1 Executive Summary
This report presents the results of our engagement with Linea to review Linea Cross-Chain Governance Executor.
The review was conducted over half a week, from October 9, 2023 to October 11, 2023, by George Kobakhidze and Rai Yang. A total of 5 person-days were spent.
Note: The commit that includes the audit remediation is 315308a2640c696937185732159b130417f29997, and the
LineaBridgeExecutor.solfile’s updated hash is
The focus was on a single contract file
LineaBridgeExecutor and an interface
IMessageService. The contract is built upon Aave’s cross-chain governance executor set of contracts that enable DAOs and other entities to take control of contracts from their native chain on destination chains, in this case the Linea network.
While the contract’s immediate use may be specifically for Lido’s governance to control the
wstETH token on Linea, this contract may be used for managing any contract with cross-chain governance.
Together with the Linea team, we identified the following priorities for our review:
- Correctness of the implementation, consistent with the intended functionality and without unintended edge cases.
- Identify known vulnerabilities particular to smart contract systems, as outlined in our Smart Contract Best Practices, and the Smart Contract Weakness Classification Registry.
3 Document Change Log
|1.1||2023-11-28||Added audit remediation commit and associated updated file hash in the scope section|
4.1 Adjust NatSpec Author ✓ Fixed
The NatSpec provided for the file has a minor inconsistency with the actual properties. The
author tag is assigned to Aave whereas this contract was written by the Linea team, though of course built on by work made by the Aave team.
* @author Aave
Adjust the NatSpec as appropriate.
5 System Overview
This system allows entities, such as DAOs, from Ethereum (or other source chains) to control contracts on destination chains using a bridge message service (like the Linea message service) and a bridge executor contract that can execute transactions on behalf of those entities. Typically, the bridge executor contract is given special permissions on target contracts, such as the
Owner role, so when bridge tasks are executed - they are executed with these permissions. In effect, the bridge executor contract acts as a cross-chain governance proxy that allows for use cases such as managing bridged token contracts and multi-chain protocol deployments. In this system, the entity that queues up and executes the tasks on the bridge executor is known as the Ethereum Governance Executor.
More technically, the
LineaBridgeExecutor is a contract that inherits the majority of its functionality from the
The functionality contained in
LineaBridgeExecutor is concerned with establishing correct access control for the rest of the core logic. Specifically, this contract defines the relevant message service address for the destination chain -
LINEA_MESSAGE_SERVICE in our case – and the access control modifier for the function that queues up tasks for the executor –
LINEA_MESSAGE_SERVICE address variable is set directly in the constructor as an immutable contract variable, a common implementation choice for such bridge executors.
onlyEthereumGovernanceExecutor modifier contains access control logic. This logic can vary based on the parameters of the destination chain. In our case, the Linea network’s message service allows contracts to query the original sender of the transaction that is being claimed on Linea, so the modifier makes use of that and ensures that any transactions not sent by the Linea message service (
msg.sender != LINEA_MESSAGE_SERVICE) or transactions not initiated by the Ethereum Governance Executor on the source chain (
IMessageService(LINEA_MESSAGE_SERVICE).sender() != _ethereumGovernanceExecutor) are reverted.
L2BridgeExecutor contract defines the access control logic around the aforementioned Ethereum Governance Executor, such as wrapping its ownership around the
queue() function and managing changes to that address. Finally, the
BridgeExecutorBase contract defines all the core logic around queuing and actually executing the transactions.
6 Security Specification
This section describes, from a security perspective, the expected behavior of the system under audit. It is not a substitute for documentation. The purpose of this section is to identify specific security properties that were validated by the audit team.
The relevant actors are listed below with their respective abilities:
- Bridge executor deployer team. Deploys and configures the
- The target contract deployer team. Deploys and configures the target contract to be managed on the destination chain (Linea).
- Ethereum Governance Executor, the contract on Ethereum that has permission to execute relevant governance tasks on Linea.
6.2 Trust Model
In any system, it’s important to identify what trust is expected/required between various actors. For this audit, we established the following trust model:
- The bridge executor deployer team is trusted to deploy the contracts with correct parameters to ensure correct functionality such as the address of the relevant managing contract on Ethereum (Ethereum Governance Executor) and the address of the Linea Message Service, which is an immutable contract variable.
- The target contract team correctly transfers the contract ownership on Linea (for example -
wstETHtoken contract ownership from Linea’s security council) to
- The Ethereum Governance Executor manages contracts on Linea owned by the
LineaBridgeExecutorcontract from Ethereum. Therefore, users should also treat (and therefore trust) the Ethereum Governance Executor as the owner of those contracts.
Appendix 1 - Files in Scope
This audit covered the following files:
Appendix 2 - Disclosure
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