fix(CCTPAdapter): Fix feeRecipient check to determine V2 vs V1 (#924)

- The casting to bytes20 will fetch the first 20 bytes of the return value. However, the functions in Solidity return abi.encoded data, which means in particular, that if an address is returned from the feeRecipient function, the returned feeRecipient.length will be equal to 32 and the address will be included in the last 20 bytes of that data (will be padded with 12 zeroes at the beginning). As a result, casting feeRecipient to bytes20 will capture only the first 8 bytes of the returned address. You can get the full address by casting the data to uint160 instead (you may need to cast it to bytes32 and uint256 first).
- The feeRecipient.length is not validated. It means that for example, in case of a call to nonexistent contract or a call to a contract with a fallback function which doesn't return any data, the code will attempt to cast empty data to bytes20. It seems that casting empty bytes object to bytes20 will return 0, but it would be safer to not rely on that behaviour and validate that the length of feeRecipient object equals 32.
- Even after fixing the issues above, this check will not be fully reliable as it could for example succeed in case when a TokenMessenger V1 contract had a fallback function returning 32 bytes. This is a very low risk, hence you may keep this logic if you accept the risk, but a fully correct solution would involve introducing an additional parameter to the constructor, which could be then used to determine the CCTP version (this could be an uint256 as it is possible that more CCTP versions will appear in the future). We understand that you wanted to avoid adding this extra parameter (and hence introducing a very large diff), but the most reliable solution would involve introducing it in the constructor.

Author: nicholaspai

contracts/libraries/CircleCCTPAdapter.sol

@@ -83,7 +83,14 @@ abstract contract CircleCCTPAdapter {
         (bool success, bytes memory feeRecipient) = address(cctpTokenMessenger).staticcall(
             abi.encodeWithSignature("feeRecipient()")
         );
-        cctpV2 = (success && address(bytes20(feeRecipient)) != address(0));
+        // In case of a call to nonexistent contract or a call to a contract with a fallback function which
+        // doesn't return any data, feeRecipient can be empty so check its length.
+        // Even with this check, its possible that the contract has implemented a fallback function that returns
+        // 32 bytes of data but its not actually the feeRecipient address. This is extremely low risk but worth
+        // mentioning that the following check is not 100% safe.
+        cctpV2 = (success &&
+            feeRecipient.length == 32 &&
+            address(uint160(uint256(bytes32(feeRecipient)))) != address(0));
     }
 
     /**