Looks like Valve got pretty well buggered. Gabe Newell:
Ever have one of those weeks? This has just not been the best couple of days for me or for Valve.
Yes, the source code that has been posted is the HL-2 source code.
Here is what we know:
1) Starting around 9/11 of this year, someone other than me was accessing my email account. This has been determined by looking at traffic on our email server versus my travel schedule.
2) Shortly afterwards my machine started acting weird (right-clicking on executables would crash explorer). I was unable to find a virus or trojan on my machine, I reformatted my hard drive, and reinstalled.
3) For the next week, there appears to have been suspicious activity on my webmail account.
4) Around 9/19 someone made a copy of the HL-2 source tree.
5) At some point, keystroke recorders got installed on several machines at Valve. Our speculation is that these were done via a buffer overflow in Outlook’s preview pane. This recorder is apparently a customized version of RemoteAnywhere created to infect Valve (at least it hasn’t been seen anywhere else, and isn’t detected by normal virus scanning tools).
6) Periodically for the last year we’ve been the subject of a variety of denial of service attacks targetted at our webservers and at Steam. We don’t know if these are related or independent.
Followup thread pinpointing the leaker?