Bridges Crumbling: Lock-and-Mint Mechanism Left Vulnerable to Attackers

Blockchains are isolated systems that maintain their own ledgers, consensus mechanisms, and finality rules. This isolation is a security feature, but it creates a practical problem for users who want to use assets on one chain with another.

Bridges exist to solve this issue by allowing users to deposit assets on one chain and receive corresponding assets on another. The core difficulty in bridge architecture is verification: someone or something must confirm that the deposit actually happened.

The lock-and-mint mechanism, used by most bridges, works as follows:

1. Lock: The user sends tokens to a smart contract on the source chain.

2. Verify: A set of validators, relayers, or an oracle observes the deposit and attests to its validity on the destination chain.

3. Mint: A smart contract on the destination chain mints synthetic tokens that represent a claim on the locked original assets.

The security of lock-and-mint bridges relies entirely on the verification step. If an attacker can convince the destination chain that a deposit occurred when it did not, they can mint unbacked tokens. This is exactly what happened in the largest bridge exploits.