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Smart Contract Assessment - Matter Labs (ZKSync)

Prepared by:

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HALBORN

Last Updated 10/20/2025

Date of Engagement: January 9th, 2023 - March 8th, 2023

Summary

100% of all REPORTED Findings have been addressed

All findings

5

Critical

1

High

0

Medium

0

Low

1

Informational

3

Table of Contents

1. INTRODUCTION

MatterLabs zkSync Era is a Layer 2 blockchain protocol that eliminates Ethereum’s inherent congestion with zero knowledge proofs. Matter Labs' creation is on a mission to accelerate the mass adoption of crypto for personal sovereignty. It is designed to unlock the full potential of trustless blockchain technology while scaling the core values of Ethereum.

MatterLabs engaged Halborn to conduct a security audit on their zero knowledge circuits and the verifier, beginning on 2023-01-09 and ending on 2023-03-08. The security assessment was scoped to the circuits provided to the Halborn team.

2. AUDIT SUMMARY

The team at Halborn was provided two months for the engagement and assigned a full-time security engineer to audit the security of the zero knowledge circuits and the verifier. The security engineer is a blockchain, smart-contract and ZK security expert with advanced penetration testing, smart-contract hacking, and deep knowledge of multiple blockchain protocols.

The purpose of this audit to achieve the following:

    • Ensure that the circuits operate as intended.

    • Identify potential security issues within the circuits.

    • Ensure that the verifier operate as intended.

    • Identify potential security issues within the verifier contracts.

In summary, Halborn identified some security risks that were mostly addressed by the MatterLabs team.

3. TEST APPROACH & METHODOLOGY

Halborn performed a combination of manual and automated security testing to balance efficiency, timeliness, practicality, and accuracy in regard to the scope of this audit. While manual testing is recommended to uncover flaws in logic, process, and implementation; automated testing techniques help enhance coverage of the bridge code and can quickly identify items that do not follow security best practices. The following phases and associated tools were used throughout the term of the audit:

    • Research into architecture and purpose.

    • Smart contract manual code review and walkthrough.

    • Circuit manual code review and walkthrough.

    • Graphing out functionality and circuit logic/connectivity/functions. (cargo-deps)

    • Manual code review of common Rust security vulnerabilities.

    • Manual code review of specific zero knowledge security vulnerabilities.

    • Scanning of circuits files for unsafe Rust code usage. (cargo-geiger)

    • Static Analysis of security for scoped circuits, and imported functions. (cargo-audit)

4. SCOPE

1. IN-SCOPE:

The security assessment was scoped to the following zero knowledge circuits:

    • /src/vm/*

    • /src/glue/sort_decommitments_requests/*

    • /src/glue/code_unpacker_sha256/*

    • /src/glue/demux_log_queue/*

    • /src/precompiles/keccak256.rs

    • /src/precompiles/sha256.rs

    • /src/glue/ecrecover_circuit/*

    • /src/glue/ram_permutation/*

    • /src/glue/storage_validity_by_grand_product/*

    • /src/glue/storage_application/*

    • /src/glue/pub_data_hasher/*

    • /src/glue/log_sorter/*

    • /src/glue/merkleize_l1_messages/*

    • /src/scheduler/*

    • /src/circuit_structures/*

    • /src/data_structures/*

    • /src/inputs/*

    • /src/recursion/*

    • /src/traits/*

    • /src/secp256k1/*

    • /src/utils.rs

Commit ID : 014e674916058e31725d6e92439fa5ff14e6677e

And the verifier:

    • /zksync/Verifier.sol

    • /zksync/Plonk4VerifierWithAccessToDNext.sol

    • /zksync/libraries/PairingsBn254.sol

    • /zksync/libraries/TranscriptLib.sol

Commit ID : fc7e86a3df404acb88d86502c944c0630a7ed288

2. REMEDIATION PR/COMMITS:

5. RISK METHODOLOGY

Every vulnerability and issue observed by Halborn is ranked based on two sets of Metrics and a Severity Coefficient. This system is inspired by the industry standard Common Vulnerability Scoring System.
The two Metric sets are: Exploitability and Impact. Exploitability captures the ease and technical means by which vulnerabilities can be exploited and Impact describes the consequences of a successful exploit.
The Severity Coefficients is designed to further refine the accuracy of the ranking with two factors: Reversibility and Scope. These capture the impact of the vulnerability on the environment as well as the number of users and smart contracts affected.
The final score is a value between 0-10 rounded up to 1 decimal place and 10 corresponding to the highest security risk. This provides an objective and accurate rating of the severity of security vulnerabilities in smart contracts.
The system is designed to assist in identifying and prioritizing vulnerabilities based on their level of risk to address the most critical issues in a timely manner.

5.1 EXPLOITABILITY

Attack Origin (AO):
Captures whether the attack requires compromising a specific account.
Attack Cost (AC):
Captures the cost of exploiting the vulnerability incurred by the attacker relative to sending a single transaction on the relevant blockchain. Includes but is not limited to financial and computational cost.
Attack Complexity (AX):
Describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Includes but is not limited to macro situation, available third-party liquidity and regulatory challenges.
Metrics:
EXPLOITABILITY METRIC (mem_e)METRIC VALUENUMERICAL VALUE
Attack Origin (AO)Arbitrary (AO:A)
Specific (AO:S)		1
0.2
Attack Cost (AC)Low (AC:L)
Medium (AC:M)
High (AC:H)		1
0.67
0.33
Attack Complexity (AX)Low (AX:L)
Medium (AX:M)
High (AX:H)		1
0.67
0.33
Exploitability EE is calculated using the following formula:

E=∏meE = \prod m_e

5.2 IMPACT

Confidentiality (C):
Measures the impact to the confidentiality of the information resources managed by the contract due to a successfully exploited vulnerability. Confidentiality refers to limiting access to authorized users only.
Integrity (I):
Measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of data stored and/or processed on-chain. Integrity impact directly affecting Deposit or Yield records is excluded.
Availability (A):
Measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. This metric refers to smart contract features and functionality, not state. Availability impact directly affecting Deposit or Yield is excluded.
Deposit (D):
Measures the impact to the deposits made to the contract by either users or owners.
Yield (Y):
Measures the impact to the yield generated by the contract for either users or owners.
Metrics:
IMPACT METRIC (mIm_I)METRIC VALUENUMERICAL VALUE
Confidentiality (C)None (C:N)
Low (C:L)
Medium (C:M)
High (C:H)
Critical (C:C)		0
0.25
0.5
0.75
1
Integrity (I)None (I:N)
Low (I:L)
Medium (I:M)
High (I:H)
Critical (I:C)		0
0.25
0.5
0.75
1
Availability (A)None (A:N)
Low (A:L)
Medium (A:M)
High (A:H)
Critical (A:C)		0
0.25
0.5
0.75
1
Deposit (D)None (D:N)
Low (D:L)
Medium (D:M)
High (D:H)
Critical (D:C)		0
0.25
0.5
0.75
1
Yield (Y)None (Y:N)
Low (Y:L)
Medium (Y:M)
High (Y:H)
Critical (Y:C)		0
0.25
0.5
0.75
1
Impact II is calculated using the following formula:

I=max(mI)+∑mI−max(mI)4I = max(m_I) + \frac{\sum{m_I} - max(m_I)}{4}

5.3 SEVERITY COEFFICIENT

Reversibility (R):
Describes the share of the exploited vulnerability effects that can be reversed. For upgradeable contracts, assume the contract private key is available.
Scope (S):
Captures whether a vulnerability in one vulnerable contract impacts resources in other contracts.
Metrics:
SEVERITY COEFFICIENT (CC)COEFFICIENT VALUENUMERICAL VALUE
Reversibility (rr)None (R:N)
Partial (R:P)
Full (R:F)		1
0.5
0.25
Scope (ss)Changed (S:C)
Unchanged (S:U)		1.25
1
Severity Coefficient CC is obtained by the following product:

C=rsC = rs

The Vulnerability Severity Score SS is obtained by:

S=min(10,EIC∗10)S = min(10, EIC * 10)

The score is rounded up to 1 decimal places.
SeverityScore Value Range
Critical9 - 10
High7 - 8.9
Medium4.5 - 6.9
Low2 - 4.4
Informational0 - 1.9

6. SCOPE

Out-of-Scope: New features/implementations after the remediation commit IDs.

7. Assessment Summary & Findings Overview

Critical

1

High

0

Medium

0

Low

1

Informational

3

Security analysisRisk levelRemediation Date
CIRCUIT NOT PROPERLY WORKING WHEN USING SHARD ID > 0CriticalSolved - 04/03/2023
HEAD STATE NOT ENFORCED TO BE ZEROLowSolved - 04/03/2023
UNUSED CIRCUIT FUNCTIONALITYInformationalSolved - 04/03/2023
QUEUE NOT ENFORCED TO BE EMPTY RIGHT AFTER POPPING ALL ELEMENTSInformationalAcknowledged - 04/03/2023
UNNEEDED INITIALIZATION OF UINT256 VARIABLESInformationalAcknowledged - 04/03/2023

8. Findings & Tech Details

8.1 CIRCUIT NOT PROPERLY WORKING WHEN USING SHARD ID > 0

//

Critical

Description
BVSS
Recommendation

8.2 HEAD STATE NOT ENFORCED TO BE ZERO

//

Low

Description
BVSS
Recommendation

8.3 UNUSED CIRCUIT FUNCTIONALITY

//

Informational

Description
BVSS
Recommendation

8.4 QUEUE NOT ENFORCED TO BE EMPTY RIGHT AFTER POPPING ALL ELEMENTS

//

Informational

Description
BVSS
Recommendation

8.5 UNNEEDED INITIALIZATION OF UINT256 VARIABLES

//

Informational

Description
BVSS
Recommendation

9. Automated Testing

Halborn strongly recommends conducting a follow-up assessment of the project either within six months or immediately following any material changes to the codebase, whichever comes first. This approach is crucial for maintaining the project’s integrity and addressing potential vulnerabilities introduced by code modifications.

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