Hack in order to install a rootkit and get the server to participate in a botnet.Who will hack an linux forum ?
2. To what end ? To gain what ?
Then it can be employed, on demand via command-and-control, as a spam bulk mailer, a DDOS attack participant, a bitcoin miner...
Rootkit doesn't need to be at o/s level, can be code added into php scripts (phpBB forum software), or otherwise run with the webserver (service) permissions.
Next thing ya know (for instance)... the domain you're operating the compromised webserver from (in this case mepiscommunity.com), you find out no one can receive your domain mailserver because the domain has been added to spamlist.