Api Rate Abuse in Strapi with Hmac Signatures

Rate abuse occurs when an attacker issues a high volume of requests to consume resources, degrade performance, or exhaust quotas. In Strapi, enabling Hmac Signatures for API authentication changes how clients prove identity, but it does not inherently enforce request volume limits. Strapi’s core behavior is to validate the Hmac signature for authenticated routes and then process the request. If rate limiting is not configured separately, an unauthenticated or poorly limited endpoint that also exposes a predictable Hmac verification path can be hammered without triggering account lockout or throttling.

Consider a Strapi endpoint that accepts POST requests to /api/orders with an Hmac signature in headers (e.g., X-API-Signature and X-API-Timestamp). An attacker who can generate valid Hmac signatures—perhaps by obtaining a static secret or by exploiting weak key management—can submit many requests per second. Strapi will verify each signature successfully and proceed with business logic (e.g., creating orders), leading to transaction spam, inventory exhaustion, or financial fraud. Even if the secret is not static, an endpoint that does not enforce per-client or per-IP request caps allows attackers to probe for timing differences or error responses that leak implementation details.

The combination of Hmac Signatures and missing rate controls also amplifies risks around signature replay. Without timestamp/nonce enforcement and strict window validation in Strapi, an attacker can capture a signed request and replay it within the allowed time window. If the endpoint lacks idempotency keys or request deduplication, this may result in duplicate operations (e.g., double charges). Moreover, when Hmac verification is done in Strapi middleware before business logic, attackers can still consume CPU cycles for each verification, enabling denial-of-service via computational exhaustion if rate limits are absent.

middleBrick scans such configurations as part of its 12 security checks, including Rate Limiting and Authentication, and flags missing request caps on endpoints that use Hmac Signatures. Findings include severity-ranked guidance: implement per-client rate limits, enforce timestamp nonces, and scope signatures to specific endpoints. By correlating runtime behavior with OpenAPI/Swagger definitions and Hmac usage patterns, middleBrick helps prioritize fixes that reduce abuse surface without requiring internal architecture details.

Remediation focuses on enforcing strict request quotas and ensuring Hmac validation incorporates nonces or timestamps to prevent replay. Below are concrete Strapi custom middleware and policy examples that you can add to your project.

1. Rate limiting with a token bucket per client identified by Hmac key ID

Use a lightweight in-memory store (or Redis in cluster setups) to track requests. This example uses a map keyed by keyId extracted from the Hmac header.

// src/middleware/rate-limit-hmac.js
'use strict';

const bucket = new Map(); // keyId => { tokens, lastRefill }
const RATE = 100; // requests per window
const WINDOW_MS = 60_000; // 1 minute

module.exports = (config, { strapi }) => {
  return async (ctx, next) => {
    const path = ctx.path;
    // Apply only to routes that use Hmac protected endpoints
    if (!path.startsWith('/api/orders')) {
      return next();
    }

    const signature = ctx.request.header['x-api-signature'];
    const timestamp = ctx.request.header['x-api-timestamp'];
    const keyId = ctx.request.header['x-api-key-id'];

    if (!keyId || !signature || !timestamp) {
      ctx.status = 401;
      ctx.body = { error: 'Missing Hmac headers' };
      return;
    }

    const now = Date.now();
    if (!bucket.has(keyId)) {
      bucket.set(keyId, { tokens: RATE, lastRefill: now });
    }

    let entry = bucket.get(keyId);
    const elapsed = now - entry.lastRefill;
    if (elapsed > WINDOW_MS) {
      entry = { tokens: RATE, lastRefill: now };
      bucket.set(keyId, entry);
    }

    if (entry.tokens <= 0) {
      ctx.status = 429;
      ctx.body = { error: 'Rate limit exceeded' };
      return;
    }

    entry.tokens -= 1;
    bucket.set(keyId, entry);
    await next();
  };
};

2. Hmac verification with nonce/timestamp replay protection

Ensure each Hmac signature includes a nonce or a recent timestamp, and store processed nonces for a bounded window to prevent reuse.

// src/policies/hmac-verify.js
'use strict';

const usedNonces = new Set();
const NONCE_TTL = 300_000; // 5 minutes

function cleanOldNonces() {
  // In production, use a proper cache with TTL (e.g., LRU cache)
  // This simplified example shows the concept.
  if (usedNonces.size > 10_000) {
    // naive cleanup: in real apps use time-based eviction
    usedNonces.clear();
  }
}

module.exports = async (ctx, next) => {
  const signature = ctx.request.header['x-api-signature'];
  const timestamp = ctx.request.header['x-api-timestamp'];
  const nonce = ctx.request.header['x-api-nonce'];

  if (!signature || !timestamp || !nonce) {
    ctx.status = 401;
    ctx.body = { error: 'Missing Hmac headers' };
    return;
  }

  const now = Date.now();
  const timeWindow = 300_000; // 5 minutes
  if (Math.abs(now - parseInt(timestamp, 10)) > timeWindow) {
    ctx.status = 401;
    ctx.body = { error: 'Timestamp out of bounds' };
    return;
  }

  if (usedNonces.has(nonce)) {
    ctx.status = 401;
    ctx.body = { error: 'Nonce already used' };
    return;
  }

  // Derive or fetch the client-specific secret based on keyId
  const keyId = ctx.request.header['x-api-key-id'];
  const clientSecret = getClientSecret(keyId); // implement securely
  if (!clientSecret) {
    ctx.status = 401;
    ctx.body = { error: 'Invalid key ID' };
    return;
  }

  const computed = crypto
    .createHmac('sha256', clientSecret)
    .update(nonce + timestamp + ctx.request.rawBody)
    .digest('hex');

  if (!crypto.timingSafeEqual(Buffer.from(computed), Buffer.from(signature))) {
    ctx.status = 401;
    ctx.body = { error: 'Invalid signature' };
    return;
  }

  usedNonces.add(nonce);
  cleanOldNonces();
  await next();
};

function getClientSecret(keyId) {
  // Fetch from secure storage, environment, or a service
  const secrets = {
    'client-a': 'super-secret-key-a',
    'client-b': 'super-secret-key-b',
  };
  return secrets[keyId] || null;
}

3. Apply the middleware and policy in Strapi

Register the rate limit middleware and Hmac policy in your Strapi application so that they run before the routes that create side effects.

// config/middleware.js
module.exports = {
  settings: {
    middlewares: [
      {
        name: '@middleBrick/rate-limit-hmac',
        config: {}
      },
      {
        name: 'hmac-verify',
        config: {}
      }
    ]
  }
};

These changes ensure that each Hmac-signed request is counted, replay attempts are rejected, and abusive clients are throttled. By combining Hmac integrity with explicit rate limits, you reduce the risk of API rate abuse while preserving the authentication benefits of Hmac Signatures.