Why are my webhooks failing with 403 errors when they're not reaching my server?

Answer

If you're experiencing webhook failures with 403 errors but your server logs show no record of these requests being received, the issue is likely caused by Cloudflare or another security service blocking the webhook requests before they reach your application.

Common Cause: Cloudflare OWASP Core Ruleset

Cloudflare's standard OWASP Core Ruleset can block Nylas webhook requests because they may exceed risk scores, particularly when webhooks include email body content. This results in:

• 403 errors logged in Nylas webhook delivery logs
• No corresponding requests appearing in your server logs
• Webhooks being marked as failed and disabled after multiple failures

How to Fix with Signature-Based Authentication

Since Nylas webhook IPs aren't static, the recommended solution is signature-based authentication:

1. Configure Cloudflare Rule: Allow POST requests to your webhook endpoint when the Nylas-Signature header is present:
   • Navigate to Security > WAF > Custom rules
   • Create a rule: Allow POST https://yourdomain.com/webhook/path if header Nylas-Signature exists
   • Set action to Skip OWASP Core Ruleset for these requests
      
2. Implement Signature Verification: In your application, verify the webhook signature:
   • Use your Nylas webhook secret with HMAC-SHA256 against the raw request body
   • Reject requests with missing or invalid signatures
   • Optionally check Nylas-Timestamp header and reject old requests

   • Maintain reasonable rate limiting on the webhook endpoint

      

Alternative Approaches

• Create broader Cloudflare exceptions for your webhook domain
• Monitor both Nylas webhook logs and your server access logs

For complete signature verification implementation details, see the Nylas webhook verification documentation.

Note: This signature-based approach provides better security than IP whitelisting and works with any WAF provider implementing OWASP rules.

Updated September 01, 2025 18:47