECRET_KEY, API_KEYS

Debug endpoints that expose internal state often include cryptographic material:

app.get('/debug/hmac', (req, res) => {
  res.json({
    currentKey: process.env.HMAC_KEY,
    lastSignatures: recentSignatures // Contains raw HMAC values
  });
});

Client-side HMAC generation creates disclosure risks when keys are embedded in JavaScript bundles:

// In browser bundle - key exposed to all users
const clientHmacKey = 'sk-...';
function generateClientHmac(message) {
  return crypto.subtle.digest('SHA-256', 
    new TextEncoder().encode(message + clientHmacKey));
}

Version-specific vulnerabilities in HMAC implementations can leak through error responses that indicate which cryptographic primitives are available, helping attackers select optimal attack strategies.

Detecting information disclosure in HMAC implementations requires both static analysis and runtime scanning. Static analysis tools examine code patterns for vulnerable comparison operations and logging statements that might expose cryptographic material.

middleBrick's HMAC-specific detection includes timing analysis to identify vulnerable string comparisons:

// Detection pattern for vulnerable comparison
const vulnerablePattern = /(?:===|==|!=|!==)[^;]*?hmac/i;
// Flags code using constant-time comparison functions
const securePattern = /(?:timingSafeEqual|secureCompare|slowEquals)/i;

The scanner analyzes error handling paths to detect information leakage:

// Checks for verbose error messages
const errorPattern = /throw new Error[^}]*?hmac/i;
// Detects stack trace exposure in error responses
const stackPattern = /err/(?:stack|trace)/i;

middleBrick's black-box scanning tests for timing side-channels by measuring response variations:

// Timing attack simulation
async function testTimingSideChannel(url, validHmac, message) {
  const timings = [];
  
  for (let i = 0; i < 256; i++) {
    const forgedHmac = validHmac.substring(0, i) + '0'.repeat(64 - i);
    const start = performance.now();
    const response = await fetch(url, {
      headers: { 'X-HMAC-Signature': forgedHmac }
    });
    timings.push(performance.now() - start);
  }
  
  // Analyze timing distribution for anomalies
  return analyzeTiming(timings);
}

Log analysis capabilities scan for patterns that might expose HMAC secrets:

// Log scanning patterns
const hmacLogPattern = /hmac|signature|key/i;
const secretPattern = /sk-[a-zA-Z0-9]{20,}/;
const base64Pattern = /[A-Za-z0-9+/]{32,}={0,2}/;

middleBrick's LLM security features detect when API documentation or error messages inadvertently expose HMAC implementation details that could be exploited by AI-powered attack tools.

The scanner's inventory management identifies all HMAC-related endpoints and their exposure levels:

// Endpoint inventory analysis
const hmacEndpoints = [
  { path: '/api/v1/auth/hmac', method: 'POST', exposure: 'high' },
  { path: '/api/v1/debug/hmac', method: 'GET', exposure: 'critical' }
];

Runtime scanning tests for key rotation exposure by attempting to use expired or rotated keys and analyzing error responses for information leakage.

Remediating information disclosure in HMAC implementations requires systematic application of secure coding practices. The foundation is constant-time comparison to eliminate timing side-channels:

// Node.js secure comparison using crypto.timingSafeEqual
const crypto = require('crypto');

function verifyHmac(receivedHmac, message, key) {
  const expectedHmac = crypto.createHmac('sha256', key)
    .update(message)
    .digest();
  
  // Constant-time comparison - same execution time regardless of input
  return crypto.timingSafeEqual(
    Buffer.from(receivedHmac, 'hex'),
    expectedHmac
  );
}

// Alternative: manual constant-time comparison
function constantTimeCompare(val1, val2) {
  let mismatch = 0;
  for (let i = 0; i < val1.length; i++) {
    mismatch |= val1[i] ^ val2[i];
  }
  return mismatch === 0;
}

Generic error handling prevents information disclosure:

// Secure error handling
function handleHmacError(err) {
  // Log full error internally with correlation ID
  const correlationId = generateCorrelationId();
  logger.error({ correlationId, err }, 'HMAC processing error');
  
  // Return generic error to client
  throw new ApiError(
    'Authentication failed',
    401,
    { correlationId } // Only safe metadata
  );
}

// Centralized error middleware
app.use((err, req, res, next) => {
  if (err instanceof ApiError) {
    res.status(err.statusCode).json({
      error: err.message,
      correlationId: err.correlationId
    });
  } else {
    res.status(500).json({
      error: 'Internal server error',
      correlationId: generateCorrelationId()
    });
  }
});

Secure logging practices prevent key exposure:

// Redact sensitive information in logs
const sensitivePatterns = [
  /hmac|signature|key/i,
  /[a-f0-9]{64}/i, // Potential HMACs
  /[A-Za-z0-9+/]{32,}={0,2}/ // Base64 secrets
];

function secureLog(message, data) {
  const redactedData = JSON.parse(JSON.stringify(data), (key, value) => {
    if (sensitivePatterns.some(pattern => pattern.test(key))) {
      return '[REDACTED]';
    }
    return value;
  });
  
  logger.info({ message, data: redactedData });
}

// Environment variable protection
const safeEnv = Object.fromEntries(
  Object.entries(process.env)
    .filter(([key]) => !key.match(/secret|key|password/i))
    .map(([key, value]) => [key, value])
);

Key management isolation prevents accidental exposure:

// Secure key storage
class HmacKeyManager {
  #keys = new Map();
  
  constructor(keyStore) {
    this.#keys = keyStore.loadKeys();
  }
  
  getSigningKey(keyId) {
    const key = this.#keys.get(keyId);
    if (!key) {
      throw new Error('Key not found');
    }
    return key;
  }
  
  // No method to list all keys or export them
}

// Key rotation without exposure
async function rotateHmacKey(keyId) {
  const newKey = await keyStore.generateKey();
  await keyStore.updateKey(keyId, newKey);
  
  // Update in-memory cache without logging
  hmacKeyManager.updateKey(keyId, newKey);
}

Input validation prevents malformed HMACs from causing information leakage through error messages:

// Strict HMAC validation
function validateHmacFormat(hmac) {
  if (typeof hmac !== 'string') {
    throw new ApiError('Invalid HMAC format', 400);
  }
  
  if (!/^[a-f0-9]{64}$/.test(hmac)) {
    throw new ApiError('Invalid HMAC format', 400);
  }
  
  return true;
}

// Sanitized error responses
function processHmacRequest(req) {
  try {
    const hmac = req.headers['x-hmac-signature'];
    validateHmacFormat(hmac);
    
    // Process request
    return verifyHmac(hmac, req.body, getHmacKey());
  } catch (err) {
    if (err instanceof ApiError) {
      throw err;
    }
    
    // Generic error - no implementation details
    throw new ApiError('HMAC processing failed', 401);
  }
}