---
title: "HMAC Signatures and Timestamp Replay Attack Protection for SMS Webhooks"
---

HMAC Signatures and Timestamp Replay Attack Protection for SMS Webhooks: SMSRiver

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QA Automation & Testing

Published 2026-09-17

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8 min read

# HMAC Signatures and Timestamp Replay Attack Protection for SMS Webhooks

Secure telephony webhooks against MITM and replay attacks. Implement HMAC-SHA256 signature verification with tolerance windows and nonces.

Key Takeaway & Executive Summary

Verify cryptographic HMAC signatures on all inbound carrier webhooks and enforce 5-minute timestamp tolerance windows to prevent replay exploits.

Because webhooks are exposed public endpoints, attackers can intercept and re-transmit previously valid payloads to forge incoming SMS messages. Implementing HMAC-SHA256 signature verification and timestamp validation is essential for production security.

## Table of Contents

- [HMAC-SHA256 Verification Protocol](#section-1)

- [Frequently Asked Questions](#frequently-asked-questions)

## 1 HMAC-SHA256 Verification Protocol

1. Carrier signs request: Aggregator computes HMAC-SHA256 hash using your shared secret over the full request URL and POST parameters.
2. Server recalculates hash: Ingest the raw request body, compute HMAC-SHA256 using the vaulted secret, and compare using timing-safe comparisons.
3. Timestamp Tolerance: Verify that the X-Webhook-Timestamp is within +/- 300 seconds of current server time to prevent replay attacks.

## Frequently Asked Questions • QA Automation & Testing

Why is timing-safe comparison required?

Standard string equality (==) returns false on the first mismatched byte, creating timing side-channel vulnerabilities. Use crypto.timingSafeEqual or hmac.compare_digest.

## Related Technical Articles & Deep Dives

[QA Automation & Testing
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Testing Webhook Idempotency: Handling Duplicate and Out-of-Order SMS Events](/articles/testing-webhook-idempotency-and-retries/)

[QA Automation & Testing
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Mocking Inbound Carrier Webhooks in Local Development Environments](/articles/mocking-carrier-webhooks-in-local-dev/)

[OTP & 2FA Engineering
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Handling Carrier DLR Webhooks and Delivery Confirmation at Scale](/articles/handling-carrier-sms-delivery-receipts/)

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