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Santhacklaus CTF 2019 - Golden Rush (Level 2 - Piggybank)

This challenge is the 2nd level of the Golden Rush series from the Santhacklaus CTF 2019. You can visit the CTF platform here and the challenge at this location: https://goldenrush.santhacklaus.xyz/level2

Read the source code

For the second level, we are given the following source code:

pragma solidity ^0.4.21;

contract PiggyBank {
    bool public isOwner = false;
    uint256 public funds;
    bytes32 private pinHash = 0x7d8db9357f1302f94064334778507bb7885244035ce76b16dc05318ba7bf624c;

    constructor() public payable {
        funds = funds + msg.value;
    }

    function() external payable {
        funds = funds + msg.value;
    }

    function unlockPiggyBank(uint256 pin) external {
        if((keccak256(abi.encodePacked(uint8(pin))) == pinHash)){
        isOwner = true;
        }
    }

    function withdrawPiggyBank() external {
        require(isOwner == true);
        msg.sender.transfer(funds);
        isOwner = false;
        funds = 0;
    }
}

This time, it seems that our Piggy Bank contract needs its isOwner varible set to true to allow us to withdraw our money. Therefore, we should probably look into the unlockPiggyBank function.

This function will set isOwner to true only if the pin argument we give him is the origin value of the hash pinHash.

Unlock the piggy

The vulnerability here is that the pin is converted to an uint8 before being tested, which means that the value we look for must be between 0 and 255. Indeed, it would not be possible otherwise to unlock the piggy bank with this function.

I will explain how I solved the challenge (beware it's quite dirty lol)

So, when I realized that I only had 256 possibilities to test, I understood that I had to bruteforce the pin, so I decided to send 256 interactions to the deployed contract with my browser console.

After connecting our Web3 object to the contract in the blockchain (see the writeup for the level 1 for more details on the commands), I realized that I will have to click on Confirm 256 times on the MetaMask pop up...

256

Since this task seemed quite labourious to me, I tried to find some idle solution to make my cursor auto-click on the button: I used xdotool to localise the position of the Confirm button on my screen and then I told him to click for me every second so so that I can play Summoners War in the meantime :D

sleep 3; xdotool getmouselocation # You have 3 seconds to get your cursor to the Confirm button
# x:792 y:597 screen:0 window:37748739
while true; do xdotool mousemove 792 597 click 1 &; sleep 1; done
# Don't forget the sleep 1 or else you're doomed...

Then a few minutes later, when xdotool had finished confirming all those operations, I moved my cursor back to my terminal and pressed ctrl+C really fast to stop the clicking!!

I stole the money by executing:

PiggyBank.withdrawPiggyBank(callback);

and finally got the flag:

flag

Only a few hours later did I realize that it was probably possible to compute the 256 hashes beforehand to know exactly which value the hash was before sending a single query to the contract with the correct value... but too late :D