Harvest Finance Hack

The Harvest Finance attack was a flash loan price manipulation exploit that took advantage of how the protocol calculated the value of its stablecoin pools. Here's the technical breakdown:

1. The attacker took out large flash loans from Uniswap (tens of millions in USDC)
2. They used these funds to manipulate the price of stablecoins in the Curve y-pool by creating large imbalances
3. During the price distortion, they deposited into and withdrew from Harvest's stablecoin (USDC, USDT) strategy vaults
4. Due to the price manipulation, they were able to deposit at one price and withdraw at a more favorable price
5. They repeated this process multiple times, draining approximately $34 million

The key vulnerability was in how Harvest calculated the share price for its vaults:

// Simplified representation of the vulnerable code
function getPricePerFullShare() public view returns (uint256) {
    // Calculate total assets including those deployed in strategy
    uint256 totalAssets = balance().add(
        IStrategy(strategy).totalAssets()
    );
    
    // Calculate share price
    uint256 totalShares = totalSupply();
    if (totalShares == 0 || totalAssets == 0) {
        return 1e18;
    }
    
    // Vulnerability: relies on external price that can be manipulated
    return totalAssets.mul(1e18).div(totalShares);
}

The main issue was that totalAssets() called by the strategy was indirectly using Curve's stablecoin exchange rate, which could be manipulated by large trades. By creating artificial price differences in the Curve pool, the attacker could exploit these rate differences to extract value from the vault.