README.md

L1 to L2 Message Failure

In Starknet, Ethereum contracts can send messages from L1 to L2 using a bridge. However, it is not guaranteed that the message will be processed by the sequencer. For instance, a message can fail to be processed if there is a sudden spike in the gas price and the value provided is too low. To address this issue, Starknet developers have provided an API to cancel ongoing messages.

Example

Consider the following code to initiate L2 deposits from L1, taking the tokens from the user:

contract L1ToL2Bridge {
    IERC20 public token; // some token to deposit on L2

    function depositToL2(address to, uint256 amount) public returns (bool) {
        require(token.transferFrom(msg.sender, address(this), amount));
        // ...
        StarknetCore.sendMessageToL2(data);
        return true;
    }
}

If an L1 message is never processed by the sequencer, users will never receive their tokens in either L1 or L2, and they need a way to cancel the message.

A recent AAVE audit highlighted this issue and required the addition of code to cancel messages.

Mitigations

When sending a message from L1 to L2, it is essential to consider the possibility that a message may never be processed by the sequencer. This can block either the contract from reaching a certain state or users from retrieving their funds. If needed, allow the use of startL1ToL2MessageCancellation and cancelL1ToL2Message to cancel ongoing messages.