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Bridges hold enormous locked value, making them prime targets. Some of the largest crypto hacks in history were bridge exploits.

Understanding the common failure modes is essential for builders and users.

Federated and multisig bridges depend on private keys held by validators. If enough keys are stolen, attackers can forge transfers and mint unbacked tokens.

The Ronin bridge hack (over $600M) stemmed from compromised validator keys.

A frequent bug is failing to properly verify the proof or message of a transfer.

If the destination contract accepts a forged or replayed proof, an attacker can mint tokens that were never locked.

The Wormhole exploit (~$320M) came from a flaw that let an attacker spoof the guardian signature check.

Any weakness in how signatures or proofs are validated can be catastrophic.

A replay attack resubmits a valid message to claim funds multiple times.

Bridges must track processed message IDs (a nonce or hash) and reject duplicates.

If a bridge trusts events without verifying they came from the real source contract, attackers can emit fake deposit events.

The bridge then releases funds for deposits that never happened.

Many bridges are upgradeable via proxies. A compromised admin key can push a malicious upgrade that drains funds.

Timelocks and multisig admin controls reduce — but do not eliminate — this risk.

Reentrancy, integer issues, and logic errors plague bridge contracts just like any DeFi protocol.

Because bridges concentrate so much value, a single bug can be devastating.

If a bridge is exploited, the wrapped tokens it issued lose their backing and can crash to near zero.

Holders of bridged assets bear this risk even if they never interacted with the exploit directly.

To reduce bridge risk:

• Prefer trust-minimized (light client / ZK) designs
• Enforce strict proof verification and replay protection
• Add rate limits and circuit breakers
• Use timelocked, multisig-guarded upgrades and audits

Bridge exploits usually trace to compromised keys, weak validation, signature flaws, replay attacks, or fake events. The biggest hacks in crypto have been bridges.

Trust-minimized designs and rigorous verification are the best defenses. Next: messaging protocols.

You learned the common exploits:

• Compromised signer keys (Ronin)
• Signature/proof verification flaws (Wormhole)
• Replay attacks and fake events
• Upgradeable contract and general smart-contract bugs
• Mitigate with trust-minimized designs, replay protection, rate limits, audits

Next: messaging protocols.