Enhancing Security in Cross-Chain Bridges

Blockchain technology has become very popular, leading to the creation of many platforms and digital currencies. Some well-known examples include Ethereum, Binance Coin, Bitcoin, and Non-Fungible Tokens (NFTs). The growth of these various platforms has resulted in a complex system that requires effective communication between different blockchains, such as converting Ether to Bitcoin.

A cross-chain bridge serves as a link that allows information and digital assets to move between different blockchains. For instance, the Polygon network bridge allows users to easily transfer tokens between Polygon and Ethereum without needing to rely on untrusted third parties.

Despite having a significant market presence with billions of dollars in transactions, cross-chain bridges face security challenges. Research shows that in the last two years, these bridges have encountered more than 29 security issues. A major source of these problems stems from weaknesses in their smart contracts. A notable example is the PolyNetwork incident, where a security flaw resulted in a loss of 600 million USD.

Vulnerabilities that are specific to cross-chain bridge smart contracts are referred to as Cross-Chain Vulnerabilities (CCVs). These weaknesses are mainly due to the unique nature of asset exchanges that occur between different blockchains. For example, some cross-chain bridges may have insufficient security checks, which can lead to unauthorized access.

Although the seriousness of these vulnerabilities is acknowledged, there has been limited research aimed at analyzing or identifying these issues, making it difficult to secure cross-chain bridges effectively. Previous studies have explored cross-chain attacks and suggested some ways to protect these systems, but many of these suggestions are not directly applicable or enforceable.

#Our Study

In response to this gap in research, we developed a new static analysis framework called SmartAxe to identify CCVs in smart contracts associated with cross-chain bridges. SmartAxe is the first tool designed specifically for this purpose at the bytecode level, which means it can analyze the underlying code before the contracts are deployed. By doing this, we aim to enhance the security of cross-chain bridge applications and reduce the risk of financial losses.

The main causes of CCVs fall into two categories: 1) incomplete access control and 2) inconsistency in how cross-chain transactions are processed. Identifying these vulnerabilities through static analysis comes with its own set of challenges.

#Challenges in Detection

The first challenge is to accurately extract access control constraints, which include security checks that should be in place to protect sensitive resources. These security checks can vary widely among different bridge contracts, making it difficult to identify what is necessary. Additionally, linking these security checks to the relevant resources can be complex.

The second challenge involves analyzing the context of cross-chain communication to detect semantic inconsistencies. This requires looking at the flow of information and data between the different blockchains. Previous analysis tools have often overlooked these important details.

To overcome these challenges, SmartAxe incorporates two main design features:

1. It models the various ways access control is implemented across different cross-chain bridges, standardizing them into a single form.
2. It aligns the control and data flows between the source and destination chains, allowing it to build graphs that represent these flows.

By doing this, SmartAxe can effectively pinpoint vulnerable functions that contain incomplete access controls or semantic inconsistencies.

#Evaluation of SmartAxe

To test how well SmartAxe works, we created a dataset of 16 cross-chain bridge applications, which included 203 smart contracts known to have vulnerabilities. Our experiments showed that SmartAxe successfully identified vulnerabilities with a precision of 84.95% and a recall of 89.77%.

With the help of SmartAxe, we conducted a large-scale evaluation of 1,703 smart contracts from 129 real-world cross-chain bridge applications. This analysis revealed 232 new vulnerabilities that had not been identified in previous research, affecting assets totaling 1,885,250 USD.

#Understanding Cross-Chain Bridges

A cross-chain bridge serves as a link for exchanging digital assets across different blockchain platforms. Typically, a cross-chain bridge is made up of three components:

1. Source Chain: The blockchain where the asset originates.
2. Cross-Chain Relayer: An intermediary that facilitates communication between the two chains.
3. Destination Chain: The blockchain where the asset is ultimately transferred.

