Building an Automated Smart Contract Vulnerability Scanner: A Technical Walkthrough

The November 7, 2023 MEV bot exploit — where a missing authentication check in function 0xf6ebebbb led to the loss of approximately 1,000 ETH ($1.89 million) — is a textbook example of a vulnerability that automated scanning tools could have caught. As the DeFi ecosystem grows alongside Bitcoin’s surge past $35,400, the need for systematic, automated smart contract security analysis has never been more pressing. This tutorial walks through building a basic vulnerability scanner that can detect common access control issues, reentrancy patterns, and unsafe external calls.

The Objective

We will build a Python-based smart contract vulnerability scanner that can analyze Solidity source code for common security issues. The scanner will use pattern matching and static analysis techniques to identify potential vulnerabilities before a contract is deployed to the blockchain. By the end of this walkthrough, you will have a tool that can detect the exact type of authentication bypass that led to the November 7 MEV bot exploit, along with several other common vulnerability classes.

This is not a replacement for professional security audits, but it provides a valuable first line of defense. Think of it as a linter for security — catching obvious issues before they reach an auditor’s desk, saving time and reducing the likelihood of costly exploits.

Prerequisites

Before starting, ensure you have the following. Python 3.8 or later installed on your system. Basic familiarity with Solidity syntax and smart contract structure. An understanding of common vulnerability patterns (reentrancy, access control, integer overflow). The following Python packages: slither-analyzer (Trail of Bits’ static analysis framework), solc (Solidity compiler interface), and rich (for formatted console output).

Install the required packages with: pip install slither-analyzer py-solc-x rich. You will also need a Solidity compiler, which solcx can download automatically.

Step-by-Step Walkthrough

Step 1: Set up the scanning framework. Create a Python class that accepts a Solidity file path, compiles the contract, and initializes the Slither analyzer. Slither provides a comprehensive API for traversing a contract’s abstract syntax tree (AST), identifying function calls, state variable access patterns, and control flow structures.

Step 2: Implement access control detection. The MEV bot exploit on November 7 was caused by a function that lacked any authentication. Your scanner should flag any public or external function that modifies state (transfers ETH, changes balances, or calls other contracts) but does not include an access control modifier like onlyOwner, onlyAdmin, or a require(msg.sender == ...) check. Slither’s detector API provides hooks for identifying functions without access control on state-modifying operations.

Step 3: Add reentrancy pattern detection. Reentrancy remains one of the most common and devastating vulnerability classes in DeFi. Your scanner should identify functions where an external call is followed by a state change — the classic reentrancy pattern. The external call hands control to an untrusted contract, which can re-enter the original function before the state is updated. Slither’s built-in reentrancy detector is a good starting point, but you should also implement custom checks for cross-function and cross-contract reentrancy.

Step 4: Detect unsafe arithmetic and type conversions. Although Solidity 0.8.x includes built-in overflow checks, many contracts still use older compiler versions or implement custom arithmetic that can overflow. Your scanner should flag unchecked arithmetic blocks and unsafe type conversions, particularly between unsigned integers of different sizes.

Step 5: Implement flash loan attack surface analysis. Flash loan attacks exploit the composability of DeFi protocols by borrowing large amounts without collateral, manipulating prices or oracle values, and repaying the loan in a single transaction block. Your scanner should identify functions that rely on spot prices from a single DEX pool, functions that perform operations based on token balances rather than internal accounting, and functions that can be called within the same transaction as a flash loan callback.

Step 6: Generate a comprehensive report. Your scanner should output a structured report listing all detected vulnerabilities, ranked by severity. Include the specific code location, a description of the vulnerability, the potential impact, and recommended remediation steps. Use the rich library to format the output with colors and tables for readability.

Troubleshooting

If your scanner produces false positives, refine your detection heuristics. Not every public function without a modifier is vulnerable — some are intentionally public. Context matters. Use Slither’s data dependency analysis to determine whether a function actually modifies security-sensitive state.

If compilation fails, ensure your Solidity version matches the contract’s pragma specification. Older contracts may use deprecated syntax or features not supported by newer compiler versions. The solcx library allows you to install and switch between specific compiler versions.

If the scanner runs slowly on large contracts, consider implementing incremental analysis that only rescans changed functions. For monorepo projects with many contracts, parallelize the scanning across multiple processes.

Mastering the Skill

Automated vulnerability scanning is the beginning, not the end, of smart contract security. To advance your skills, study real-world exploits in detail. The SlowMist Blockchain Hacked Archive catalogs hundreds of incidents with technical analysis. Contribute to open-source security tools like Slither, Echidna (fuzzing), and Mythril (symbolic execution). Participate in audit competitions on platforms like Code4rena and Sherlock, where you can practice identifying vulnerabilities in real protocols.

The November 7 MEV bot exploit cost $1.89 million because of a single missing access control check. Automated tools exist to catch exactly these types of issues. Building, maintaining, and running them is not optional — it is a professional obligation for anyone deploying smart contracts that handle real value.

This article is for educational purposes only. Smart contract security is a complex discipline, and automated tools cannot guarantee the absence of vulnerabilities. Always engage professional security auditors before deploying contracts that handle significant value.