If you do not have Steam Guard installed and protecting your Steam account, you should probably go ahead and do that now. It seems that according to several reports over the weekend, there have been some users who claimed that their Steam accounts have been hacked.
Now it isn’t so much that these users had bad passwords, but rather due to what seemed like a security glitch in the system. Basically all the hacker had to do was input the user’s Steam username and request a password reset. Now typically a verification code would be sent to your email account to verify that it is you who requested the reset.
However in this case, it was discovered that by not entering a code, it allowed the hacker to go ahead and create a new password, thus compromising the account in the process and taking it over. This did not seem to affect accounts with Steam Guard active, so like we said if you have not installed Steam Guard, you should go ahead and do that.
Valve has since appeared to have fixed the exploit although to date they have yet to acknowledge or issue a statement on the matter. According to this Reddit thread, it looks like there have been a bunch of pro DotA 2 players who have been affected by this hack as well.