Thread by @BlockSecTeam

Exploit | Our monitoring system reported that Nomad Bridge @nomadxyz_ was attacked (etherscan.io/address/0xb923…), and the loss is around 150M USDT.

#crosschain #CryptoSecurity

2/ As a cross-bridge project, Nomad adopts a merkle-proof technology to verify that user requests are valid. By calling **process** function, a user can pass the request message to the contract.

3/ The verification procedure in the **process** function firstly finds the corresponding Merkle root by the hash of the message and the Merkle root is then passed into the **acceptableRoot** function to see if it is legal or not.

4/ Normally, the **acceptableRoot** function requires the Merkle root to be already proved and not processed. If the root is not of both cases, the function will try to find whether the root is confirmed at sometime before by searching **confirmAt** mapping.

5/ If the root is not confirmed, the corresponding **confirmAt** mapping slot should be zero. However, It is not. The confirmAt[0] is actually set to 1.

6/ Since an uninitialized storage slot is always considered as zero, the attacker can actually pass any message that has never shown before to bypass the verification procedure.