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Connext A-5

Security Audit

February 22, 2024

Version 1.0.0

Presented by 0xMacro

Table of Contents

Introduction

This document includes the results of the security audit for Connext's smart contract code as found in the section titled ‘Source Code’. The security audit was performed by the Macro security team on February 22, 2024.

The purpose of this audit is to review the source code of certain Connext Solidity contracts, and provide feedback on the design, architecture, and quality of the source code with an emphasis on validating the correctness and security of the software in its entirety.

Disclaimer: While Macro’s review is comprehensive and has surfaced some changes that should be made to the source code, this audit should not solely be relied upon for security, as no single audit is guaranteed to catch all possible bugs.

Overall Assessment

The following is an aggregation of issues found by the Macro Audit team:

Severity Count Acknowledged Won't Do Addressed
Low 1 - - 1
Code Quality 2 - 2 -

Connext was quick to respond to these issues.

Specification

Our understanding of the specification was based on the following sources:

Source Code

The following source code was reviewed during the audit:

Specifically, we audited the following contracts within this repository, in the `packages/deployments/contracts/contracts/messaging/` directory:

Contract SHA256
packages/deployments/contracts/contracts/messaging/connectors/ConnectorsLib.sol

a361c5a7e92990dede4bffa3661412f006117d0eda792f4ff224949c4ba04b73

packages/deployments/contracts/contracts/messaging/connectors/scroll/BaseScroll.sol

9fc2d494c996e1bac7dc5feb1342f91a76c940e55662c4d8cf11676c29f7e673

packages/deployments/contracts/contracts/messaging/connectors/scroll/ScrollHubConnector.sol

4453f048ffac4b44c35c1a92c944a6f9748b0128a75ec8bccb80fe43a7f526dc

packages/deployments/contracts/contracts/messaging/connectors/scroll/ScrollSpokeConnector.sol

5118ef237666be28f6dc5d582a25503726db4aa0a424370cbae12e45464fe5c9

Issue Descriptions and Recommendations

Click on an issue to jump to it, or scroll down to see them all.

Security Level Reference

We quantify issues in three parts:

  1. The high/medium/low/spec-breaking impact of the issue:
    • How bad things can get (for a vulnerability)
    • The significance of an improvement (for a code quality issue)
    • The amount of gas saved (for a gas optimization)
  2. The high/medium/low likelihood of the issue:
    • How likely is the issue to occur (for a vulnerability)
  3. The overall critical/high/medium/low severity of the issue.

This third part – the severity level – is a summary of how much consideration the client should give to fixing the issue. We assign severity according to the table of guidelines below:

Severity Description
(C-x)
Critical

We recommend the client must fix the issue, no matter what, because not fixing would mean significant funds/assets WILL be lost.

(H-x)
High

We recommend the client must address the issue, no matter what, because not fixing would be very bad, or some funds/assets will be lost, or the code’s behavior is against the provided spec.

(M-x)
Medium

We recommend the client to seriously consider fixing the issue, as the implications of not fixing the issue are severe enough to impact the project significantly, albiet not in an existential manner.

(L-x)
Low

The risk is small, unlikely, or may not relevant to the project in a meaningful way.

Whether or not the project wants to develop a fix is up to the goals and needs of the project.

(Q-x)
Code Quality

The issue identified does not pose any obvious risk, but fixing could improve overall code quality, on-chain composability, developer ergonomics, or even certain aspects of protocol design.

(I-x)
Informational

Warnings and things to keep in mind when operating the protocol. No immediate action required.

(G-x)
Gas Optimizations

The presented optimization suggestion would save an amount of gas significant enough, in our opinion, to be worth the development cost of implementing it.

Issue Details

L-1

Missing refundAddress may result in lost assets

Topic
Input validation
Status
Impact
Low
Likelihood
Medium

In the ScrollHubConnector contract, the _sendMessage() implementation tries to decode refundAddress parameter value from the _encodedData input argument. If missing then refundAddress is set to address(0).

function _sendMessage(bytes memory _data, bytes memory _encodedData) internal override checkMessageLength(_data) {
  address _refundAddress = (_encodedData.length > 0) ? abi.decode(_encodedData, (address)) : address(0);
  bytes memory _calldata = abi.encodeWithSelector(Connector.processMessage.selector, _data);
  L1_SCROLL_MESSENGER.sendMessage{value: msg.value}(
    mirrorConnector,
    ZERO_MSG_VALUE,
    _calldata,
    gasCap,
    _refundAddress
  );
}

Scroll’s L1Messenger contract would use _refundAddress value to return with surplus msg.value. If this _refundAddress is address(0), those surplus native assets will be forever lost.

Remediations to consider

  • Revert if the value for refundAddress is not provided within _encodedData input argument, or
  • refundAddress should be initialized with an address under control when it is not provided so that assets can be retrieved later.
Q-1

Unnecessary storage read in ScrollSpokeConnector

Topic
Unnecessary code
Status
Wont Do
Quality Impact
Low

In the ScrollSpokeConnector, _sendMessage() function implementation when invoking sendMessage() on L2_SCROLL_MESSENGER provides and references gasCap storage variable from the parent GasCap contract.

function _sendMessage(bytes memory _data, bytes memory) internal override checkMessageLength(_data) {
  bytes memory _calldata = abi.encodeWithSelector(Connector.processMessage.selector, _data);
  L2_SCROLL_MESSENGER.sendMessage(mirrorConnector, ZERO_MSG_VALUE, _calldata, **gasCap**);
}

However, gasCap value is not used or relied upon in the L2_SCROLL_MESSENGER.sendMessage().

Therefore, consider providing a constant value of 0 instead of gasCap.

Q-2

Documentation improvements

Topic
Documentation
Status
Wont Do
Quality Impact
Low
  1. Notice natspec comment for ScrollHubConnector should refer to L2 Scroll Spoke Connector instead of L2 Scroll Hub Connector.

    Scroll Hub Connector contract in charge of sending messages to the L2 Scroll ~~Hub~~Spoke Connector through the
    L1 Scroll Messenger, and receiving messages from the L2 Scroll ~~Hub~~Spoke Connector through the L1 Scroll Messenger
    
  2. Notice and dev natspec comments for IL1ScrollMessenger.relayMessageWithProof() are incorrect.

    * @notice Execute L1 => L2 message. // replace with: Relay a L2 => L1 message with message proof.
    * @dev Make sure this is only called by privileged accounts. // there are no requirements for callers to be privileged
    

Disclaimer

Macro makes no warranties, either express, implied, statutory, or otherwise, with respect to the services or deliverables provided in this report, and Macro specifically disclaims all implied warranties of merchantability, fitness for a particular purpose, noninfringement and those arising from a course of dealing, usage or trade with respect thereto, and all such warranties are hereby excluded to the fullest extent permitted by law.

Macro will not be liable for any lost profits, business, contracts, revenue, goodwill, production, anticipated savings, loss of data, or costs of procurement of substitute goods or services or for any claim or demand by any other party. In no event will Macro be liable for consequential, incidental, special, indirect, or exemplary damages arising out of this agreement or any work statement, however caused and (to the fullest extent permitted by law) under any theory of liability (including negligence), even if Macro has been advised of the possibility of such damages.

The scope of this report and review is limited to a review of only the code presented by the Connext team and only the source code Macro notes as being within the scope of Macro’s review within this report. This report does not include an audit of the deployment scripts used to deploy the Solidity contracts in the repository corresponding to this audit. Specifically, for the avoidance of doubt, this report does not constitute investment advice, is not intended to be relied upon as investment advice, is not an endorsement of this project or team, and it is not a guarantee as to the absolute security of the project. In this report you may through hypertext or other computer links, gain access to websites operated by persons other than Macro. Such hyperlinks are provided for your reference and convenience only, and are the exclusive responsibility of such websites’ owners. You agree that Macro is not responsible for the content or operation of such websites, and that Macro shall have no liability to your or any other person or entity for the use of third party websites. Macro assumes no responsibility for the use of third party software and shall have no liability whatsoever to any person or entity for the accuracy or completeness of any outcome generated by such software.