Smart contract security audit checklist

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  1. Smart Contract Security Audit Checklist

Introduction

Smart contracts are self-executing contracts with the terms of the agreement directly written into code. They are a cornerstone of decentralized applications (dApps) on blockchain platforms like Ethereum, Binance Smart Chain, and others. While offering immense potential for automation, transparency, and security, smart contracts are also vulnerable to exploits. A single vulnerability can lead to significant financial losses, reputational damage, and erosion of trust. Therefore, a thorough security audit is crucial before deploying any smart contract to a live environment. This article provides a comprehensive checklist for conducting smart contract security audits, geared towards beginners and aiming to cover essential aspects. Understanding these points, even at a high level, is the first step towards building more secure dApps. This checklist is not exhaustive, but it serves as a strong foundation for a robust security assessment.

Phase 1: Preparation & Scope Definition

Before diving into the code, careful preparation is paramount.

  • Define Audit Scope: Clearly outline which contracts are in scope for the audit. Include version numbers and dependencies. Document all external dependencies and their versions. Consider the interaction between contracts; an audit must cover the entire system, not just individual components.
  • Understand the Business Logic: Deeply understand the *intended* functionality of the smart contract. What problem is it solving? What are the expected inputs and outputs? What are the critical assumptions? Misunderstanding the logic leads to inaccurate vulnerability assessments. Refer to whitepaper and design documentation.
  • Identify Critical Functions: Pinpoint the functions that handle sensitive operations, such as token transfers, access control, and state changes. These are prime targets for attackers.
  • Threat Modeling: Conduct a threat model to identify potential attack vectors. What are the likely ways an attacker might try to exploit the contract? Common threat models include STRIDE (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege). Threat Modeling Guide
  • Establish Audit Criteria: Define the specific security standards and best practices that the audit will adhere to. These can include industry standards like the OWASP Smart Contract Top 10 [1], or internal security policies.
  • Tool Selection: Choose appropriate tools for static analysis, dynamic analysis, and formal verification (discussed later).

Phase 2: Static Analysis

Static analysis involves examining the contract's code without executing it. This is the first line of defense against common vulnerabilities.

  • Code Review: A manual review of the code by experienced security auditors is essential. Pay attention to code style, readability, and maintainability. Complex code is harder to audit.
  • Static Analysis Tools: Utilize tools like Slither [2], Mythril [3], Oyente [4], and Securify [5] to automatically detect potential vulnerabilities. These tools can identify issues like:
   * Integer Overflow/Underflow:  Check for potential overflows or underflows in arithmetic operations. Use SafeMath libraries or Solidity 0.8.0+ which includes built-in overflow/underflow protection. [6]
   * Reentrancy:  A critical vulnerability where a malicious contract can recursively call back into the vulnerable contract before the first invocation completes, potentially draining funds.  Use checks-effects-interactions pattern or reentrancy guards. [7]
   * Timestamp Dependence: Avoid relying on block timestamps for critical logic, as miners can manipulate them within a certain range.
   * Transaction Ordering Dependence (TOD):  Vulnerabilities arising from the unpredictable order of transactions in a block.  Consider using commit-reveal schemes. [8]
   * Uninitialized Storage Variables:  Ensure all storage variables are properly initialized to prevent unexpected behavior.
   * Delegatecall Vulnerabilities:  If using `delegatecall`, carefully verify the called contract's code to prevent malicious code execution.
   * Denial of Service (DoS):  Identify potential DoS attacks, such as gas limit issues or infinite loops.
  • Control Flow Analysis: Examine the flow of execution through the contract to identify potential logic errors and unexpected behavior.
  • Data Flow Analysis: Track the flow of data through the contract to identify potential data manipulation vulnerabilities.

Phase 3: Dynamic Analysis

Dynamic analysis involves executing the contract and observing its behavior.

