How to Exploit ETH Using Delegatecall and Selfdestruct – Security Insights

DELEGATECALL basically says that I'm a contract and I'm allowing (delegating) you to do whatever you want to my storage. DELEGATECALL is a security risk for the sending contract which needs to trust that the receiving contract will treat the storage well.

DELEGATECALL was a new opcode that was a bug fix for CALLCODE which did not preserve msg.sender and msg.value. If Alice invokes Bob who does DELEGATECALL to Charlie, the msg.sender in the DELEGATECALL is Alice (whereas if CALLCODE was used the msg.sender would be Bob).

Details

When D does CALL on E, the code runs in the context of E: the storage of E is used.

When D does CALLCODE on E, the code runs in the context of D. So imagine that the code of E is in D. Whenever the code writes to storage, it writes to the storage of account D, instead of E.

contract D {
  uint public n;
  address public sender;

  function callSetN(address _e, uint _n) {
    _e.call(bytes4(sha3("setN(uint256)")), _n); // E's storage is set, D is not modified 
  }

  function callcodeSetN(address _e, uint _n) {
    _e.callcode(bytes4(sha3("setN(uint256)")), _n); // D's storage is set, E is not modified 
  }

  function delegatecallSetN(address _e, uint _n) {
    _e.delegatecall(bytes4(sha3("setN(uint256)")), _n); // D's storage is set, E is not modified 
  }
}

contract E {
  uint public n;
  address public sender;

  function setN(uint _n) {
    n = _n;
    sender = msg.sender;
    // msg.sender is D if invoked by D's callcodeSetN. None of E's storage is updated
    // msg.sender is C if invoked by C.foo(). None of E's storage is updated

    // the value of "this" is D, when invoked by either D's callcodeSetN or C.foo()
  }
}

contract C {
    function foo(D _d, E _e, uint _n) {
        _d.delegatecallSetN(_e, _n);
    }
}

When D does CALLCODE on E, msg.sender inside E is D as commented in the code above.

When an account C invokes D, and D does DELEGATECALL on E, msg.sender inside E is C. That is, E has the same msg.sender and msg.value as D.

You can quickly test above in Solidity Browser.

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}