QuillAudit CTF challenges — Writeup

My solutions to all retired challenges can be found here.

1. Confidential Hash

Challenge Description:

This contract checks whether the keccak256 hash of aliceHash and bobHash is same to inputted hash. The given task is to find the keccak256 hash of aliceHash and bobHash, which is to get true from checkthehash function. hash function can calculate the keccak256 hash from two inputted bytes32 variables.

Vulnerability Description:

Confidential contract is initialising the private keys and hashes as private varibles in contract itself which is the vulnerability here because these variables are placed in storage with slot numbers. Anyone can just count the slot in which the private key is located and call it. In this case the slots are 4(aliceHash) and 9(bobHash).

Attack steps:

1. Setup the contract and attacker
2. Load the hashes of alice and bob from the respective slots in storage using load function.
3. Calculate the keccak256 hash of aliceHash and bobHash now using hash function of the contract.
4. Verify the calculated hash using checkthehash function of the contract which will return true.

Proof of Concept:

I used Foundry to test the Exploit. In foundry load function can get the variables from the deployed contract’s storage. Below is the test contract I used.

// SPDX-License-Identifier: UNLICENSED
pragma solidity 0.8.7;

import “@forge-std/Test.sol”;
import “../src/ConfidentialHash.sol”;

// @author: viking71
// Vulnerability: Confidential contract is initialising the private keys and hashes as private varibles in contract itself which is the vulnerability here because these variables are placed in 
// storage with slot numbers. Anyone can just count the slot in which the private key is located and call it. In this case the slots are 4(aliceHash) 
// and 9(bobHash). In foundry load function can get the varibles from the deployed contract’s storage. 
contract ConfidentialTest is Test {

 Confidential public confidential;
 address public attacker;

 function setUp() public {
   attacker =…