  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.
