In August 2021, the cryptocurrency industry witnessed one of the largest digital asset thefts in history when a hacker exploited a vulnerability in Poly Network. By taking advantage of flaws in the platform’s smart contracts, the attacker transferred more than $600 million worth of cryptocurrencies to wallets under their control. At the time, it was considered the largest decentralized finance (DeFi) hack ever recorded, sending shockwaves through the crypto community.

What happened next was even more extraordinary. Instead of laundering the funds or disappearing, the hacker began communicating directly with Poly Network by embedding messages into blockchain transactions. The attacker claimed the goal had been to expose critical security weaknesses rather than permanently steal the money. Because blockchain transactions are publicly visible, millions of people watched the unusual conversation unfold in real time as the hacker and the platform exchanged messages.

Over the following days, the individual—dubbed “Mr. White Hat” by parts of the cryptocurrency community—returned almost all of the stolen assets. Poly Network thanked the hacker for returning the funds, announced a $500,000 bug bounty, and even offered the individual the role of Chief Security Advisor. The hacker later declined to keep the reward money, saying it should instead be used to improve the platform’s security.

The incident remains controversial. While some viewed the attacker as an ethical hacker who exposed a serious vulnerability, others pointed out that taking control of hundreds of millions of dollars without authorization was still an illegal act, regardless of the funds being returned. Security researchers also noted that the hacker may have found it difficult to cash out such a large amount because the stolen assets were being closely tracked by blockchain analysts and cryptocurrency exchanges.