The ALEX Lab Foundation, a Bitcoin DeFi application, was hacked and lost over $4.3 million in various tokens. The attackers likely gained access to a private key controlling ALEX's XLink bridge, which allows users to transfer tokens between different blockchains. The team has identified the attacker and offered a 10% bounty for the return of 90% of the stolen funds. Major exchanges have frozen funds associated with the hacker to prevent further misuse. Private key compromises are a common attack vector for hackers.