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ZK Sync - ZK Safe - 1Kx

Prepared by:

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HALBORN

Last Updated 09/20/2024

Date of Engagement: May 20th, 2024 - May 31st, 2024

Summary

100% of all REPORTED Findings have been addressed

All findings

1

Critical

0

High

0

Medium

1

Low

0

Informational

0

Table of Contents

1. Introduction

The 1kx team engaged Halborn to conduct a security assessment on their smart contracts and circuits, beginning on May 20, 2024, and ending on May 31, 2024. The security assessment was scoped to the smart contracts and circuits inside their zksafe GitHub repository, located at https://github.com/1kx-network/zksafe/blob/main, commit 5d55130d7a9deddf15025a479dff65ba8582def6.

2. Assessment Summary

The team at Halborn was provided two weeks for the engagement and assigned two full-time security engineers to assess the security of the smart contracts and the zero knowledge circuits. Both security engineers are blockchain, smart-contract, and ZK security experts with advanced penetration testing, smart-contract hacking, and deep knowledge of multiple blockchain protocols.

The purpose of this assessment is to achieve the following:

    • Ensure that the system operates as intended.

    • Identify potential security issues.

    • Identify lack of best practices within the codebase.

    • Identify systematic risks that may pose a threat in future releases.

    • Identify common ZK issues found in ZK circuits.

In summary, Halborn identified one security issue that was addressed by the 1kx team.

3. Test Approach and 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 assessment. While manual testing is recommended to uncover flaws in logic, process, and implementation; automated testing techniques help enhance coverage of the code and can quickly identify items that do not follow the security best practices. The following phases and associated tools were used during the assessment:

    • Research into architecture and purpose.

    • Smart contract manual code review and walkthrough.

    • Graphing out functionality and contract logic/connectivity/functions (solgraph).

    • Manual assessment of use and safety for the critical Solidity variables and functions in scope to identify any arithmetic-related vulnerability classes.

    • Manual testing by custom scripts.

    • Static Analysis of security for scoped contract, and imported functions (slither).

    • Testnet deployment (Foundry).

    • Running ZK tests with nargo.

4. 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.

4.1 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

4.2 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 (I:N)
Low (I:L)
Medium (I:M)
High (I:H)
Critical (I: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}

4.3 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

5. SCOPE

Files and Repository
(a) Repository: zksafe
(b) Assessed Commit ID: 5d55130
(c) Items in scope:
  • contracts/ZkSafeModule.sol
  • circuits/src/main.nr
Out-of-Scope:
Remediation Commit ID:
Out-of-Scope: New features/implementations after the remediation commit IDs.

6. Assessment Summary & Findings Overview

Critical

0

High

0

Medium

1

Low

0

Informational

0

Impact x Likelihood

HAL-01

Security analysisRisk levelRemediation Date
Loss of ETH when executing a transaction from the moduleMediumSolved - 05/31/2024

7. Findings & Tech Details

7.1 Loss of ETH when executing a transaction from the module

//

Medium

Description

Due to adding the payable keyword and not implementing a way to pull out the ETH sent, if the call to sendZkSafeTransaction carries ETH, then those funds will remain locked within the module.

Proof of Concept

It can be seen in the function definition that it accepts calls with ETH attached due to having the payable keyword:

https://github.com/1kx-network/zksafe/blob/5d55130d7a9deddf15025a479dff65ba8582def6/contracts/ZkSafeModule.sol#L117C14-L117C21

    function sendZkSafeTransaction(
        GnosisSafe safeContract,
        // The Safe address to which the transaction will be sent.
        Transaction calldata transaction,
        // The proof blob.
        bytes calldata proof
    ) public payable virtual returns (bool)

However, through the Safe module, there is no way to pull such tokens out, as it purpose is to validate transactions and executing them in the context of the Safe contract, not to hold funds. Moreover, the only place where it would make sense to do that would be in the last call to the Safe contract's execTransactionFromModule:

        return safeContract.execTransactionFromModule(
            transaction.to,
            transaction.value,
            transaction.data,
                transaction.operation
            );

But

  1. Such a function does NOT accept ETH, as it is not payable.

  2. To pass ETH, it would need to add the syntax {value : transaction.value in between the function name and the first (, which is not the case.

Because of that, the ETH that a tx carries will be locked within the module forever.

BVSS
Recommendation

Remove the payable keyword to prevent the transaction from carrying ETH. This makes sense, as the funds that the approved transaction will use will be those of the Safe contract, not the ones carried with the send one.

Remediation

SOLVED: The 1kx team solved this issue as recommended, removing the payable keyword from the function definition.

Remediation Hash
https://github.com/1kx-network/zksafe/commit/f8b8fef8d23abe3d5d45e92101b6a0a8a8b0617c

8. Automated Testing

Halborn used automated testing techniques to enhance the coverage of certain areas of the smart contracts in scope. Among the tools used was Slither, a Solidity static analysis framework. After Halborn verified the smart contracts in the repository and was able to compile them correctly into their ABIs and binary format, Slither was run against the contracts. This tool can statically verify mathematical relationships between Solidity variables to detect invalid or inconsistent usage of the contracts' APIs across the entire code-base.


Overall, the reported issues were not mostly low/informational issues that did not pose a real threat to the system, so they were not considered to be part of this report.

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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