When a user wants to exchange one token for another, they initiate the transaction through the cross-chain bridge. This process generally involves three steps:

1. Asset Deposit on Source Chain: The user deposits their asset, which is locked in the source chain, and a confirmation event is emitted.
2. Cross-Chain Communication: The off-chain relayer verifies the deposit and communicates the necessary information to the destination chain.
3. Asset Withdrawal on Destination Chain: Once the deposit is confirmed, the user can withdraw the equivalent asset from the destination chain.

#Types of Vulnerabilities

Cross-chain vulnerabilities (CCVs) can significantly impact the security of these smart contracts. Some common types include:

1. Access Control Incompleteness: This occurs when important security checks are missing or not implemented properly. As a result, unauthorized users may gain access to critical functions.
2. Cross-Bridge Semantic Inconsistency: In this case, the logic and meaning of actions between the source and destination chains do not align properly, leading to potential errors or exploits.

#Previous Research and Limitations

Despite the clear risks presented by CCVs, research on identifying these vulnerabilities has been limited. While various security tools exist for analyzing smart contracts, they typically focus on single blockchain systems and do not consider cross-chain interactions.

Tools designed for static analysis can effectively find potential vulnerabilities, but they often lack the specific models needed to analyze cross-chain bridge contracts. As a result, many previously proposed methods do not address the unique challenges posed by cross-chain transactions.

The most relevant previous work has focused on detecting attacks in real-time, rather than analyzing the underlying contract code beforehand to prevent such attacks. Our approach with SmartAxe fills this gap by providing a dedicated tool for identifying vulnerabilities before they can be exploited.

#How SmartAxe Works

SmartAxe is designed to conduct thorough static analysis of smart contracts across different blockchains. Here is a simplified overview of how it functions:

1. Basic Control Flow Analysis: SmartAxe builds a detailed view of how the smart contracts operate, identifying the flow of control from one function to another.
2. Access Control Completeness Checking: It examines the extracted access control constraints and compares them with established models to find any access control omissions.
3. Cross-Bridge Semantic Inconsistency Finding: SmartAxe checks for consistency in the way transactions are processed across both chains, ensuring that the logic remains intact.
4. Vulnerability Trace Discovery: Lastly, it analyzes how vulnerabilities can be exploited and identifies the paths that could lead to unauthorized access or errors.

#Accuracy and Performance

SmartAxe has demonstrated strong performance in its evaluations. It successfully identified a significant number of vulnerabilities with a high level of accuracy, showcasing its utility in enhancing the security of cross-chain bridges.

#Real-World Implications

The findings from our research have important implications for developers and users of cross-chain bridges. With the rise of decentralized finance (DeFi) and various blockchain applications, ensuring the security of cross-chain interactions is crucial.

#Case Studies

To illustrate the effectiveness of SmartAxe, we can look at two case studies:

1. Case Study 1: A contract on a popular cross-chain bridge was found to have incomplete access control. The authorization and withdrawal functions did not correctly validate the type of tokens being used, creating potential loopholes that could be exploited.

2. Case Study 2: Another contract revealed an oversight in validating user inputs for liquidity deposits. Without proper checks in place, an attacker could submit false information to claim false deposits.

Both cases highlight how critical it is to implement strict security measures in smart contracts and how tools like SmartAxe can help identify these vulnerabilities.

#Conclusion

The rise of blockchain technology and the complexity of cross-chain transactions introduce a variety of security challenges. Cross-chain bridges serve a crucial role in facilitating transactions but are also susceptible to vulnerabilities that can have serious financial consequences.

Our research highlights the need for robust tools such as SmartAxe to identify and address these vulnerabilities before they can be exploited. By employing a systematic approach to analyzing smart contracts, SmartAxe provides a valuable solution for improving the security of cross-chain bridges and safeguarding the assets of users.

With the blockchain ecosystem continuing to grow and evolve, ongoing research and development are essential to ensure that these systems are secure and trustworthy. By investing in tools and frameworks that promote security and transparency, we can help pave the way for a safer and more reliable future in blockchain technology.