Admidio is an open-source user management solution. Prior to 5.0.6, 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. 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. This vulnerability is fixed in 5.0.6.

Affected: Packagist: admidio/admidio

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