SAML IDP POST to /saml_login url returns 403 response

You have configured SAML Authentication in AEM and after logging in to the IDP you get a 403 response from AEM during the SAML POST to /saml_login or /content/saml_login (or other URL configured for the iDP to post back to).

The problem only happens when authenticating via the dispatcher enabled URL.

In the dispatcher.log you find this text: 

Cause

The dispatcher /filter section needs to allow POST requests to */saml_login

Resolution

Add a the following rule to the dispatcher farm configuration (.any files) /filter section: