Vulnerable Vault

An intentionally vulnerable smart contract.

Vulnerability brief

The project consists of a simple token vault.

• Users can deposit funds
• after enough time they can withdraw funds and receive yields based on several factors

tree
.
├── src/                                    
│   └── Vault.sol             # A vulnerable vault, with a bad accounting logic
│   └── VaultToken.sol        # Generic ERC20 token implementation
├── test/                                    
│   └── Vault.t.sol           # Solution PoC test file

One of the yield calculation's deciding factors is the contracts balance. The problem with that is that the balance, calcualted by token.balanceOf(address(this)), can be manipulated by an attacker sending funds directly, i.e. transfer() instead of using deposit(), to a situation in which funds it sends directly bump the possible yield gains to be more than what is transferred.

This is the vulnerable function:

    function calculateYield(address _staker) public view returns (uint256) {
        Stake memory stake = stakes[_staker];
        uint256 stakingDuration = block.timestamp - stake.startTime;
        uint256 scaleFactor = 100000;
        uint256 balanceBasedBonus = (getVaultBalance() * 5) / 100;

        // The longer the stake duration the bigger the yield
        // The higher the stake amount the bigger the yield
        // The more total balance the vault has, the bigger the yield
        uint256 yield = ((stakingDuration * stake.amount * balanceBasedBonus) /
            (totalSupply * scaleFactor)) + balanceBasedBonus;
        return yield;
    }

An attacker might realize that the yield calculation can be abused under certain conditions, and follow these steps:

• Normally deposit some funds in the vault
• Wait a time period (e.g. 6 months) to gain yield
• Before withdrawal, directly transfer tokens to the vault to tamper with it's yield calculation to the attacker's benefit
• Withdraw all invested funds, with fees covering the direct transfer losses, and steal extra tokens due to the tampered yield calculation.

Required installations:

forge install OpenZeppelin/openzeppelin-contracts --no-commit

Run an innocent usecase:

forge test -vv --match-test testNormalUse

Run the attack PoC:

forge test -vv --match-test testAttack