Blockchain site Poly Network said hackers had exploited a vulnerability in its system and taken thousands of digital tokens such as Ether.
The Poly Network blockchain site allows users to move tokens linked to one of these ledgers to a different network, making them work together.
“The amount of money you have hacked is one of the biggest in defi [decentralised finance] history,” the company noted in a letter addressed to the hacker that was posted on Twitter.
“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,” Poly Network said in the letter.
Changpeng Zhao, chief executive of Binance, tweeted that his firm was aware of the hack, and was “co-ordinating with all our security partners to proactively help”.
Hours after the hack, the attacker started returning the funds.
Roughly $260 million worth of cryptocurrency has been returned to Poly Network in a range of coins, according to separate analyses by blockchain forensics company Chainalysis and crypto tracking firm Elliptic.
A person claiming to have perpetrated the hack said they did it "for fun" and wanted to "expose the vulnerability" before others could exploit it, according to digital messages shared by Elliptic and Chainalysis.
It was "always the plan" to return the tokens, the purported hacker wrote, adding: "I am not very interested in money."
They started sending back small transfers totalling a few dollars to the online wallets controlled by Poly - but then began making much larger deposits, totalling hundreds of millions.
In scale, the hack is on par with huge recent breaches at exchanges such as Coincheck and Mt Gox.
