Fan Fun - EVM Contracts Audit | Fan Fun

Summary

100% of all REPORTED Findings have been addressed

About this report

Assessment Type

Assurance Assessment

Date of Engagement

February 26th, 2025 - February 28th, 2025

Source Code

contracts contracts

Assessed Commit ID

5281e99 3a72d60

Severity Breakdown

5Acknowledged
5Risk Accepted
8Solved
18All Findings

Critical0
High0
Medium2
- 1Solved
- 1Risk A.
Low5
- 4Risk A.
- 1Solved
Informational11
- 5Ack.
- 6Solved

Introduction#

Fan.Fun engaged Halborn to conduct a security assessment of their Fandotfun project beginning on February 27th and ending on March 3rd. The security assessment was scoped to the smart contract provided in the GitHub repository. Commit hash and further details can be found in the Scope section of this report.

FanDotFun is a decentralized protocol for creator-fan engagement through time-decaying keys.

Assessment Summary#

Halborn was provided 3 days for the engagement and assigned one full-time security engineer to review the security of the smart contract in scope. The engineer is a blockchain and smart contract security expert with advanced penetration testing and smart contract hacking skills, and deep knowledge of multiple blockchain protocols.

The purpose of the assessment is to:

Identify potential security issues within the smart contract.
Ensure that smart contract functionality operates as intended.

In summary, Halborn identified some improvements to reduce the likelihood and impact of risks, which were partially addressed by the Fan.Fun team. The main ones were the following:

Implement Slippage Protection and/or return excess funds sent by subscribers.
Consider adopting a pull pattern for claiming decayed keys.
Review fee calculations in claim() or prevent protocol fees from being 0.

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).

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 ( $m_e$ )	METRIC VALUE	NUMERICAL VALUE
Attack Origin (AO)	Arbitrary (AO:A)	1
Attack Origin (AO)	Specific (AO:S)	0.2
Attack Cost (AC)	Low (AC:L)	1
	Medium (AC:M)	0.67
	High (AC:H)	0.33
Attack Complexity (AX)	Low (AX:L)	1
	Medium (AX:M)	0.67
	High (AX:H)	0.33

Exploitability

E

is calculated using the following formula:

$E = \prod m_e$

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 ( $m_I$ )	METRIC VALUE	NUMERICAL VALUE
Confidentiality (C)	None (C:N)	0
	Low (C:L)	0.25
	Medium (C:M)	0.5
	High (C:H)	0.75
	Critical (C:C)	1
Integrity (I)	None (I:N)	0
	Low (I:L)	0.25
	Medium (I:M)	0.5
	High (I:H)	0.75
	Critical (I:C)	1
Availability (A)	None (A:N)	0
	Low (A:L)	0.25
	Medium (A:M)	0.5
	High (A:H)	0.75
	Critical (A:C)	1
Deposit (D)	None (D:N)	0
	Low (D:L)	0.25
	Medium (D:M)	0.5
	High (D:H)	0.75
	Critical (D:C)	1
Yield (Y)	None (Y:N)	0
	Low (Y:L)	0.25
	Medium (Y:M)	0.5
	High (Y:H)	0.75
	Critical (Y:C)	1

Impact

I

is calculated using the following formula:

$I = max(m_I) + \frac{\sum{m_I} - max(m_I)}{4}$

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 ( $C$ )	COEFFICIENT VALUE	NUMERICAL VALUE
Reversibility ( $r$ )	None (R:N)	1
	Partial (R:P)	0.5
	Full (R:F)	0.25
Scope ( $s$ )	Changed (S:C)	1.25
Scope ( $s$ )	Unchanged (S:U)	1

Severity Coefficient

C

is obtained by the following product:

$C = rs$

The Vulnerability Severity Score

S

is obtained by:

$S = min(10, EIC * 10)$

The score is rounded up to 1 decimal places.

