Impact

Users who used to be subscribed to a private stream and have been removed from it since retain the ability to edit messages/topics, and delete messages that they used to have access to, if other relevant organization permissions allow these actions. For example, a user may be able to edit or delete their old messages they posted in such a private stream. An administrator will be able to delete old messages (that they had access to) from the private stream.

Workaround

If the organization permissions for message and topic edition, as well as message deletion are disabled, regular users can't exploit the vulnerability. The ability of administrators to delete messages can't be disabled.