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Hundreds of millions of dollars' worth of Ether was reportedly stolen in the hack.
Hack of DeFi platform could be one of the largest crypto heists ever.

Hackers who stole more than $600 million in cryptocurrency from a protocol known as Poly Network have returned more than half of that sum, after Poly Network pleaded with the culprits to return their haul Tuesday.

"Dear hacker," an open letter from Poly Network read. "The amount of money you have hacked is one of the biggest in defi history. Law enforcement in any country will regard this as a major economic crime and you will be pursued. ... The money you stole are from tens of thousands of crypto community members, hence the people.

"You should talk to us to work out a solution," it concluded.

On Wednesday, the hackers sent a message to Poly Network saying they were "ready to return" the funds, according to CNBC. Poly Network replied with three crypto addresses for the money.

Poly Network is a decentralized-finance (DeFi) platform that operates on the Binance Smart Chain, Ethereum and Polygon blockchains. It said Tuesday that a preliminary investigation found a vulnerability between contract calls, and denied rumors that the exploit was caused by a single keeper.

Poly Network said its team traced the stolen assets to three addresses, and asked token issuers to blacklist tokens coming from those addresses.

Tether Chief Technology Officer Paolo Ardoino tweeted that the stablecoin company froze $33 million worth of its tokens that were taken in the hack.

The heist, estimated at about $611 million in various crypto assets, including hundreds of millions of dollars' worth of Ether ETHUSD, -2.97% and Binance coins, would rank as among the largest in crypto history.

Last week, Securities and Exchange Commission Chairman Gary Gensler called for new regulations on cryptocurrency and DeFi, warning that the digital-asset class is "rife with fraud, scams and abuses."