Impact

This is present when Multicall is used on any contract which reads the value of msg.value. Multicall is a pattern to call several contract endpoints in one transaction, using delegatecall. A contract endpoint may implicitly assume that it is called in a single transaction, by looking at msg.value. Since the value is defined per transaction, calling the same endpoint twice in one multicall means that the same msg.value may be read several times, even though the value was only transferred once.

For example, if a token contract accepts ETH in exchange for tokens in a swap function, and the contract implements multicall, an attacker may call swap several times in one transaction. Let's say the attacker sends along 1 ETH in the multicall transaction, which would normally give them 100 tokens. Each call to the swap function will read msg.value and transfer 100 tokens to the attacker. If the attacker calls swap 10 times in one multicall, they will get 1,000 tokens in exchange for 1 ETH.

Patches

Add extensive warnings in the Multicall contract, or remove payability from it. Additionally use address(this).balance

Workarounds

if using msg.value a migration is needed.

References

Are there any links users can visit to find out more?

Multicall being payable is seriously dangerous. #52

For more information

If you have any questions or comments about this advisory:

• Open an issue in github.com/manifoldfinance/defi-threat/