How to secure webhooks from SMSVerifier API against spoofing and replay attacks?

Secure SMSVerifier API webhooks by validating HMAC signatures, checking timestamps to prevent replay, and enforcing HTTPS endpoints to prevent spoofing and interception.

Understanding Webhook Threats: Spoofing & Replay

Webhooks are automated callbacks from SMSVerifier to your server, delivering critical data such as OTP codes or verification status. However, these endpoints are exposed to the internet, which makes them vulnerable to two common attacks:

• Spoofing: An attacker sends fake webhook requests pretending to be SMSVerifier, injecting fraudulent data or triggering unwanted processes.
• Replay attacks: An attacker intercepts a legitimate webhook request and re-sends it multiple times to cause repeated actions or confusion.

Without proper validation, malicious actors could exploit webhook endpoints to bypass verification steps, cause data corruption, or launch denial-of-service attacks.

Signature Verification: Ensuring Authenticity

SMSVerifier secures webhook payloads by signing them using a shared secret key and HMAC (Hash-based Message Authentication Code). This signature is included in the HTTP header X-SMSVerifier-Signature.

Your server should compute the HMAC of the received payload using your webhook secret and compare it to the signature header. If they match, the request is authentic and unaltered.

Here is a typical verification flow:

1. Receive webhook payload and extract the X-SMSVerifier-Signature and X-SMSVerifier-Timestamp headers.
2. Concatenate the timestamp and payload body in the prescribed format.
3. Compute the HMAC SHA256 digest using your webhook secret key.
4. Compare the computed digest with the signature header.

Replay Attack Mitigation Techniques

Replay attacks exploit the fact that the same webhook payload can be resent multiple times to your server. To mitigate these risks:

• Timestamp Validation: Check the X-SMSVerifier-Timestamp header and reject requests older than a configurable time window (e.g., 5 minutes).
• Nonce or Unique ID: Store unique identifiers from webhook payloads or headers to detect and block duplicates.
• Short Expiry Window: Limit the validity period of a webhook event to minimize the window for replay.

Best Practices for Secure Webhooks

Implementing Security in SMSVerifier Webhooks

SMSVerifier provides all necessary headers to implement robust webhook validation:

Here is a sample Node.js snippet demonstrating signature verification and timestamp validation:

const crypto = require('crypto');

function verifyWebhook(req, secret) {
    const signature = req.headers['x-smsverifier-signature'];
    const timestamp = req.headers['x-smsverifier-timestamp'];
    const body = JSON.stringify(req.body);

    // Reject old requests (older than 5 minutes)
    if (Math.abs(Date.now()/1000 - timestamp) > 300) {
        return false;
    }

    // Compute HMAC SHA256
    const hmac = crypto.createHmac('sha256', secret);
    hmac.update(timestamp + '.' + body);
    const digest = hmac.digest('hex');

    // Constant-time comparison
    return crypto.timingSafeEqual(Buffer.from(digest), Buffer.from(signature));
}

Implement this verification step at the start of your webhook handler to accept only legitimate requests.

Frequently asked questions