RACE #28 Of The Secureum Bootcamp Epoch∞

This is a Write-Up of RACE-28, Quiz of the Secureum Bootcamp (opens in a new tab) for Ethereum Smart Contract Auditors. It was designed by our recently joined Secureum Mentor Palina (opens in a new tab), Research and Verification Engineer at Runtime Verification Inc (opens in a new tab).

Participants of this quiz had to answer 8 questions within the strict time limit of 16 minutes. If you’re reading this in preparation for participating yourself, it’s best to give it a try under the same time limit!

As usual, I waited for submissions to close before publishing it and, to stay true to the original, I omitted syntax highlighting. Feel free to copy it into your favorite editor, but do so after starting the timer!

April 2, 2024 by patrickd

Code

Note: All 8 questions in this quiz are based on the ERC4626Vault contract snippet. This is the same contract snippet you will see for all the 8 questions in this quiz.

ERC4626Vault code snippet illustrates an implementation of an ERC4626 (opens in a new tab)-based tokenized yield-bearing vault that represents shares of a single underlying ERC-20 token. Assume that all strictly required ERC4626 functions and events that are not shown are implemented correctly. Assume that the underlying (asset) token contract can be any token following an ERC-20 compatible standard.

pragma solidity ^0.8.13;


import {ERC20} from "https://github.com/transmissions11/solmate/blob/main/src/tokens/ERC20.sol";
import {SafeTransferLib} from "https://github.com/transmissions11/solmate/blob/main/src/utils/SafeTransferLib.sol";
import {FixedPointMathLib} from "https://github.com/transmissions11/solmate/blob/main/src/utils/FixedPointMathLib.sol";


contract ERC4626Vault is ERC20 {
    using SafeTransferLib for ERC20;
    using FixedPointMathLib for uint256;

    ERC20 public immutable asset;

    constructor(
        ERC20 _asset,
        string memory _name,
        string memory _symbol
    ) ERC20(_name, _symbol, 18) {
        asset = _asset;
    }

    function deposit(uint256 assets, address receiver) public virtual returns (uint256 shares) {
        require((shares = previewDeposit(assets)) != 0, "ZERO_SHARES");
        asset.safeTransferFrom(msg.sender, address(this), assets);
        _mint(receiver, assets);
        emit Deposit(msg.sender, receiver, assets, shares);
    }

    function mint(uint256 shares, address receiver) public virtual returns (uint256 assets) {
        assets = previewMint(shares);
        _mint(receiver, shares);
        asset.safeTransferFrom(msg.sender, address(this), assets);
        emit Deposit(msg.sender, receiver, assets, shares);
    }

    function withdraw(
        uint256 assets,
        address receiver,
        address owner
    ) public virtual returns (uint256 shares) {
        shares = previewWithdraw(assets);
        if (msg.sender != owner) {
            uint256 allowed = allowance[owner][msg.sender];
            if (allowed != type(uint256).max) allowance[owner][msg.sender] = allowed - shares;
        }
        _burn(owner, shares);
        emit Withdraw(msg.sender, receiver, owner, assets, shares);
        asset.safeTransfer(receiver, assets);
    }

    function redeem(
        uint256 shares,
        address receiver,
        address owner
    ) public virtual returns (uint256 assets) {
        if (msg.sender != owner) {
            uint256 allowed = allowance[owner][msg.sender];
            if (allowed != type(uint256).max) allowance[owner][msg.sender] = allowed - shares;
        }
        require((assets = previewRedeem(shares)) != 0, "ZERO_ASSETS");
        _burn(owner, shares);
        emit Withdraw(msg.sender, receiver, owner, assets, shares);
        asset.safeTransfer(receiver, assets);
    }

    function totalAssets() public view virtual returns (uint256) {
        return asset.balanceOf(address(this));
    }

    function convertToShares(uint256 assets) public view virtual returns (uint256) {
        return totalSupply == 0 ? assets : assets.mulDivDown(totalSupply, totalAssets());
    }

    function convertToAssets(uint256 shares) public view virtual returns (uint256) {
        return totalSupply == 0 ? shares : shares.mulDivDown(totalAssets(), totalSupply);
    }

