Server hacked by single website

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http://forums.cpanel.net – Sir, We are running updated version of cpanel/whm. I have a question, How hacker can hack whole server by hacking single joomla website (old version)? I undersatnd he can hack CMS, so he should access only to this account files. So how he got whole server access? all other website access? Your reply will be appreciated. Thanks Regards, Simon (HowTos)