How can I implement webhook security tokens or signatures when receiving SMSVerifier API callbacks?

To secure SMSVerifier API callbacks, implement webhook security by validating shared secret tokens or verifying HMAC signatures included in request headers to ensure authenticity and prevent spoofing.

Why secure SMSVerifier webhook callbacks?

Webhooks are HTTP requests sent from SMSVerifier to your server to notify you of new SMS messages or OTP codes received on virtual numbers. Because these callbacks trigger critical business logic—such as user authentication or transaction verification—it's vital to ensure they come from a trusted source.

Without any security measures, your webhook endpoint could be targeted by malicious actors sending fake or spoofed requests. This can lead to unauthorized account access, fraudulent transactions, or corrupted application state.

Using security tokens for webhook validation

A simple and effective method to secure your webhook endpoint is to use a shared secret token. This token acts as a password that only SMSVerifier and your server know.

When configuring your webhook callback URL in the SMSVerifier dashboard or API, you can set a secret token. SMSVerifier then includes this token as a custom HTTP header or a query parameter in each webhook request.

On your server, you verify that every incoming webhook request contains the correct token before processing. If the token is missing or invalid, you reject the request immediately.

Verifying HMAC signatures on callbacks

For stronger security, SMSVerifier supports HMAC (Hash-based Message Authentication Code) signatures. This involves signing the entire webhook payload with a secret key using a cryptographic hash function like SHA256.

Upon sending a webhook callback, SMSVerifier computes the HMAC signature and includes it in a dedicated HTTP header (e.g., X-SMSVerifier-Signature).

Your server should reproduce this signature by hashing the received request body using the shared secret key and compare it with the header value. If the signatures match, the request is authentic and untampered.

import hmac
import hashlib

def verify_signature(secret, payload, header_signature):
    computed = hmac.new(secret.encode(), payload, hashlib.sha256).hexdigest()
    return hmac.compare_digest(computed, header_signature)

const crypto = require('crypto');

function verifySignature(secret, payload, headerSignature) {
    const computed = crypto.createHmac('sha256', secret).update(payload).digest('hex');
    return crypto.timingSafeEqual(Buffer.from(computed), Buffer.from(headerSignature));
}

<?php
function verify_signature($secret, $payload, $header_signature) {
    $computed = hash_hmac('sha256', $payload, $secret);
    return hash_equals($computed, $header_signature);
}
?>

Best practices for webhook security

• Use HTTPS endpoints exclusively to encrypt webhook traffic and prevent interception.
• Validate both the presence and correctness of security tokens or signatures on every request.
• Respond with HTTP 403 Forbidden status for any failed verification to reject unauthorized calls.
• Log failed verification attempts for monitoring and incident response.
• Rotate your webhook secret tokens periodically and update SMSVerifier settings accordingly.
• Keep webhook endpoint URLs unguessable by using random, unique paths.