THORChain Contract Problems – Approvals Can Drain RUNE Balance


THORChain is in hot waters again less than 24 hours after the last hack on July 23. This time it’s actually more serious. An interaction with any malicious contract can allow the attacker(s) to drain your wallet off the RUNE balance! It has worked as follows for now: attacker mass drops a token called UNIH to wallets, it’s worth something so users try to swap it on Uniswap. Before anything can be traded, you have to approve it. But if you do, the attacker can transfer your THORChain RUNE balance to his own wallet!

How is it even possible? Sadly, it appears that THORChain developers made a critical mistake while writing the code or they just wanted to save their users a few dollars by combining two tx in one. But it was a fatal diversion from solidity design principles and went contrary to what the documentation cautioned against. The surprising is that numerous audits conducted so far failed to report the issue also.

The developers at THORChain have used the transferTo function in the code, allowing any contract which interacts with it to transfer the RUNE balance. It uses tx.origin to authenticate the tx on your behalf. Also, it doesn’t check the allowance in the contract before transferring – sort of an infinite approval. It’s a strict no-no. Even the THORChain code documentation notes the danger of using this approach, but somehow they still went with it.

THORChain Code Documentation Notes The Issue
THORChain Code Documentation Notes The Issue

Apparently, the THORChain attacker has dropped the UNIH to thousands of wallets and is now draining their RUNE balance to his own. A clear assessment of the total amount stolen isn’t directly possible. THORChain RUNE price is now showing a 23%+ decline in the last 24 hours and is currently changing hands at around $3.63. It’s essential that users don’t approve smart contracts that aren’t trusted, though the THORChain should certainly do a better job at ensuring the security of the users as they can’t be assumed to know and safeguard themselves against complex vulnerabilities. 

THORChain contract exploits
THORChain contract exploits© Cryptoticker

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