Hackers perpetrated what is likely the biggest theft ever in the world of decentralized finance, stealing about $600 million in cryptocurrency from a protocol known as PolyNetwork that lets users swap tokens across multiple blockchains, Bloomberg notes.

PolyNetwork said in a letter posted on Twitter that tens of thousands of people were affected by the hack. About $33 million of the stablecoin Tether that was a part of the theft has reportedly been frozen by Tether’s issuer, making it unavailable to the attacker.

According to Bloomberg, it isn’t clear from the PolyNetwork website who runs the protocol, which governs transactions that run on the application.  DeFi, or decentralized finance has surged in popularity in the past few years in the wake of a boom in the development of applications that let people trade, borrow and lend funds to each other without intermediaries.

Security researcher SlowMist said it has found the attacker’s email, IP address and device fingerprints, the team reported on Twitter.  It added that “this is likely to be a long-planned, organized and prepared attack.”

Crypto exchanges including Binance are involved in helping PolyNetwork, Binance CEO Changeng Zhao said on Twitter.

With DeFi apps attracting billions in investor funds, they’ve also become frequent targets of attacks, Bloomberg says.  According to crypto security company CipherTrace, DeFi-related hacks this year made up more than 60% of the total hack and theft volume of crypto attacks, up from 20% in 2020. At $156 million, the amount netted from DeFi-related hacks in the first five months of 2021 already surpasses the $129 million stolen in DeFi-related hacks throughout all of 2020, CipherTrace said.

Bloomberg notes that about $80 billion is locked in DeFi applications, making them an attractive target, according to tracker DeFi Pulse.