Biconomy: Nexus

Summary

_getEnableModeDataHash() doesn't include a nonce, thereby allowing enable mode signatures to be replayed.

Vulnerability Details

When Nexus account owners send a transaction with enable mode in PackedUserOperation.nonce, validateUserOp() calls _enableMode() to install the validator as a new module.

Nexus.sol#L108-L109

To ensure that the account owner has allowed the validator to be installed, the validator (ie. module shown below) is hashed alongside its data (ie. moduleInitData) in _getEnableModeDataHash(), and subsequently checked to be signed by the owner in enableModeSignature in _checkEnableModeSignature():

ModuleManager.sol#L166-L171

However, the hash returned by _getEnableModeDataHash() does not include a nonce:

ModuleManager.sol#L388-L398

This allows the owner's signature to be used repeatedly.

As a result, if a validator that was previously installed through _enableMode() is uninstalled by the owner, a malicious relayer/bundler can re-use the previous signature to re-install it through validatorUserOp() again, despite not having the owner's permission.

Impact

Due to signature replay, validators that have been uninstalled by Nexus account owners can be re-installed without their permission.

This is especially problematic as validators are used by Nexus accounts for access control - being able to re-install a validator without the owner's permission might affect the Nexus account's permissions and allow attackers to execute transactions on behalf of the account.

Recommendations

Include a nonce in _getEnableModeDataHash() to ensure that enable mode signatures cannot be replayed.