Critical	High	Medium	Low	Informational
9 - 10	7 - 8.9	4.5 - 6.9	2 - 4.4	0 - 1.9

Scope#

(a) Repository:contracts

(b) Assessed Commit ID:5281e999e633416ab5a4ef49dbe8b25f02d7e32f

Show out-of-scope files (1)

4 files

src

3 files

IKeys.sol

solidity

Keys.sol

solidity

KeysLibrary.sol

solidity

(a) Repository:contracts

(b) Assessed Commit ID:3a72d60f407d9f42a2ad1c8ad2448953c8a9ad27

Show out-of-scope files (1)

4 files

src

3 files

IKeys.sol

solidity

Keys.sol

solidity

KeysLibrary.sol

solidity

2a150926a8553a89cd410354a016bf42b03efeee 5c65c252f02fe64e7cf81f1860c9d4efe4d718a0 aa06bf691bd14b523dc59e40d794a219c76d37c3 9beca85202a8a5d26dca0bda50e2072cabf03cf4 6e556707b13a449f827f4590dd3cbb31407dc82d bdeda88a9684c2fcdb58810d3552ef38ffe32783 710372e4032368a3b13f67279a8bb01761cef492 6e5ad3433d15cf79a8e41e59c71c6aae3459f9da

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

Assessment Summary & Findings Overview#

#	Title	Severity	Score	Status
HAL-01	Front-Running Risks: Potential DoS, Sandwich Attacks, and Slippage in Buy/Sell	Medium	5.4	Solved03/06/2025
HAL-02	Creators Could Prevent Their Subscribers From Claiming	Medium	5.0	Risk Accepted03/10/2025
HAL-03	Protocol Cannot Have 0% Fees Due to Adjusted Calculation in Claim()	Low	3.1	Risk Accepted03/10/2025
HAL-04	Malicious Subscribers Could Potentially DoS Claims	Low	2.8	Risk Accepted03/10/2025
HAL-05	Missing Emergency Withdrawal Capability	Low	2.5	Solved03/06/2025
HAL-06	Missing Pausing Mechanism	Low	2.5	Risk Accepted03/10/2025
HAL-07	Centralization Risks	Low	2.3	Risk Accepted03/10/2025
HAL-08	Subscribers Can Buy Zero Keys	Informational	1.9	Acknowledged03/10/2025
HAL-09	Single-step Ownership Transfer Process	Informational	1.7	Acknowledged03/10/2025
HAL-10	Owner Can Renounce Ownership	Informational	1.3	Acknowledged03/10/2025
HAL-11	Missing Visibility Attribute	Informational	0.0	Solved03/07/2025
HAL-12	Lack of Event Emission	Informational	0.0	Solved03/06/2025
HAL-13	Magic Numbers in Use	Informational	0.0	Solved03/07/2025
HAL-14	Style Guide Optimizations	Informational	0.0	Solved03/07/2025
HAL-15	Events Missing Indexed Fields	Informational	0.0	Acknowledged03/10/2025
HAL-16	Incomplete NatSpec Documentation	Informational	0.0	Solved03/07/2025
HAL-17	Missing Input Validation	Informational	0.0	Acknowledged03/10/2025
HAL-18	Inconsistent License Declarations	Informational	0.0	Solved03/07/2025

Findings & Tech Details#

HAL-01Solved

Front-Running Risks: Potential DoS, Sandwich Attacks, and Slippage in Buy/Sell

Medium

5.4AO:A/AC:M/AX:L/R:N/S:U/C:N/A:M/I:M/D:M/Y:LAO:A/AC:M/AX:L/R:N/S:U/C:N/A:M/I:M/D:M/Y:L

Description

Recommendation

Remediation Comment

Addressed in:

Remediation Comment

Addressed in:

Inconsistent License Declarations

Informational

0.0AO:A/AC:L/AX:L/R:N/S:U/C:N/A:N/I:N/D:N/Y:NAO:A/AC:L/AX:L/R:N/S:U/C:N/A:N/I:N/D:N/Y:N

Description

Recommendation

Remediation Comment

Addressed in:

6e5ad3433d15cf79a8e41e59c71c6aae3459f9da

8. Automated Testing#

Disclaimer#

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.