We disclosed a major vulnerability to the Multichain
project (formerly "AnySwap"). The threat was one of the largest, if not the largest, in crypto history, and the attack vector is novel. It turns out that some ERC20 tokens, most notably WETH, that has a permissive fallback function will allow arbitrary function calls to succeed. The reasons for this are complex, but have to do with low-level EVM semantics, the Ethereum ABI standard, and the way the Solidity compiler works. The article describes how an attacker could steal > $1B worth of WETH from Multichain and Fantom, after responsibly disclosing this issue to the Multichain team.