Common Security Vulnerabilities

Re-Entrancy

// Vulnerable Contract 
contract EtherStore {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw() public {
        uint bal = balances[msg.sender];
        require(bal > 0);

        (bool sent, ) = msg.sender.call{value: bal}("");
        require(sent, "Failed to send Ether");

        balances[msg.sender] = 0;
    }

    // Helper function to check the balance of this contract
    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

// Fixed Contract 
contract EtherStore {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw() public {
        // check 
        uint bal = balances[msg.sender];
        require(bal > 0);
        
        // effect
        balances[msg.sender] = 0;

        // external calls
        (bool sent, ) = msg.sender.call{value: bal}("");
        require(sent, "Failed to send Ether");
    }

    // Helper function to check the balance of this contract
    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

Always verify code even if it's a library!

Two Rights Might Make A Wrong - Paradigm

function batch(bytes[] calldata calls, bool revertOnFail) external payable returns (bool[] memory successes, bytes[] memory results) {
    successes = new bool[](calls.length);
    results = new bytes[](calls.length);
    for (uint256 i = 0; i < calls.length; i++) {
        (bool success, bytes memory result) = address(this).delegatecall(calls[i]);
        require(success || !revertOnFail, _getRevertMsg(result));
        successes[i] = success;
        results[i] = result;
    }
}

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