  Cross-Chain bridges have become the most popular solution to support asset
interoperability between heterogeneous blockchains. However, while providing
efficient and flexible cross-chain asset transfer, the complex workflow
involving both on-chain smart contracts and off-chain programs causes emerging
security issues. In the past year, there have been more than ten severe attacks
against cross-chain bridges, causing billions of loss. With few studies
focusing on the security of cross-chain bridges, the community still lacks the
knowledge and tools to mitigate this significant threat. To bridge the gap, we
conduct the first study on the security of cross-chain bridges. We document
three new classes of security bugs and propose a set of security properties and
patterns to characterize them. Based on those patterns, we design Xscope, an
automatic tool to find security violations in cross-chain bridges and detect
real-world attacks. We evaluate Xscope on four popular cross-chain bridges. It
successfully detects all known attacks and finds suspicious attacks unreported
before. A video of Xscope is available at https://youtu.be/vMRO_qOqtXY.
