A webhook endpoint that processes any POST request it receives, without checking who actually sent it, is an unauthenticated write path into your system. Anyone who finds the URL can send fake events — fake payments, fake order confirmations, fake anything the endpoint reacts to.
Why signature verification is the fix
Providers that take webhooks seriously sign each payload with a secret only you and the provider know, and send that signature in a header. Verifying it means recomputing the expected signature from the raw request body and comparing — reject anything that doesn’t match, before your handler logic ever runs.
The gap: not every provider ships a clean SDK helper
Some platforms document webhook verification thoroughly and ship an official helper function. Others don’t — leaving you to implement HMAC verification by hand, following whatever convention they use (raw body vs parsed JSON, timestamp tolerance windows to prevent replay attacks, which header carries the signature). Getting any of these details wrong silently breaks verification or, worse, makes it falsely pass.
Summary
Never trust a webhook payload just because it arrived at the right URL. Verify the signature against the raw body before processing anything, add a timestamp tolerance window to block replays, and treat any provider without a clean official SDK helper as a place where it’s easy to get the details subtly wrong.