If you are here because of a message in your minecraft server console, this wasn't us and is a harassment campaign from a griefing group. Your server is not hacked they are just sending messages in your chat from the dynmap plugin/mod. To fix this open the dynmap config and change the allowchat setting to false like this:

 - class: org.dynmap.SimpleWebChatComponent
    allowchat: false
    # If true, web UI users can supply name for chat using 'playername' URL parameter.  'trustclientname' must also be set true.
    allowurlname: false