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HTB Business CTF 2024 - Recruitment [blockchain]

Difficulty : Very easy

Team : Phreaks2600

Source files

Do you think you have what it takes to live up to this crew? apply and prove it.

Introduction

The challenge need us to “apply” by sending some specific value to fill the requirements.

Recon

This challenge was my first blockchain challenge solved in CTF (except in root-me). I used a VM with debian to solve this but is possible on any linux distro (maybe windows but idk)

Reading the smart contract

We can apply by interacting with the function application(uint16 input1, string memory input2) by supplying two parameters.

Reading the contract Recruitment.sol:

function application(uint16 input1, string memory input2) public {
        // In order to be eligible, you must match the following set of skills:
        // - Hacker
        // - Stealth Specialist
        // - Engineer
        // - Demolition Specialist

        // Let's start!
        // Some preliminary checks: we do not hire unlucky people.
        require(block.timestamp % 2 == 0, "Natural selection people say..");

        // CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart)
        require(tx.origin == msg.sender, "Are you even human?");

        // Now let's start for real.
        // 1. Are you an hacker?
           require(input1 == 1337, "You lack hacking skills.");
        // yeah you definitely are.

        // 2. Are you stealthy?
        require(block.number < 20, "You lack stealth skills.");

        // 3. Are you an engineer?
        require(gasleft() <= 50000, "You lack engineering skills.");

        // 4. Are you a demolition specialist?
        require(keccak256(abi.encodePacked(input2)) == keccak256(abi.encodePacked("BOOM")), "You lack demolition skills"

        // Congratulations! Welcome to the crew.
        crew[msg.sender] = true;
        // here is your reward :)
        payable(msg.sender).transfer(1 wei);
        }

We understand that to get recruited we need to:

  • send an interaction when the timestamp is odd (we can’t influence on that, just call the function a few time)
  • input1 == 1337
  • block.number < 20 : we have to solve in under 20 transaction
  • gasleft <= 50000
  • input2 == BOOM

Solve

Getting familiar with the instance

Blockchain challenges on hackthebox are not on sepolia network but on a local networ using anvil

Starting the instance we got two ip, one for the RPC endpoint and one to get the flag and other informations.

We have got two IPs but the first time i saw it i didn’t understand the frick it was for. Then i used netcat to contact them and one gave me a response:

1 - Connection information
2 - Restart Instance
3 - Get flag
action? 1

Private key     :  0x0a5c3d2e7717a2f9292d5f13084352a800cad51eb923b169184b22f9863ad554
Address         :  0xF7b5424a61fFD3Afa8B2a1350d94Ff06D1f13eF7
Target contract :  0x03d65b9e5740f7a8b8c9aAfAc74fB26Ff2328eC4
Setup contract  :  0x6224fF48Af65663fB8b10CF830ECAC6F75B674f6

But i was still not familiar with HackTheBox’s game so i decided to try basic command on the second IP which did not respond at first.

For that i used the INCREDIBLE, ONLY and ONE cast cli utility from foundry-rs framework.

I can for instance check the balance of my contract with this noob tier command without environment variables : cast balance 0xF7b5424a61fFD3Afa8B2a1350d94Ff06D1f13eF7 --rpc-url http://<IP>:<PORT>

So now i understand that i got my address, my private key and the target contract address and how to interact with it.

To make commands easier, i will set local environment variables in a file name .env:

PRIVATE_KEY=0x0a5c3d2e7717a2f9292d5f13084352a800cad51eb923b169184b22f9863ad554
RPC_URL=http://94.237.61.244:38424/
TARGET=0x03d65b9e5740f7a8b8c9aAfAc74fB26Ff2328eC4
ATTACKER=0xF7b5424a61fFD3Afa8B2a1350d94Ff06D1f13eF7

And apply the variables: source .env

I also noticed that cast blockchain spells with cast send doesn’t increase block number if the transaction fail due to the require in the target contract. So i can try multiple time until i got it right..

Getting recruited

For applying we need to interact with the function application which has the signature : application(uint16,string)

  • To prove we are not unlucky, just run the transaction couple of times until it work. If it doesn’t, the transaction will just revert.

  • To pass the captcha, we need to have tx.origin == msg.sender. To be short, msg.sender is the address of the “account” running the transaction, where the ether will be taken for gas transaction for instance. And tx.origin is the address of the contract which has interacted with the destination contract.

    • If it is a smart contract, tx.origin == address of the smart contract.
    • If it is a person, tx.origin == msg.sender, address of the account publishing the transaction.

  • To fill hacker and demolition skills we need to call the contract wit values: 1337 and "BOOM".

  • To fill the engineer skills we can set –gas-limit close to 50000 but above of it. I had got some trouble wtih this part and thanks to HTB support i found that my gas limit was too low (50000). So i send with –gas-value = 60000.

Here is the final command:

cast send $TARGET --gas-limit 60000 --from $ATTACKER --rpc-url $RPC_URL --private-key $PRIVATE_KEY "application(uint16,string)" 1337 "BOOM"

We can fire this command until there are no more require error and get the flag.

HTB{th3y_s4id_W3lc0m3_Ab0ard}

Ressources

https://book.getfoundry.sh/reference/cast/cast-send

https://docs.soliditylang.org/en/latest/cheatsheet.html