    function previewDeposit(uint256 assets) public view virtual returns (uint256) {
        return convertToShares(assets);
    }

    function previewMint(uint256 shares) public view virtual returns (uint256) {
        return totalSupply == 0 ? shares : shares.mulDivUp(totalAssets(), totalSupply);
    }

    function previewWithdraw(uint256 assets) public view virtual returns (uint256) {
        return totalSupply == 0 ? assets : assets.mulDivDown(totalSupply, totalAssets());
    }

    function previewRedeem(uint256 shares) public view virtual returns (uint256) {
        return convertToAssets(shares);
    }

}

Question 1 of 8

Is it possible for an attacker to drain this vault contract?

A. Yes, due to integer underflow in withdrawal
B. Yes, because mint function is reentrant
C. Yes, due to the wrong rounding direction
D. No, it is not possible

Solution

Correct is B, C.

Palina's answer: There’s no integer overflow in the withdrawal() function since the contract is written in Solidity 0.8. There is reentrancy in mint() if the underlying asset is an ERC777 token as the Check-Effects-Interactions pattern is not followed. previewWithdraw(), which calculates the number of shares a user has to supply to receive a given number of the underlying tokens, rounds down instead of rounding up which might lead to the loss of funds.

I'd add that even a contract written with Solidity 0.8.x can have integer overflows, but that would require the use of unchecked or assembly blocks - neither of which are used in the withdrawal() function.

As the info-box above the code mentioned, we are to "assume that the underlying (asset) token contract can be any token following an ERC-20 compatible standard". ERC777 (opens in a new tab) tokens are an ERC-20 backward-compatible standard that implements "hooks" which are functions that get called when tokens are sent (tokensToSend) or received (tokensReceived). That means that the sender (ie. owner of tokens being sent) or recipient can take over the execution flow and exploit contract state that hasn't been fully updated yet (representing an exploitable reentrancy vector).

Rounding should always happen in favor of the protocol to prevent profitable attacks. That means that, when withdrawing a specified amount of assets, the shares to redeem in exchange should be rounded up. When redeeming a specific amount of shares, the amount of returned assets should be rounded down. When depositing a certain amount of assets, the number of shares should be rounded down. When minting a specified amount of shares, the number of assets to be deposited should be rounded up.

The previewWithdraw() function returns the number of shares that need to be burned in order to return the specified amount of assets. As such, the number of shares should be rounded up instead of down.

Question 2 of 8

Which is true about the rounding directions according to the standard?

A. withdraw should round up, redeem should round down
B. withdraw should round down, redeem should round up
C. withdraw and redeem should round down
D. None of the above, all computations must be exact

Solution

Correct is A.

Palina's answer: According to the EIP (https://eips.ethereum.org/EIPS/eip-4626 (opens in a new tab)), withdraw should round up, while redeem should round down, thereby ensuring that rounding favors the protocol rather than the user.

Also see Question 1 solution.

Question 3 of 8

What is the EIP conformant slippage protection for minting shares in ERC4626Vault?

A. ERC4626Vault is not vulnerable to slippage by design
B. Make mint internal
C. mint should have an extra parameter minShares to be compared with the actual amount of minted shares, e.g., require(assets > minShares);
D. Slippage protection could be added to a router-like contract that is interacting with ERC4626Vault instead

Solution

Correct is D.

Palina's answer: Slippage is one of the major security concerns related to the ERC4626 standard. Making mint internal would prevent users from calling it, thereby restricting them from using basic ERC4626 functionality, instead of addressing the slippage issue. According to the EIP (https://eips.ethereum.org/EIPS/eip-4626 (opens in a new tab)), mint function should have an interface that does not include the additional minShares parameter, making option C non-compliant to the standard. The correct answer is, therefore, D, as implemented in the ERC4626Router (https://github.com/ERC4626-Alliance/ERC4626-Contracts/blob/main/README.md#erc4626router-and-base (opens in a new tab)).

