SuperEx Educational Series: A Comprehensive Breakdown of Cross-Chain Bridge Security Black Holes, Attack Methods, and Industry Responses

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Today, we continue with the 《Blockchain Security Risks — Hacker Series》 educational content. We’ve already gone through two lessons in this series, and I believe that anyone who has carefully read our content has already developed a very intuitive understanding of the dangers posed by blockchain security vulnerabilities.

However, I also believe everyone has a question: the previously discussed “Sybil Attacks,” “51% Attacks,” and “Eclipse Attacks” do not seem very common. Some users even left messages asking us to explain hacker attack methods that have become popular in the past two years. Faced with this request, of course we will satisfy everyone’s curiosity.

So, the theme of today’s lesson is: “The Security Black Holes and Attack Types of Cross-Chain Bridges.” If you carefully review the hacker incidents over the past two years, you will find that whenever the topic shifts to “the infrastructure hackers love to attack the most,” cross-chain bridges are always at the top of the list.

Cross-chain bridges carry asset flows between chains and are known as the “highways of the crypto world,” but for the same reason, they have become the largest “cash machine” for hackers. According to Chainalysis data, cross-chain bridges alone caused more than $2 billion in losses in 2024.

• Why are cross-chain bridges so dangerous？
• How exactly do hackers attack them?
• Which attacks can be prevented?
• Do cross-chain bridges still have a future?

This SuperEx educational series will give you a complete understanding of the full picture of cross-chain bridge attacks, major attack categories, real attack logic, core risks, industry solutions, and—most importantly—the most practical safety guide for everyday investors.

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Why Are Cross-Chain Bridges the Biggest Security Black Hole in Web3?

Cross-chain bridges are used for “transferring assets,” such as sending your USDT from BNB Chain to Solana, or locking ETH on L1 and minting an equivalent asset on L2.

Essentially, a cross-chain bridge must solve two things at the same time:
– Verify whether a state on one chain (such as locking assets) truly happened;
– Safely mint or release assets on another chain.

It sounds simple, but the problems are huge:

1. Multi-chain heterogeneity — not all chains speak the same language

Different blockchains have different consensus mechanisms, virtual machines, and data structures. For a bridge to “understand” different chains, extremely complex verification logic is required, and vulnerabilities grow explosively.

2. Asset centralization — a natural giant vault

Cross-chain bridges usually store large amounts of locked assets. If something goes wrong, it’s not hundreds of thousands—it’s tens of millions or even hundreds of millions of dollars. The cost of attacking is low, the reward is huge, so hackers swarm toward them.

3. Multi-layer architecture = multi-point failure

A cross-chain bridge typically consists of several layers:
– The contract layer
– Relayers/validators
– Off-chain guardians
– Multisig systems
– Asset minting mechanisms

If any layer has a vulnerability, the entire system may collapse.

4. Audits cannot cover all cross-chain combinations

Single-chain code can be audited, but a cross-chain bridge is the combination of multiple chains, and the cost and complexity of auditing expand exponentially.

When the attack surface expands from “one blockchain system” to “multiple blockchains + relayer networks + multisig + cross-chain protocols,” the risk naturally escalates exponentially.

Therefore, hackers’ attitude toward cross-chain bridges is simple: because there’s a lot of money, because they’re easy to attack, they must be attacked.

Overview of Common Cross-Chain Bridge Attack Types

Cross-chain bridge attacks are extremely diverse, but can generally be classified into four categories:

1. Validator/Multisig Compromise Attacks

Examples:
– Private key leakage
– Multisig being hacked
– Signature logic flaws
– Validator nodes being forked or deceived

Typical case: Ronin Bridge (600 million USD): attackers stole 5 out of 9 multisig keys and directly forged withdrawals.

2. Smart-contract vulnerability attacks

Cross-chain bridge contracts are far more complex than regular DeFi contracts, and logic errors are common, such as:
– Insufficient validation
– Hash replay attacks
– Event forgery
– Reentrancy
– Flaws in proof verification logic

Typical case: Wormhole (320 million USD): due to missing checks in the verification function, hackers forged a “fake proof” and tricked the entire bridge.

3. Consensus-layer attacks (against source or target chain)

Consensus-layer attacks include:
– 51% attacks
– Chain reorg attacks
– Eclipse attacks
– Submission of malicious blocks

As we can see, 51% attacks and eclipse attacks are also types of cross-chain bridge attacks.

