MWChat contains a flaw that may allow a remote attacker to execute arbitrary commands. The issue is due to start_lobby.php not properly sanitizing user input supplied to the CONFIG[MWCHAT_Libs] variable. This may allow an attacker to include a file from a remote host that contains arbitrary commands which will be executed by the vulnerable script.
Upgrade to version 6.8 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.