Affected Version:

• 1.7.2 or greater
• 2.5

In method setUserGroups, inside file libraries\joomla\user\helper.php

// Set the titles for the user groups.
for ($i = 0, $n = count($results); $i < $n; $i++) {
$user->groups[$results[$i]->id] = $results[$i]->title;
}


This code will overwrite the index of the groups parameter pass into the function. Also in the method addUserToGroup in this class, the group information is store as

// Add the group data to the user object.
$user->groups[$title] = $groupId;


not the otherway around.