Several possible attacking modes on XRP ledger
https://www.xrpchat.com/topic/7229-a-question-about-validators/?do=findComment&comment=69555 There are several degrees of damage, that trusted validators can do (if they decide to give up their hard-earned trust), in increasing difficulty order: If a set of them, each of which only trust others in the same set, with less than 20% of them trust validators not in that set, decide to collude, they can create fork. If more than 20% of them decide to collude, they can prevent transactions from being validated. If more than 80% of them decide to collude, they can rewrite history as they wish. However, as pointed out by mDuo13@, creating a fork is easy, but making people accept it is hard.