Missing authorization vulnerability has been discovered in 2Checkout payment gateway of WHMCS from 8.13.0 before 8.13.7, from 9.0.0 before 9.0.8, all other EOL versions from 4.5.0. The vulnerability allows an unauthenticated user to get WHMCS customer's data via 2Checkout payment gateway's endpoint under specific conditions.

Deserialization of untrusted data in WHMCS 9.0.0 before 9.0.8 and 8.0.0 before 8.13.7 allows remote attackers to execute arbitrary code.

Insufficient ownership check in `clientarea.php` allows an authenticated client area user to submit requests using another user’s `addonId` without any ownership validation leading to unauthorized access to the victim's account.