Valve have announced that they have addressed a serious security issue with Steam which would allow hackers to gain access to the account using Steams password recovery system.
Valve say they worked quickly to resolve the security issue which they were informed of on the 25th July, although some affected players may span back to the 21st July.
Valve confirmed that they are also reaching out to players who appear to have had suspicious password activity on their accounts recently.
Reports are that the known attacks targeted some prominent streamers and Counter-Strike: Global Offensive players.
The security issue is explained in the following video