VAC automatically bans Linux users who run Steam and have "catbot" in their system's name.
Apparently there is a TF2 cheat that changes your Steam username to catbot##### so Valve just automatically bans anything that contains catbot even if it just the name you've assigned the computer running Linux (not your Steam's username).
kritzsie commented Dec 31, 2017
Confirmed with one long-standing account and one fresh account, both under the same Linux username starting with "catbot".
But consider yourselves lucky! Valve have a history of hunting down users who don't adapt to policy changes and banning their accounts, often worth thousands of dollars, with no indication as to why. I have been caught in a ridiculous but unrelated permanent community and trade ban for trying to sell a large amount of items on the community market, even though Steam support never bothered to confirm this.
Don't be surprised when Steam support discard your ticket due to "privacy policy" issues. I know I wasn't.
kisak-valve commented Dec 31, 2017
Good day, I've received word from the VAC team that this is intentional and not open for discussion on Github.
In general VAC issues are not handled on Github in any capacity and further issue reports on this may result in being banned from the Valve Software issue trackers.
UPDATE
A Valve employee replied on Reddit at the beginning of January said that this information was incorrect:
Hi! Valve employee here. The bug report is incorrect. VAC will not ban you for simply having catbot in your user name (either your steam profile or on one or more of your linux accounts).
The bug report--and I suspect many of the posts in this thread--are a tactic employed by cheaters to try and sow discord and distrust among anticheat systems.
VAC has many different types of detections and we cannot discuss what they do publicly because doing so makes them less effective. However, one thing I can disclose is that all detections require that the detection occur while a user is actively cheating and connected to a VAC-secured server.
Linux historically hasn't been a problem for cheating--the base rate of cheating is significantly lower on Linux than it is on Windows. Unfortunately, a "healthy" community of cheaters grew up around catbot on linux and their impact on TF became large enough that they simply could no longer be ignored. Those banned users are very annoyed that VAC has dropped the hammer on them.
Kisak moderates many of valve's github bug repositories for us in an attempt to keep the bugs high quality and actionable. The VAC team asked him to close the issue in question and to indicate that github was not an appropriate location to discuss VAC bans. He did so, and we support this action.
For proof that I am a Valve employee, you can check my posting flair in the other subs I post on (/r/CSGO and /r/tf2) or a mod can message me and we can work on confirmation using my work email and PMs.
