A bug in OpenSea, the popular NFT marketplace, has let hackers buy rare NFTs for well below market value, in some cases leading to hundreds of thousands of dollars in losses for the original owners — and hundreds of thousands of dollars in profits for the apparent thieves.
The bug appears to have been present for weeks and seems to be referenced in at least one tweet from January 1st, 2022. But exploitation of the bug has picked up significantly in the past day: blockchain analytics company Elliptic reported that in a 12-hour stretch before the morning of January 24th, it was exploited at least eight times to “steal” NFTs with a market value of over $1 million.
One of the NFTs, Bored Ape Yacht Club #9991, was purchased using the exploit technique for 0.77 ETH ($1,760) and quickly resold for 84.2 ETH ($192,400), netting the attacker a profit of more than $190,000. An Ethereum address linked to the reseller had received more than 400 ETH ($904,000) in payouts from OpenSea in the same 12-hour period.
“It’s a subjective thing whether you consider this to be a loophole or a bug, but the fact is that people are being forced into sales at a price they wouldn’t otherwise have accepted right now,” said Tom Robinson, chief scientist and co-founder of Elliptic.
According to a Twitter thread by software developer Rotem Yakir, the bug is caused by a mismatch between the information available in NFT smart contracts and the information presented by OpenSea’s user interface. Essentially, the attackers are taking advantage of old contracts that persist on the blockchain but are no longer present in the view provided by the OpenSea application.
OpenSea users sell NFTs by setting a “list price” for potential buyers to see. Due to the nature of smart contracts, if a buyer accepts that list price, the NFT is automatically transferred to them. If an owner wants to re-list an NFT for a higher sale price, the proper way to do this is to cancel the first listing, which costs a “gas fee” that might be in the tens or even hundreds of dollars, so some users had skirted around this by transferring the NFT to another wallet, then back to the original wallet. While this technique apparently removed the listing from the information in OpenSea’s front-end display, the original listing remained active on the blockchain and could allegedly be found through the OpenSea API.
The bug was discovered as early as December 31st, 2021, according to CoinDesk. A tweet from almost two weeks ago on January 12th, 2022, details the forced sale of NFTs via the same method.
It’s unclear whether OpenSea is treating the situation as an open security flaw or a result of user error. The company did not respond to a request for comment by time of publication.