twilio webhook signature mismatch error ngrok tunnel

Fix Twilio webhook signature mismatch behind an ngrok tunnel

TL;DR

Signature mismatches behind ngrok usually mean Twilio's request is validated against the wrong URL or the body was altered. Validate against the exact public ngrok URL Twilio called, using the raw body and your auth token. Tunnels do not break signatures; URL mismatches do.

The error

Twilio webhook error
Signature mismatch: computed signature does not match X-Twilio-Signature

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Fix it

Step 1: Confirm the exact public URL Twilio called

Check the ngrok forwarding URL and compare with the webhook URL configured in the Twilio console.

Expected: They match exactly, including https and any path.

Step 2: Validate against that exact URL

In your signature check, use the full public URL Twilio used, not your internal address.

Expected: The URL input to validation matches what Twilio signed.

Step 3: Use the raw body and your auth token

Compute the signature over the raw form-encoded body with your Twilio auth token as the key.

Expected: The computed signature matches the header.

Step 4: Check for body-altering middleware

Make sure no middleware parses or re-encodes the form body before validation.

Expected: Validation sees the untouched bytes.

Step 5: Test with a real Twilio request

Send a test SMS or call to trigger the webhook.

Expected: Signature validates and the handler runs.

When this applies

• Twilio webhooks fail signature checks behind ngrok
• Signatures validate locally but not through the tunnel
• You are developing Twilio webhooks with a tunnel

When it doesn't

• No requests arrive at all (check the ngrok tunnel is up)
• The signature passes but handling fails (check your logic)
• You are in production (use a stable public URL, not a tunnel)

Compatibility

Twilio webhooks with request validation. Tunnels: ngrok and similar.

Variant phrasings

twilio signature validation failed ngrok

Same failure. The tunnel URL is the critical input; everything else is standard validation.

x-twilio-signature mismatch

A mismatch with correct code means the URL or body input is wrong. Check both.

twilio webhook 403 signature

Some setups return 403 on failed validation. The fix is the same signature inputs.

Why it happens

Twilio signs the exact URL it called plus the raw POST body. Behind a tunnel, developers often validate against the internal URL or let middleware alter the body, so the inputs differ from what Twilio signed. The tunnel itself preserves bytes fine; the validation inputs are what drift.

Edge cases

• ngrok URLs change on restart; update the Twilio console URL every time it changes
• URL-encoded bodies must be validated pre-decode; decoding first breaks the signature
• Forwarded headers from the tunnel do not affect the signature; only URL and body matter

If it still fails

• Send a test to a controlled inbox and read the full headers before changing config.
• Check sender reputation and blocklists in parallel with content fixes.
• Verify authentication with a validator tool instead of guessing at the records.
• Change one variable at a time; changing content and config together hides the cause.
• If delivery fails at one receiver only, their filtering is the suspect, not your setup.

Prevention

• Validate SPF, DKIM, and DMARC with a checker after every DNS change.
• Warm up new sending domains and IPs gradually.
• Monitor bounce and complaint rates weekly.
• Keep suppression lists synced across all sending tools.
• Test to seed inboxes before big campaigns.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_Qk91m2DTZixAClnqmcpj2w