  • Unit Testing: Write comprehensive unit tests to cover all critical functions and edge cases. Use frameworks like Truffle [9] or Hardhat [10]. Aim for high code coverage.
  • Fuzzing: Use fuzzing tools like Echidna [11] to automatically generate random inputs and test the contract for crashes or unexpected behavior.
  • Symbolic Execution: Tools like Manticore [12] use symbolic execution to explore all possible execution paths of the contract.
  • Gas Usage Analysis: Analyze the gas cost of each function to identify potential gas optimization opportunities and prevent DoS attacks. High gas costs can make the contract unusable.
  • Runtime Verification: Monitor the contract's behavior during runtime to detect anomalies and potential attacks.

Phase 4: Specific Vulnerability Checks

Beyond the general techniques, focus on specific vulnerability categories.

  • Access Control: Verify that access control mechanisms are correctly implemented. Only authorized users should be able to perform sensitive operations. Use modifiers effectively. [13]
  • Arithmetic Errors: (Reiterated for emphasis) Carefully review all arithmetic operations for potential overflows, underflows, and rounding errors.
  • Logic Errors: Thoroughly test the contract's logic to ensure it behaves as expected in all scenarios. Pay attention to edge cases and boundary conditions.
  • Front Running: Analyze for potential front-running attacks, where an attacker observes a pending transaction and submits their own transaction with a higher gas price to execute first. [14]
  • Oracle Manipulation: If the contract relies on external oracles, assess the security of those oracles and the potential for manipulation.
  • Improper Event Emission: Ensure events are emitted correctly to provide accurate and reliable information about contract state changes.
  • Unhandled Exceptions: Ensure all potential exceptions are handled gracefully to prevent unexpected behavior.
  • Storage Collisions: Carefully consider storage layout to avoid potential collisions that could lead to data corruption.
  • Gas Limit Issues: Ensure that transactions do not exceed the block gas limit, which could cause them to fail.

Phase 5: Formal Verification (Advanced)

Formal verification uses mathematical techniques to prove the correctness of a smart contract. This is a more rigorous but also more complex approach.

  • Specification Language: Define the contract's behavior using a formal specification language.
  • Model Checking: Use model checking tools to verify that the contract meets its specification.
  • Theorem Proving: Use theorem proving tools to formally prove the correctness of the contract's code. Tools like Certora Prover [15] are becoming increasingly popular.

Phase 6: Reporting & Remediation

  • Detailed Report: Create a detailed audit report that outlines all identified vulnerabilities, their severity, and recommended remediation steps. Include code snippets and clear explanations.
  • Severity Assessment: Categorize vulnerabilities based on their severity (Critical, High, Medium, Low).
  • Remediation: Implement the recommended remediation steps to fix the vulnerabilities.
  • Retesting: After remediation, retest the contract to ensure that the vulnerabilities have been fixed and no new vulnerabilities have been introduced.
  • Documentation: Update the contract's documentation to reflect any changes made during the audit and remediation process. Good documentation is essential for long-term maintainability and security.

Tools and Resources

  • Solidity Compiler: [16]
  • Remix IDE: [17]
  • Truffle Suite: [18]
  • Hardhat: [19]
  • Slither: [20]
  • Mythril: [21]
  • Oyente: [22]
  • Securify: [23]
  • Echidna: [24]
  • Manticore: [25]
  • Certora Prover: [26]
  • OWASP Smart Contract Top 10: [27]
  • ConsenSys Diligent: [28]
  • Samczsun's Blog: [29] (Excellent analysis of smart contract exploits)
  • OpenZeppelin Contracts: [30] (Secure, pre-built smart contract components)
  • ChainSecurity: [31] (Security Auditing Firm)
  • Trail of Bits: [32] (Security Auditing Firm)
  • Quantstamp: [33] (Security Auditing Firm)
  • Runtime Verification: [34] (Formal Verification Tools)
  • Gas Optimization Tips: [35]

Disclaimer

This checklist is intended as a guide and should not be considered a substitute for professional security auditing. The complexity of smart contract security requires expertise and experience.


Security audit Whitepaper Gas optimization Reentrancy attack Integer overflow Access control Formal verification Static analysis Dynamic analysis Solidity

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