Some originally feared that the theft was enabled by an exploit on Hyperliquid itself, shortly after another Hyperliquid-based project was compromised, but the theft appears to have been a key leak rather than an exploit on the protocol.
Hyperliquid user loses $21 million to private key leak
An attacker apparently obtained access to a victim's private key, enabling them to drain $21 million in various crypto assets. The attacker quickly bridged the stolen funds to ETH, then bounced through various addresses in hopes of disguising their origin and making the funds more challenging to recover.