Malaware Adds in server player list

A man joined my server and had a link on the Tab list. After joining the other server, apparently he got a file and when he joined back to me, he could see the link on the player list. A new file was also created in MY server folder where I host my vanilla server. (completely vanilla).

Below is the log file that was created when he joined. After I removed this, he said he could not see the message anymore. Thought it might be useful to include it

The link that was being shown to him can be seen at: http://imgur.com/d3RvrKF I am dark red, he is red. I can not see the messages, in aqua, he can.