Today's hotfix for Windows Steam users solved an issue where players were able to steal items from other players. We looked into the issue and were able to fix it quickly, so we released a hotfix. Please update your game to remain safe. This hotfix does not recover items stolen during the time of this exploit. We apologize for the frustration.
We hope to have the hotfix ready for Mac and Linux Steam users within the next day or two. We'll post another announcement when that hotfix is ready.