Recent years have witnessed significant advancements in cross-chain
technology. However, the field faces two pressing challenges. On the one
hand, hacks on cross-chain bridges have led to monetary losses of around
3.1 billion USD, highlighting flaws in security models governing
interoperability mechanisms and the ineffectiveness of incident response
frameworks. On the other hand, users and bridge operators experience
restricted privacy, which broadens the potential attack surface.
In this paper, we present the most comprehensive study to date on the
security and privacy of blockchain interoperability. We employ a
systematic literature review, yielding a corpus of 212 relevant
documents, including 58 academic papers and 154 gray literature
documents, out of a pool of 531 results. We systematically categorize 57
interoperability solutions based on a novel security and privacy
taxonomy. Our dataset, comprising academic research, disclosures from
bug bounty programs, and audit reports, exposes 45 cross-chain
vulnerabilities, 25 theoretical attacks, and 93 mitigation strategies.
Leveraging this data, we analyze 14 notable bridge hacks accounting for
over 2.9 billion USD in losses, mapping them to the identified
vulnerabilities.
Our findings reveal that a substantial portion (65.8%) of stolen funds
originates from projects secured by intermediary permissioned networks
with unsecured cryptographic key operations. Privacy-wise, we
demonstrate that achieving unlinkability in cross-chain transactions is
contingent on the underlying ledgers providing some form of
confidentiality. Our study offers critical insights into the security
and privacy of cross-chain systems. We pinpoint promising future
research directions, underscoring the urgency of enhancing security and
privacy efforts in cross-chain technology. The identified improvements
can mitigate the financial risks associated with bridge hacks, fostering
user trust in the blockchain ecosystem and, consequently, wider
adoption.