07.SWC-107_Reentrancy

SWC-107_Reentrancy

State Variable Default Visibility

• Description: One of the major dangers of calling external contracts is that they can take over the control flow. In the reentrancy attack (a.k.a. recursive call attack), a malicious contract calls back into the calling contract before the first invocation of the function is finished. This may cause the different invocations of the function to interact in undesirable ways.
• Remediation: The best practices to avoid Reentrancy weaknesses are:
  • Make sure all internal state changes are performed before the call is executed. This is known as the Checks-Effects-Interactions pattern
  • Use a reentrancy lock (ie. OpenZeppelin’s ReentrancyGuard.

vulnerability contract

pragma solidity ^0.5.0;

contract ModifierEntrancy {

  mapping (address => uint) public tokenBalance;
  string constant name = "Nu Token";
  Bank bank;
  
  constructor() public{
      bank = new Bank();
  }

  //If a contract has a zero balance and supports the token give them some token
  function airDrop() hasNoBalance supportsToken  public{
    tokenBalance[msg.sender] += 20;
  }
  
  //Checks that the contract responds the way we want
  modifier supportsToken() {
  	// reentrancy vulneability
    require(keccak256(abi.encodePacked("Nu Token")) == bank.supportsToken());
    _;
  }
  
  //Checks that the caller has a zero balance
  modifier hasNoBalance {
      require(tokenBalance[msg.sender] == 0);
      _;
  }
}

contract Bank{

    function supportsToken() external returns(bytes32) {
    	// reentrancy
        return keccak256(abi.encodePacked("No Token"));
    }

}