Vertcoin (VTC) experienced a 51% attack on December 1, with 603 blocks being removed from the blockchain and replaced with 553 ‘attacker’ blocks. Five double spend transactions occurred simultaneous with the attack, but only 125 Vertcoins (VTC) worth $29 were doublespent. The attack may have been much worse, but the devs noticed an attack may be coming and notified Bittrex, the biggest exchange which offers Vertcoin (VTC), and then Bittrex halted all deposits and withdrawals.
Approximately a year ago Vertcoin (VTC) experienced a 51% attack which resulted in the loss of over $100,000, over the course of many different attacks. This prompted the devs to change Vertcoin’s (VTC) mining algorithm to Lyra2REv3 in an attempt to prevent another attack.
Apparently Lyra2REv3 is prone to attacks as well though, as proved by the current incident. The hacker who performed this 51% attack did not even need their own mining equipment, they simply rented the equipment on NiceHash at the relatively low cost of 0.5-1 Bitcoin (BTC). If the costs were on the lower end of this spectrum, then the attack was mostly paid for by the Vertcoin (VTC) block rewards that the attacker received, which were equivalent to 0.44 Bitcoin (BTC).
Fortunately, the Vertcoin (VTC) devs noticed that there was an anomalous spike in the network hash rate, and quickly recommended that Bittrex halt all deposits and withdraws. Indeed, the block reorg was just over the 600 block confirmation time which Bittrex uses, so it seems this attack was aimed at Bittrex.
Ultimately the attacker did not get away with much money, maybe at the most $29. It is possible that Bittrex halting deposits and withdraws thwarted this attack, or that the hacker was simply testing the viability of a 51% attack on Vertcoin (VTC).
Regardless, it is clear that Vertcoin (VTC) needs to change its algorithm again, since it remains susceptible to 51% attacks.