DOS.md

Denial of Service

Smart contract below is vulnureable to Denial of Service (Gas limit reached)

contract FundTransfer {
  mapping (address => uint) public balances;

  function transfer(address _to, uint _amount) public {
    require(balances[msg.sender] >= _amount);
    balances[msg.sender] -= _amount;
    balances[_to] += _amount;

    // perform expensive computations
    for (uint i = 0; i < 10**6; i++) {
      // do something expensive
    }
  }
}

Why it's vulnerable?

This contract allows users to transfer funds to each other. However, it is vulnerable to a DoS attack because the transfer() function performs a large number of expensive computations, which can cause the gas limit to be reached.

Impact?

If the gas limit is reached, the transaction will fail and the funds will not be transferred.

How to fix?

To prevent this vulnerability, the transfer() function should be updated to avoid performing expensive computations that could cause the gas limit to be reached. Here is an updated version of the contract that avoids expensive computations:

contract FundTransfer {
  mapping (address => uint) public balances;

  function transfer(address _to, uint _amount) public {
    require(balances[msg.sender] >= _amount);
    balances[msg.sender] -= _amount;
    balances[_to] += _amount;
  }
}

In this updated version of the contract, the transfer() function does not perform any expensive computations, which means that it will not cause the gas limit to be reached. This prevents the contract from being vulnerable to a DoS attack.