Question 4 of 8

According to the standard, deposit(uint256 assets, address receiver) mints -Vault shares to receiver by depositing exactly assets of underlying tokens. Assume that the exchange rate between the underlying ERC20 asset and a Vault share, as returned by previewDeposit and convertToShares, is 2:1. How many Vault shares will the user get when calling deposit(64, msg.sender) in ERC4626Vault?

A. 32
B. 64
C. 128
D. 160

Solution

Correct is B.

Palina's answer: Even though the asset and share should be trading 2:1, there is a typo, which causes _mint function in deposit to mint assets instead of shares. The user depositing 64 assets will therefore get 64 shares in return.

Question 5 of 8

Which of the following invariants hold in the ERC4626Vault contract?

A. The amount of user shares does not decrease unless the user calls withdraw or redeem
B. Sum of all user shares is equal to the underlying token balance of the vault
C. Number of shares in circulation equals the total of shares minted minus the total of shares burned
D. Sum of all user shares is equal to the total supply of shares

Solution

Correct is C, D.

Palina's answer: A is incorrect because the user can transfer shares to another account using the transfer function that is also present in ERC4626 contracts. Sum of all user shares is equal to the total number of shares minted, i.e., total supply of shares, so D is correct and B isn’t. The number of shares that is in circulation is the difference between the number of shares that got minted minus the number of shares that were redeemed.

Question 6 of 8

Which statements are true about redeem and withdraw functions in the ERC4626Vault contract?

A. redeem and withdraw differ in rounding direction
B. withdraw sends the exact amount of assets to receiver
C. withdraw and redeem only differ in visibility
D. redeem burns the exact amount of shares from owner

Solution

Correct is B, D.

Palina's answer: redeem and withdraw are supposed to differ in rounding direction, since redeem should round down and withdraw should round up, however, due to a bug in this contract, they both round down. withdraw does send the exact amount of assets to receiver and redeem burns the exact number of shares from owner according to both the standard and this implementation, hence, B and D are correct. withdraw and redeem don’t differ in visibility, but do differ in functionality, making C incorrect.

Question 7 of 8

When there are no virtual shares and assets, inflation attacks on ERC4626 vaults are possible because of:

A. Rounding up when converting shares to assets
B. Rounding down when converting assets to shares
C. Share tokens of the vault increasing in price
D. Donation of assets to the vault to manipulate the exchange rate between assets and shares

Solution

Correct is B, D.

Palina's answer: Inflation attack allows an exploiter to profit at the expense of other users. It is possible since the calculation of users' shares being minted are rounded down, so the attacker can dilute other users' shares by manipulating the exchange rate between assets and shares. One of the ways to skew the exchange rate in the attacker's favor is by donating (not depositing) a large amount of assets into the vault: this increases the total assets in the vault without affecting the amount of shares in circulation, leading to a much smaller amount of shares being minted for the next user. Source: https://blog.openzeppelin.com/a-novel-defense-against-erc4626-inflation-attacks (opens in a new tab)

Question 8 of 8

What are the issues with the underlying ERC20 token integration in the ERC4626Vault contract?

A. If ERC4626Vault uses fewer than or the same number of decimals as the underlying ERC20 asset token, precision loss can occur during conversions between assets and shares
B. In withdraw, transfer should be used instead of safeTransfer to transfer assets to the receiver
C. Tokens taking fee on transfer might break accounting
D. None of the above, there are no issues with the integration

Solution

Correct is A, C.

Palina's answer: in the constructor, ERC4626Vault invokes parent’s ERC20 constructor providing 18 as the value of decimals, which allows for discrepancy between the underlying asset’s decimals and those of ERC4626Vault. Precision loss during conversions between assets and shares can occur due to rounding and is more pronounced if the Vault has fewer decimals than the underlying ERC20. safeTransfer is a safer alternative to transfer that checks the possible Boolean return value of a call, so it is used correctly in the contract. deposit and mint functions do not account for the possibility that assets might change a fee on transfer — in this case, (assets - fee) would get deposited in the contract, while the shares calculation accounts for assets being deposited. A safer alternative is to use the actual difference between the contract’s balance before and after the transfer in the calculation of shares the user is entitled to.

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