Why do cross-chain bridges get hacked?
Bridges hold large pooled balances and depend on proving to one chain that something happened on another. That proof is the weak point. Bridges have been drained through compromised validator keys, forged or replayed messages, flawed verification logic and bad initialisation, and two of the largest hacks in the year to mid-2023 hit bridges, accounting for $912 million.
How bridges fail
- Validator or signer key compromise. If a threshold of keys approves withdrawals, stealing those keys is enough. The Ronin Bridge theft in 2022 worked this way.
- Forged messages. Verification that can be bypassed, for example because a default or zero value passes as valid.
- Replayed messages. The same deposit or withdrawal processed more than once.
- Accounting errors. Minted tokens not fully backed by locked ones.
- Upgrade and admin control. A single key that can change bridge logic.
Securing a bridge
- Spread trust across independent signers on separate infrastructure, with monitored, rate-limited withdrawals.
- Verify every message for source, uniqueness and integrity.
- Make pausing fast and permission it carefully.
- Audit the contracts on every chain, and test the off-chain components. See bridge audits.
Frequently asked questions
Are some bridge designs safer than others?
Bridges that verify the other chain's consensus on-chain depend less on a small set of trusted signers than multisig bridges do, but they bring complex verification code that must be right. Every design has a trust assumption, and security depends on protecting it.
Who attacks bridges?
Everyone, but nation-state groups have been prominent. The Ronin Bridge theft in 2022 was attributed to Lazarus Group.
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