On June 2, 2018, ZenCash (now Horizen) suffered a 51% attack resulting in double-spend losses of over $500K. The attacker gained majority hashrate control and reorganized 38 blocks, executing double-spends of 13,000 and 6,600 ZEN across exchanges. The attack likely involved rented hashpower from NiceHash.

The ZenCash encountered a 51% cyber attack or double-spend attack at around 8:26 PM on June 2nd (June 3rd 00:26 UTC). This attack resulted in the loss of Zen encrypted tokens worth approximately US $550,000. The ZenCash team responded immediately and implemented preventive mitigation measures, which greatly increased the difficulty of future cyber attacks.