Dos with revert

Question

While I was reading a guide on smart contract security (by consensys) I found this:

// INSECURE
contract Auction {
    address currentLeader;
    uint highestBid;

    function bid() payable {
        require(msg.value > highestBid);

        require(currentLeader.send(highestBid)); // Refund the old leader, if it fails then revert

        currentLeader = msg.sender;
        highestBid = msg.value;
    }
}

The vulnerability here (quoted from the guide):

When it tries to refund the old leader, it reverts if the refund fails. This means that a malicious bidder can become the leader while making sure that any refunds to their address will always fail. In this way, they can prevent anyone else from calling the bid() function, and stay the leader forever.

How can someone make all the incoming transactions(the refunds) to their address fail? In case of a contract I guess the answer is having a revert inside the fallback function. In the case of simple address how can someone do it? I am asking cause it really blew my mind since I have never heard of this before(cause it's kind of pointless i guess).

Cheers!

Answer 1

An address can also be another contract, which is executed when the Auction tries to send funds to it. That other contract can just execute an illegal instruction or use up all the gas to make the refund transaction fail.

Sending funds to account addresses cannot fail as far as I know (at least if the sender has sufficient funds).

Answer 2

Just to add a little bit more information after the correct existing answer. I don't see a way to fail a transaction to an EOA (Externally Owned Address) as long as you give enough gas.

When you send Ether to an address you don't possibly know whether it's a contract address or an EOA. Or maybe it's a brand new address and it's neither (yet).

If you send Ether to a non-used address the transaction can't fail but it stays in the address. After that someone may even create a contract which uses that address and which has a fallback revert but the Ether is already in the address so it can't be reverted. This is one of the ways to send Ether to a contract with no option to deny the Ether.