I'm running a vanilla minecraft server. I'm wondering if there is anything in place that stops cracked clients from connecting? Or would any problems be related to the client itself?
If you go to the server.properties file, you will be able to see a line that says
Usually when you join a server with this true, it checks with Mojang servers whether the account being used is valid. If not, it will return with the error:
Failed to Login : Invalid session (Try restarting your game)
When this is false, it will not authenticate with Mojang servers and anyone is allowed to enter at free will, resulting in security problems.
To prevent cracked clients from entering your server, leave this to True.