Api Rate Abuse in Loopback with Hmac Signatures

Rate abuse occurs when an attacker issues a high volume of requests to consume resources, cause denial of service, or bypass usage limits. Loopback is a Node.js framework that often uses Hmac Signatures to ensure request integrity and authenticity. While Hmac Signatures help prevent tampering and replay, they do not inherently enforce rate limits. When rate limiting is implemented at a layer that does not account for per-key or per-client tracking, or when the signature is computed over a subset of request properties that an attacker can vary, the protection against abuse becomes incomplete.

Consider an endpoint that accepts POST /transfer and validates an Hmac Signature header computed over selected headers and the request body. If the API applies global rate limits only by IP address, an attacker can rotate source IPs or use distributed sources to bypass IP-based controls. If the rate limit is applied before signature validation, an attacker can send many unsigned or invalid-signed requests that consume CPU and connection resources. If applied after validation but without a consistent keying strategy (e.g., using API key + timestamp window), an attacker can reuse valid signatures within the allowed window to exceed intended quotas.

Another scenario involves replay within the timestamp window. If the server accepts requests with a small time drift and does not maintain a strict one-time-use cache for nonces or a monotonic counter per signer, an attacker can capture a legitimate signed request and replay it multiple times until the rate window resets. In Loopback, if the Hmac computation does not include a nonce or a strictly monotonic sequence number, there is no mechanism for the server to detect replays, enabling abuse even when signatures are valid.

Additionally, weak signature scope can contribute to abuse. For example, if the signature covers only the body but not key headers such as X-User-Id or X-Client-Tier, an attacker could keep the body unchanged while altering those headers to exploit higher rate tiers or privileged operations. Inadequate differentiation between public and privileged operations in the rate policy also means that endpoints with Hmac Signatures might still allow unauthenticated-style abuse if the rate limit for authenticated requests is too generous.

Effective mitigation requires tying rate limits to the signer identity extracted from the Hmac, enforcing replay protection with nonces or monotonic counters, validating signatures before consuming significant resources, and applying differentiated limits per operation sensitivity. middleBrick scans such configurations and flags missing per-client rate controls, weak signature scope, and missing replay defenses under the Rate Limiting and Authentication checks.

Remediation centers on binding rate limits to the canonical signer identity, ensuring signature validation precedes resource-intensive work, and preventing replays. The following examples show a robust pattern for Loopback using Node.js crypto and a middleware approach.

First, compute the Hmac over a canonical string that includes a nonce and a timestamp, and verify before proceeding:

const crypto = require('crypto');

function verifyHmac(req, res, next) {
  const signature = req.get('X-Signature');
  const timestamp = req.get('X-Timestamp');
  const nonce = req.get('X-Nonce');
  const apiKey = req.get('X-Api-Key');

  if (!signature || !timestamp || !nonce || !apiKey) {
    return res.status(401).json({ error: 'Missing authentication headers' });
  }

  // Reject old requests to prevent replay
  const now = Math.floor(Date.now() / 1000);
  const skew = 300; // 5 minutes
  if (Math.abs(now - parseInt(timestamp, 10)) > skew) {
    return res.status(401).json({ error: 'Request timestamp out of skew' });
  }

  const payload = `${apiKey}:${timestamp}:${nonce}:${req.rawBody}`;
  const expected = crypto.createHmac('sha256', Buffer.from(process.env.HMAC_SECRET, 'base64'))
                        .update(payload)
                        .digest('hex');

  if (!crypto.timingSafeEqual(Buffer.from(signature, 'hex'), Buffer.from(expected, 'hex'))) {
    return res.status(401).json({ error: 'Invalid signature' });
  }

  // Attach identity for downstream rate limiting
  req.principal = { apiKey, timestamp: parseInt(timestamp, 10), nonce };
  return next();
}

Second, implement rate limiting keyed by the signer identity (apiKey) and include nonce tracking to prevent replays within the time window. Use a sliding window or token bucket appropriate for your traffic profile:

const rateLimit = require('rate-limiter-flexible');

// Key by apiKey; adjust points and duration to your policy
const keyRateLimiter = new rateLimit.RateLimiterRedis({
  storeClient: redisClient,
  points: 100, // 100 requests
  duration: 60, // per 60 seconds
  keyPrefix: 'rl_hmac'
});

async function rateLimitByPrincipal(req, res, next) {
  if (!req.principal) {
    return res.status(401).json({ error: 'Not authenticated' });
  }

  const { apiKey, nonce } = req.principal;
  const nonceKey = `nonce:${apiKey}:${nonce}`;

  // Simple replay cache; in production use a fast KV with TTL aligned with window
  const isReplay = await redisClient.get(nonceKey);
  if (isReplay) {
    return res.status(401).json({ error: 'Replay detected' });
  }

  try {
    await keyRateLimiter.consume(apiKey);
    // Mark nonce as seen for the current time window; keep TTL slightly longer than window
    await redisClient.set(nonceKey, 'used', 'EX', 120);
    return next();
  } catch (rlRejected) {
    return res.status(429).json({ error: 'Rate limit exceeded' });
  }
}

Third, ensure the Hmac signature scope includes all elements that define the operation context: headers that influence authorization or pricing, and a monotonic sequence when practical:

function buildPayload(req) {
  const apiKey = req.get('X-Api-Key');
  const timestamp = Math.floor(Date.now() / 1000);
  const nonce = req.get('X-Nonce') || generateNonce();
  // Include headers that affect authorization or cost
  const method = req.method.toUpperCase();
  const path = req.path;
  const body = req.rawBody || '';
  return `${apiKey}:${timestamp}:${nonce}:${method}:${path}:${body}`;
}

function signPayload(payload) {
  return crypto.createHmac('sha256', Buffer.from(process.env.HMAC_SECRET, 'base64'))
              .update(payload)
              .digest('hex');
}

Finally, enforce that unauthenticated or unsigned requests are rejected early, and that public endpoints have distinct, lower rate caps compared to authenticated privileged endpoints. Use separate middleware chains and clearly document the scope of the Hmac protection so clients know which headers and body components are included in the signature.