Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

Belt Finance has a strategy token. This represents shares within the pool of assets. Each token is given out proportionally (pro rata) for assets put into the strategy contract. The strategy token is an interest baring asset as well.

To understand the bug, we need to understand the withdrawal flow of the contract. There are two main variables for keeping track of funds: balanceSnapshot and wantLockedInHere. wantLockedInHere is the balance of the contract not being put to work to generate yield on the assets. balanceSnapshot holds the balance of the contract.

When calling withdrawal, two paths can be hit. First, if the contract has enough funds in wantLockedInHere it will send this. Otherwise, it will liquidate the yield-generating asset and decrease the value of balanceSnapshot.

When making a withdrawal from the contract directly, instead of through a different contract, there is a double counting bug that occurs. In particular, both balanceSnapshot and wantLockedInHere will be subtracted from. Why does this happen when calling directly? There's an if statement that fails to actually NOT withdraw the money but update the state variables.

By making these variables very small, the contract has much MORE assets than it believes. Because the perceived value is so low, the attackers amount of shares appears to be much higher than it actually is. Now, when the attacks deposits money again, the contract will mint too many shares because of how low the balance appears to be. A attacker could call earn to get the real value of the contract. Practically, this means that this attack can be performed multiple times.

Finally, an attacker calls withdraw to claim all of the shares they have earned from the contract. They now have more money than what they started with.

Calling contracts in weird ways causes problems! A great bug find for a 1 million dollar payout.