Summary

The contracts MOR, L2TokenReceiver and L1Sender inherit ERC165. This standard is used to publish and detect what interfaces a smart contract implements. Read this for more. If we take a look at MOR and L2TokenReceiver, we can see that they implement this standard the correct way:

But there is no such function in L1Sender.sol.

Vulnerability Details

Having no supportsInterface function, means that the ERC165 standard is not implement correctly. This contract implements IL1Sender and ERC165. Lets say someone wants to check if this contract implements the IL1Sender interface. The lack of supportsInterface function, means that the return value will be false, which is wrong.

Impact

When supportsInterface is called, the output will always be false.

Proof of Concept

Do the following changes to L1Sender.sol:

• Import the IERC165 interface at the top of the file:

import {ERC165, IERC165} from "@openzeppelin/contracts/utils/introspection/ERC165.sol";

• Create the following pure functions:

Full file can be found in this Gist.

Now add the following test cases to L1Sender.test.ts

Run npx hardhat test.

As we can see, the first test fails(which is the IL1Sender) and the second(IERC165) passes. The second passes because in ERC165 the supportsInterface functions is this:

However this is not the expected behaviour. Both of the tests should pass.

Tools Used

Manual Review, Hardhat

Recommendations

Make sure to implement correctly the supportsInterface function:

Note that we add a check for IERC165 here as well, because this function is override.

Now run npx hardhat test again: