Cross-Chain Signature Replay Attack

[code423n4]

Lines of code

https://github.com/code-423n4/2023-01-biconomy/blob/main/scw-contracts/contracts/smart-contract-wallet/paymasters/verifying/singleton/VerifyingSingletonPaymaster.sol#L77-L90

Vulnerability details

Cross-Chain Signature Replay Attack

Impact

User operations can be replayed on smart accounts accross different chains. This can lead to user's loosing funds or any unexpected behaviour that transaction replay attacks usually lead to.

Proof of Concept

As specified by the EIP4337 standard to prevent replay attacks ... the signature should depend on chainid. In VerifyingSingletonPaymaster.sol#getHash the chainId is missing which means that the same UserOperation can be replayed on a different chain for the same smart contract account if the verifyingSigner is the same (and most likely this will be the case).

Tools Used

Manual review

Recommended Mitigation Steps

Add the chainId in the calculation of the UserOperation hash in VerifyingSingletonPaymaster.sol#getHash

    function getHash(UserOperation calldata userOp)
    public view returns (bytes32) { // @audit change to view
        //can't use userOp.hash(), since it contains also the paymasterAndData itself.
        return keccak256(abi.encode(
                userOp.getSender(),
                userOp.nonce,
                keccak256(userOp.initCode),
                keccak256(userOp.callData),
                userOp.callGasLimit,
                userOp.verificationGasLimit,
                userOp.preVerificationGas,
                userOp.maxFeePerGas,
                userOp.maxPriorityFeePerGas
        block.chainid // @audit add chain id
            ));
    }

[gzeoneth]

might consider as dupe of #469

[c4-judge]

gzeon-c4 marked the issue as primary issue

[c4-sponsor]

livingrockrises marked the issue as sponsor confirmed

[c4-judge]

gzeon-c4 changed the severity to 2 (Med Risk)

[c4-judge]

gzeon-c4 marked the issue as selected for report

[vladbochok]

@gzeoneth @livingrockrises

User operations can be replayed on smart accounts accross different chains

The author refers that the operation may be replayed on a different chain. That is not true. The "getHash" function derives the hash of UserOp specifically for the paymaster's internal usage. While the paymaster doesn't sign the chainId, the UserOp may not be relayed on a different chain. So, the only paymaster may get hurt. In all other respects, the bug is valid.

The real use case of this cross-chan replayability is described in issue #504 (which, I believe, was mistakenly downgraded).

[livingrockrises]

true. besides chainId , address(this) should be hashed and contract must maintain it's own nonces per wallet otherwise wallet can replay the signature and use paymaster to sponsor! we're also planning to hash paymasterId as add-on on top of our off-chain validation for it.

I have't seen an issue which covers all above. either cross chain replay or suggested paymaster nonce