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So if you're a server owner make sure your server is up to date and change your rcon password as a precaution. It may be worth your time checking your logs for any suspicious rcon activity, and check your user lists to make sure no admins have been added.
If you're a player and you can't join your favourite server - then it is out of date. I know this is a shitty time to force a server update but this really couldn't wait.