Submitted by OffsideLabs, also found by anon and ustas
https://github.com/code-423n4/2023-10-zksync/blob/72f5f16ed4ba94c7689fe38fcb0b7d27d2a3f135/code/system-contracts/contracts/libraries/TransactionHelper.sol#L183-L187
https://github.com/code-423n4/2023-10-zksync/blob/72f5f16ed4ba94c7689fe38fcb0b7d27d2a3f135/code/system-contracts/contracts/DefaultAccount.sol#L78-L107
The L2 DefaultAccount.validateTransaction function and transaction encoding process in the bootloader do not enforce EIP-155. If the reserved[0]  of the legacy tx is zero, the EIP-155 will be disabled. The chainId will not be included in the transaction signature.
There are two potential exploitation scenarios:
EIP-155 will be disabled when the following two conditions are met:
The value of the reserved field cannot be set arbitrarily, but if the user inputs a transaction with tx_type=0 and the v in the signature is 27 or 28, then reserved[0] will be set to 0.
Code here.
Enforce EIP-155 signature verification in the contract DefaultAccount.validateTransaction.
miladpiri (zkSync) confirmed, but disagreed with severity and commented:
Alex the Entreprenerd (judge) decreased severity to QA and commented:
Alex the Entreprenerd (judge) increased severity to Medium and commented:
Note: For full discussion, see here.
