Left 4 Dead has been updated to a new version today by Valve. You can learn more about this update including patch notes for the PC version.
This patch addresses a major exploit in the game that was recently reported and appears to lead to some file uploads which could theoretically allow any attackers to gain access to the source PC and remotely execute malicious code on their hardware. This vulnerability has been fixed with the new patch.
Left 4 Dead Update Patch Notes February 1
- Fixed a potential remote code execution exploit through malformed file uploads. Reported via HackerOne.