Admidio: Event participation IDOR - non-leaders can register other users for events via user_uuid parameter

Vulnerability

In modules/events/events_function.php, the event participation logic allows any user who can participate in an event to register OTHER users by manipulating the user_uuid GET parameter.

Line 47: $getUserUuid = admFuncVariableIsValid($_GET, 'user_uuid', 'uuid', ...)
Line 424: if ($event->possibleToParticipate() || $participants->isLeader($gCurrentUserId))

The condition uses || (OR), meaning if possibleToParticipate() returns true (event is open for participation), ANY user - not just leaders - can specify a different user_uuid and register/cancel participation for that user.

The code then operates on $user->getValue('usr_id') (the target user from user_uuid) rather than the current user.

Impact

• Register unwilling users for events (potential harassment/spam)
• Cancel other users' event participation
• Manipulate event participant counts and comments
• If events have participation limits, fill slots with unwanted registrations

Fix

For non-leader users, force user_uuid to the current user:

if (!$participants->isLeader($gCurrentUserId)) {
    $getUserUuid = $gCurrentUser->getValue('usr_uuid');
}