Brute Force Attack in Strapi with Hmac Signatures

A brute force attack against a Strapi endpoint that relies on Hmac Signatures can occur when the signature is either not required for certain actions or is derived from a predictable or low-entropy secret. Strapi itself does not provide built-in Hmac signing for REST or GraphQL endpoints; developers typically add this in custom controllers or policies. If the Hmac verification is performed after basic route matching, an attacker can send many requests with different payloads or identifiers while observing timing differences or error messages that confirm validity. This is especially risky when the signature is computed over a subset of parameters (e.g., only a timestamp or resource ID) and does not cover a per-request nonce or client-specific secret.

Consider a Strapi controller that accepts an Hmac-Signed request to retrieve or update a user profile. If the signature is computed as hmac_sha256(secret, userId + timestamp) and the timestamp has a coarse granularity (e.g., one-second resolution), an attacker can replay or slightly mutate requests within the same second. If Strapi returns distinct errors for invalid signatures versus valid-but-forbidden resources, the attacker can iteratively guess user IDs and use Hmac verification to confirm when a signature matches the server’s expectations, effectively turning authentication into a brute force vector.

In GraphQL, if introspection is enabled and type definitions expose mutation inputs that accept signed tokens without strict constraints, an attacker can automate submission of many candidate payloads. For example, a mutation like updateSettings(input: {data: "...", signature: "..."}) can be hammered with variations while monitoring response times or error codes. Per the 12 security checks run by middleBrick, this maps to BOLA/IDOR and Authentication checks, where weak signature usage allows enumeration or privilege escalation. The presence of Hmac does not inherently prevent brute force; without proper rate limiting, entropy in the secret, and inclusion of a nonce or per-request context, attackers can probe the unauthenticated attack surface quickly (5–15 seconds for a scan) and identify exploitable endpoints.

Real-world patterns such as CVE-2022-24776 (Server-Side Request Forgery in Strapi plugins) illustrate how unchecked parameter handling can amplify risks when signatures are not tightly bound to request context. Additionally, if the Hmac secret is embedded in client-side code or transmitted in logs, it can be recovered and reused, enabling scalable brute force or credential reuse attacks. middleBrick’s LLM/AI Security checks do not apply here, but its Authentication and BOLA/IDOR checks highlight the importance of binding signatures to request-specific data and enforcing strict input validation.

To mitigate brute force risks when using Hmac Signatures in Strapi, ensure the signature covers all critical parameters, includes a per-request nonce or timestamp with sufficient granularity, and is verified before any business logic. Use a strong secret stored securely (e.g., environment variables) and enforce rate limiting on endpoints that accept signed requests. The following examples illustrate a robust approach.

Example 1: Hmac signing for a Strapi controller action (Node.js)

const crypto = require('crypto');

const SECRET = process.env.HMAC_SECRET; // store securely, not in code

function generateHmac(payload) {
  const message = `${payload.userId}:${payload.resourceId}:${payload.nonce}:${Math.floor(Date.now() / 1000)}`;
  return crypto.createHmac('sha256', SECRET).update(message).digest('hex');
}

function verifyHmac(requestPayload, receivedSignature) {
  const expectedSignature = generateHmac(requestPayload);
  // Use timing-safe compare to avoid timing attacks
  return crypto.timingSafeEqual(
    Buffer.from(expectedSignature),
    Buffer.from(receivedSignature)
  );
}

// In a Strapi controller
module.exports = {
  async updateSettings(ctx) {
    const { userId, resourceId, data, nonce, timestamp, signature } = ctx.request.body;

    if (!userId || !resourceId || !data || !nonce || !timestamp || !signature) {
      return ctx.badRequest('Missing required fields');
    }

    // Ensure timestamp is recent (e.g., within 2 minutes)
    const now = Math.floor(Date.now() / 1000);
    if (Math.abs(now - parseInt(timestamp, 10)) > 120) {
      return ctx.badRequest('Stale timestamp');
    }

    const payload = { userId, resourceId, nonce, timestamp };
    if (!verifyHmac(payload, signature)) {
      return ctx.unauthorized('Invalid signature');
    }

    // Proceed with update, ensuring the userId matches the authenticated context
    const entry = await strapi.entityService.update('api::settings.settings', resourceId, {
      data: { data },
    });

    return entry;
  },
};

Example 2: Enforcing nonce and rate limiting in Strapi policy

const usedNonces = new Set(); // In production, use a fast KV store with TTL

module.exports = async (ctx, next) => {
  const { nonce, timestamp, signature } = ctx.request.body;

  // Basic validation
  if (!nonce || !timestamp || !signature) {
    return ctx.badRequest('Missing security parameters');
  }

  // Reject replayed nonces
  if (usedNonces.has(nonce)) {
    return ctx.badRequest('Replay detected');
  }

  // Store nonce with expiration (simplified)
  usedNonces.add(nonce);
  setTimeout(() => usedNonces.delete(nonce), 300_000); // 5 minutes

  // Proceed to signature verification in controller or next policy
  await next();
};

Recommendations aligned with middleBrick findings

• Include a nonce or per-request unique value in the Hmac input to prevent replay and brute force attempts.
• Ensure the signature covers user context, resource identifiers, and a fine-grained timestamp to bind the request to a specific window.
• Apply rate limiting at the platform or gateway level to restrict rapid requests from a single source.
• Validate and sanitize all inputs before they reach controllers to reduce BOLA/IDOR surface, as highlighted by middleBrick’s Authentication and BOLA/IDOR checks.
• Use crypto.timingSafeEqual for signature comparison to avoid timing-based side channels.

These steps reduce the risk that an attacker can brute force user IDs or resources by exploiting predictable or weakly bound Hmac Signatures in Strapi.