Poly Network made the controversial offer after the hacker pledged to send back the money. The attack is one of the largest hacking heists in history. Poly Network said the person had exploited a vulnerability in its system. The attack was uncovered on Tuesday when Poly Network publicly pleaded with the hacker to help. A hacker who stole just over $600m (£433m) worth of cryptocurrency was offered $500,000 and immunity as a reward for returning the money. Most of the money has now been given back, although the hacker says they are not interested in the reward. Is rewarding the hacker encouraging cyber crimes ?