Biconomy - Smart Contract Wallet contest - ayeslick's results

Lines of code

https://github.com/code-423n4/2023-01-biconomy/blob/main/scw-contracts/contracts/smart-contract-wallet/SmartAccount.sol#L342

Vulnerability details

Impact

By using a smart contract to return the expected magic value an operator can bypass authentication checks in SmartAccount. Bypassing the authentication would allow an operator to call other contracts using someone else's SmartAccount.

An operator can craft a signature such that it points to an address he controls. When the contract calls isValidSignature on his contract the contract returns the appropriate value. This part of the function doesn’t check if the signer matches the owner when it checks the contract signature.

Proof of Concept

operator creates a contract that returns ERC1271's magic value operator crafts signature such that v == 0 The isValidSignature function is called on the operator's contract which returns the magic value The contract executes the operators transaction

Recommended Mitigation Steps

Require at least two signatures when using a contract signature so that one of the signatures can be used to verify the call came from the owner.