Cross-chain bridges rely heavily on the finality of blockchains, and attacking a chain can cause “confirmed events” to be rolled back, leading to cross-chain double-spends.

4. Cross-chain message forgery or tampering

Once a relayer or oracle is compromised, attackers can send fake “locked assets” messages to another chain, allowing them to mint assets without authorization.

Eight Major Cross-Chain Bridge Attack Types — In-Depth Analysis

Attack Type 1: Multisig Private Key Theft / Threshold Signature Compromise

Attack logic: most bridges use multisig to approve cross-chain messages.Example: “Lock 100 ETH → Mint 100 wrapped ETH” As long as the majority of signatures are obtained, the message is considered valid. A hacker only needs to obtain enough private keys to:Forge lock messages → Unauthorized minting → Withdraw all assets.

Real case: Ronin Bridge (600 million USD)

Attackers successfully controlled 5 of the 9 Sky Mavis keys and initiated fake withdrawals.

Attack Type 2: Smart Contract Verification Vulnerabilities

Attack logic: bridges often rely on “proof systems” to verify whether an event occurred on another chain.

As long as the verification logic has flaws, hackers can forge event proofs.

Classic case: Wormhole, 320 million USD loss

Flaw: verification contract did not check signature integrity.

Hackers created a “forged message” and bypassed verification, minting 120k fake ETH on Solana.

Attack Type 3: Reentrancy Attacks

If a bridge has an “external call → state update order flaw,” hackers can repeatedly withdraw assets.

Attack Type 4: Oracle Manipulation / Price Manipulation

Some bridges rely on oracle prices for collateral or minting. If manipulated, attackers can mint excessive assets.

Attack Type 5: Hash Replay Attacks

Even if the proof is valid, replaying it on another chain can mint assets repeatedly.

Attack Type 6: Cross-Chain Message Forgery

If the relayer is compromised, it can send forged “locked assets” messages to the target chain.

Attack Type 7 & 8: 51% Attack & Eclipse Attack (Already Explained Before)

How the Industry Is Solving Cross-Chain Bridge Security Risks

The problem of cross-chain bridges is not a single vulnerability, but a systemic architectural flaw. Therefore, the industry has formed three major solution paths over the past three years:

Route 1: Cross-Chain Verification with Zero-Knowledge Proofs (Most Promising Future Solution)

Core concept: use cryptographic proofs verified by Chain B to prove Chain A’s state — no relayers, no multisig trust.

Simply put:
– No custodial assets
– No trusted parties
– No trust assumptions
– No possibility of forged messages

Route 2: Light-Client Cross-Chain — Naturally Secure but Expensive

Light clients verify another chain’s block headers, enabling chains to authenticate each other without relayers.

Cosmos IBC is considered the safest cross-chain system because it derives security directly from the chain’s underlying consensus.

Route 3: Multi-Validator Networks

Examples:
– Map Protocol
– LayerZero
– New Wormhole
– Axelar
– Chainlink CCIP

Large validator networks collectively verify messages → safer than multisig.

Route 4: Do Not Bridge Assets — Bridge Liquidity

For example, Thorchain, Connext, LiFi exchange assets through liquidity pools without cross-chain minting.

Cross-Chain Safety Guide for Regular Users

Rule 1: Never bridge large amounts of money.
Rule 2: Avoid new bridges, small bridges, unaudited bridges.
Rule 3: Choose bridges with backing and scale.
Rule 4: Prefer official bridges.
Rule 5: Move funds to a safe wallet immediately after bridging.
Rule 6: Never bridge tokens whose origins you don’t understand.
Rule 7: Monitor the bridge’s TVL and message delays.

Conclusion: The Risks, Future, and Bottom Lines Investors Must Remember

Cross-chain bridges are an inevitable component of a multi-chain world, but they are also Web3’s weakest link.

Whether permission management, key storage, verification logic, upgrade processes, or multisig validator operations — a flaw in any link can cause hundreds of millions in losses.

The industry is moving toward three trends:
– Zero-knowledge cross-chain
– Light-client cross-chain
– Large validator networks

In other words: cross-chain bridges will not disappear, but they will become far more secure than they are today.