A fragile solution: cross-chain bridge vulnerabilities

In this post, we’re exploring the vulnerabilities inherent in cross-chain bridges.

Cross-chain bridges have become essential to the decentralized finance (DeFi) ecosystem, addressing the long-standing issue of interoperability between disparate blockchain networks. However, their growing prominence has also exposed critical vulnerabilities, making them attractive targets for malicious actors.

A need for reliable cross-chain solutions

In the current crypto ecosystem, blockchains remain largely isolated from one another, making it difficult to transfer assets seamlessly between chains. The most common solution to this problem has been cross-chain bridges, which facilitate the movement of assets and data across different blockchains.

By locking assets on one chain and minting equivalent tokens on another, bridges enable users to interact with multiple ecosystems. This interoperability is especially critical for the growth and integration of decentralized applications (dApps) across diverse platforms.

Bridge exploits result in heavy losses

However, despite their utility, cross-chain bridges have proven to be vulnerable to hacks. To date, cross-chain bridges have been hacked for more than $2.8 bln - representing almost 40% of the entire value hacked in Web3, according to DefiLlama. These are the most prominent bridge hacks that occurred in recent years.

Multichain bridge exploit

In July 2023, Multichain, a cross-chain router protocol, experienced unauthorized withdrawals totaling nearly $130 mln across its Fantom, Moonriver and Dogechain bridges. The team advised users to suspend the use of Multichain services and revoke all contract approvals related to the protocol. 

HECO Chain bridge exploit

In November 2023, the HECO Chain bridge, linked to the HTX exchange, was exploited, resulting in the transfer of over $86.6 mln in digital assets to suspicious addresses. Assets such as stablecoins, ETH, SHIB and LINK were affected. HTX announced plans to fully compensate users for any losses incurred. 

Force Bridge exploit

In June 2025, Force Bridge, a cross-chain protocol on the Nervos Network, was targeted in an exploit leading to the loss of over $3 mln in crypto assets. The attackers siphoned various tokens, including USDT, ETH, USDC, DAI and WBTC, converting them to ETH and funneling the funds through Tornado Cash. The Nervos Network's contributor, Magickbase, paused the bridge service and initiated an investigation.

These incidents underscore major vulnerabilities in cross-chain bridge design and implementation.

Key vulnerabilities in cross-chain bridges

Centralized control points

Many cross-chain bridges are operated using a small group of validators or a multisignature wallet managed by a limited number of parties. While this may simplify development and coordination, it creates a single point of failure. If a majority of these validators are compromised—through phishing, malware, social engineering, or insider collusion—attackers can effectively seize control of the bridge. This was the case in the Ronin Bridge hack, where attackers gained access to five of nine validator keys, resulting in the theft of over $600 million.The danger lies not in just a coding error, but in trusting too few entities to secure a critical system.

Smart contract flaws

Smart contracts form the core logic of cross-chain bridges—responsible for locking, minting, and releasing assets across networks. However, even small bugs or overlooked conditions in the code can be catastrophic.In the Wormhole and Nomad bridge attacks, flaws in the contract logic were exploited to bypass verification mechanisms and mint or withdraw assets without collateral. Such flaws may stem from incorrect initialization, missing signature checks, or flawed assumptions about external data.The challenge is that smart contracts, once deployed, are difficult to patch without interrupting operations. Without thorough auditing and formal verification, bridges can become low-hanging fruit for attackers.

Inadequate key management

Even if bridge logic is sound, poor operational security around private keys can render it moot. Key management vulnerabilities include:

• Storing keys on insecure servers
• Using hot wallets instead of secure hardware modules (HSMs)
• Weak backup or recovery procedures
• Lack of role separation and access control

If a validator’s key is stolen, an attacker may not need to exploit any contract at all—they can simply sign valid transactions. This risk is magnified in bridges where only a few keys are needed to approve transfers, as in multisig configurations.

Lack of real-time monitoring

Many bridges are deployed without active monitoring systems that can detect anomalies or halt operations during suspicious activity. Without real-time visibility into token flows and bridge behaviors, attackers can drain funds rapidly before teams can react.A delay of even a few minutes between an exploit and its detection can result in losses of millions of dollars. Some newer designs now incorporate automated circuit breakers, alert systems, or withdrawal limits to mitigate this risk.However, many older or minimally funded bridges remain passive—making them vulnerable to rapid and silent exploitation.

Absence of rate limiting

Rate limiting is a simple yet underused defense mechanism in bridge design. By capping the maximum volume of assets transferable in a given time window (e.g., per hour or per transaction), a protocol can mitigate the damage of a breach.Without such limits, attackers can drain entire treasuries in one transaction, as seen in multiple bridge exploits. Rate limits, combined with anomaly detection, can serve as a last line of defense—buying precious time for teams to respond.Unfortunately, many bridges prioritize speed and liquidity over safety, choosing no-limit architectures that leave them wide open in the event of a compromise.

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