Poly Network cross-chain bridge exploited for ~$611M

On August 10, 2021, an attacker exploited a flaw in Poly Network's cross-chain smart contracts and drained roughly $611 million across Ethereum, BNB Chain, and Polygon, then the largest DeFi theft on record. In a plot twist, the hacker began returning the funds the next day, claimed it was all to expose vulnerabilities, and was eventually offered a $500,000 bug bounty and a "chief security advisor" role; nearly everything was returned by August 25, save the $33M in USDT that Tether froze.