SoK: Not Quite Water Under the Bridge: Review of Cross-Chain Bridge Hacks

Abstract: The blockchain ecosystem has evolved into a multi-chain world with various blockchains vying for use. Although each blockchain may have its own native cryptocurrency or digital assets, there are use cases to transfer these assets between blockchains. Systems that bring these digital assets across blockchains are called bridges, and have become important parts of the ecosystem. The designs of bridges vary and range from quite primitive to extremely complex. However, they typically consist of smart contracts holding and releasing digital assets, as well as nodes that help facilitate user interactions between chains. In this paper, we first provide a high level breakdown of components in a bridge and the different processes for some bridge designs. In doing this, we identify risks associated with bridge components. Then we analyse past exploits in the blockchain ecosystem that specifically targeted bridges.