Groundhogg < 4.9.1 - Authenticated (Sales Person+) Privilege Escalation via Contact Identity Rebinding leading to Administrator Account Takeover to 'user_id' Parameter (v3 /contacts) chained with v4 /emails/test

Description

The Groundhogg — CRM, Newsletters, and Marketing Automation plugin for WordPress is vulnerable to Privilege Escalation via Contact Identity Rebinding in all versions up to, and including, 4.9 The vulnerability exists because the `create_contact` function in the v3 REST endpoint (`POST /gh/v3/contacts`) is gated solely by the `add_contacts` capability and forwards the full request payload — including the security-bearing `user_id` column — into the upsert path of `Contacts_DB::add()`, which bypasses the ownership guard that `Contacts_DB::update()` enforces, allowing an attacker to rebind any existing contact record to an arbitrary WordPress user ID. This makes it possible for authenticated attackers with Sales Representative-level access and above to upsert their own contact row to point to an Administrator's user ID, then invoke the v4 email-test endpoint (`POST /gh/v4/emails/test`) — also accessible to the Sales Representative role via the `send_emails` capability — to generate an `{auto_login_url}` one-time permissions key bound to the rebound contact, and consume that link to call `wp_set_auth_cookie()` and gain a fully authenticated session as the WordPress Administrator.