Curve, Vyper

Curve Finance is a central protocol within the DeFi ecosystem. The protocol was written in the Vyper language because of its gas efficiency. Most people assumed that the exploits were due to a known read-only reentrancy within Curve. However, the more people dove into the issues, the more they realized this ran deeper. It looked like a reentrancy issue in Curve. But, how is this possible? It had been audited multiple times! From investigation, it turns out that Vyper had a compiler bug on their reentrancy protection. Since Curve was relying upon the reentrancy protection working, this undermines the whole security of the protocol. The bug had been in the compiler for over a year. How did the compiler mess this up? According to the here, there is a mismatch in the slots that are being checked for reentrancy. This means that the protection was per function instead of per contract, which is really bad for the protocol. From the commit hash, it appears that a check was missing to see if a reentrancy lock had already been made. This resulted in a lock being made per function, which makes the reentrant lock possible to work around. BlockSec's image shows the change that made the code vulnerable. Initially, it was crazy to me that basics tests did not call this. However, a developer would write a test to call the same function back-to-back. In this case, the protection would have worked. Instead, one function in the contract then another one would have been called in order to test this. Performing the extra test cases and a test suite can pay off! A crazy bug led to the destruction of this. I wonder if people will use Vyper in the future or if they will only use Solidity.