Smart contract proxy analysis

Abstract

The evolution of Smart Contract protocols both in respect to size and complexity has led to the creation of new design patterns, centered around modularity, maintainability and upgradeability. One such emerging pattern in the Ethereum space is the Diamond Pattern. The Diamond Pattern is analogous to a reverse-proxy in Web2 infrastructure, as it provides a singular endpoint to a smart contract protocol whose implementation is split across multiple smart contracts. The state (storage) across the implementation contracts is consolidated in the proxy contract through the use of the delegatecall opcode. Although mechanisms exist to ensure implementation contracts can operate over segmented sections of the storage (state), a portion of the state will always remain shared and mutable. Incompatibilities in the manipulation of these storage variables across implementation contracts can introduce unique vulnerabilities which can go unnoticed when observing a single contract. Current state of the art static analysis tools do not take into account the unique intricacies of having shared mutable state across multiple smart contracts. This study introduces a general technique for multi-contract analysis under delegatecall , through the modularisation of the Gigahorse analysis framework and the propagation of storage facts between smart contracts during analysis execution. Following this we present a new tool called SOuL-Splitter, which generates multi-contract evaluation test sets through automated decomposition of existing smart contracts. Overall, we find that our analysis technique is highly effective, with some vulnerabilities exhibiting over a 70% improvement in recall as compared with their single contract counterparts. We also find evidence of increased adoption of the Diamond pattern in the Ethereum space, validating the need for, and value of, this research.