Groundhogg <= 4.9 - Authenticated (Custom+) Privilege Escalation to 'user' Parameter

The Groundhogg — CRM, Newsletters, and Marketing Automation plugin for WordPress is vulnerable to Privilege Escalation in all versions up to, and including, 4.9 This is due to a missing ownership and capability check on the `user` parameter within the `process_edit()` function, which allows any authenticated user with the `edit_contacts` capability to reassign a contact record's linked WordPress user ID to any arbitrary account without requiring the `edit_users` or `promote_users` capabilities. This makes it possible for authenticated attackers, with sales_rep-level access and above, to escalate their privileges to administrator by linking a contact to an administrator's WordPress user ID, then creating a note containing the `{auto_login_link}` replacement tag to trigger generation of a valid auto-login permissions-key URL for the administrator-linked contact, and finally visiting that URL to authenticate as the targeted administrator. The auto-login URL is stored in the note content and is readable back by the attacker via the `view_notes` and `add_notes` capabilities that the sales_rep role holds by default.