Backup and Staging by WP Time Capsule < 1.21.16 - Authentication Bypass

Proof of Concept

The issue resides in wptc-cron-functions.php line 12 where it parses the request. This parse_request function calls the function decode_server_request_wptc which check if the raw POST payload contains a certain string. If it does, it calls wptc_login_as_admin and you'll be logged in as an administrator.