My antivirus or firewall software reports my new Steam purchases contains a trojan or potentially dangerous program - what is happening?
We encourage protecting your PC from viruses, spyware and other forms of malicious software. However, it is not necessary to run anti-virus and other security applications while you are engaged in playing a game on Steam.
Anti-virus applications often interfere with Steam and can cause a range of problems from connection issues to games not launching correctly. Please set your anti-virus to Game Mode or disable it before launching Steam if you are experiencing issues with your Steam games.
The Steam application or Steam game has been mistakenly identified as a virus or "trojan" by certain software as a result of the development of potentially-malicious programs which use the same filenames as valid Steam files (this is a very common practice amongst malicious software writers - many viruses already masquerade as valid operating system files).
We are working with the antivirus and firewall software vendors to ensure fewer false positives involving Steam appear in the future.
In the meantime, please review the symptoms below to determine whether or not this issue is impacting your system:
McAfee AntiVirus incorrectly identifies some third party game files as malicious and will quarantine the files during the download process. This often results in the message "This game is currently unavailable" when trying to launch the game.
Unfortunately, most editions of McAfee AntiVirus have no way to exclude files from real-time scans, so the only work-around is to disable real-time scans. To do this, open McAfee and select "Virus and Spyware Protection", then select the "Real-Time Scanning" option and click the button labelled "Turn Off".
Once Real-time scanning is disabled, you can restore your game files by following the instructions to Verify Game Cache Files.
Real-time scanning can be re-enabled once you are done playing, but unfortunately this will re-quarantine the game files, requiring this process to be repeated.
NOD32 incorrectly identifies some third party game files as malicious and will quarantine the files during the download process. This often results in the message "This game is currently unavailable" when trying to launch the game.
When using the "Friends" feature, some Norton Firewall users have reported that their firewall software reports that Steam may be malicious software attempting to access their system. This is a false positive.
Occasionally the Friends server sends your computer a message asking if you're still online - if you closed Steam (especially if you killed it from the task manager), this message can be intercepted by the Norton firewall.
Since Friends sometimes uses the same port number as a known trojan horse virus, the firewall incorrectly identifies the message as being malicious - despite the fact that the traffic does not mimic a trojan in any other respect.
Kaspersky AV may prevent the in-game overlay from functioning in some or all games. This may occur silently, or Kaspersky may warn you that Steam is acting as an Invader application trying to load itself in another process.
In your Kaspersky settings go the Threads and exclusions, Trusted Zone, and add Steam.exe as a trusted application. Make sure you check the settings to Do not restrict application activity as well as Do not scan opened files for full compatibility.
Kaspersky 7.0 users must enable the Compatibility Mode for Programs Using Self-Protection Methods option, also disable the option for Application Integrity Control.