HACKING VIA HOSTING PROVIDER.POSSIBLE ?

Hello. I am newbie in hacking,pentesting,security.I want to know can some company's (CompanyA) server/servers can be hacked through vulnarable/hacked server/servers of another company(CompanyB) if both these companies hosted on the same hosting provider ? CompanyA and CompanyB are not related to each other. If this possible teoretically then how it can be done ? I would like to know fundamentals of this type of hacking.Thanks.

Yes.. anything can be done.. in that scenario, you have to escalate privilages somehow and gain root of a box..