The problem:
1) For example, I host 2 domains for 2 customers - domain_1.com and domain_2.com
2) 1st customet who own first domain can park sub.domain_2.com (!!! - subdomain of second domain) on top of his own domain_1.com
If i own &pornogirls.com& i can park &support.hostingcompany.com& on top of &pornogirl.com& even if i do not own &hostingcompany.com& domain
ha ha ha
