Reentrancy Guard Checker

WHAT IT DOES

Reentrancy Guard Checker is a specialized security tool that scans EVM contracts specifically for reentrancy vulnerabilities—one of the most common and dangerous smart contract attack vectors. Enter a contract address and optionally provide the source code to get a focused analysis. The tool detects external calls (CALL opcodes), checks for reentrancy guard patterns like OpenZeppelin's ReentrancyGuard and nonReentrant modifiers, and identifies state changes after external calls that could enable reentrancy attacks. Results are categorized as "Vulnerable," "Warning," or "Safe," with clear explanations and location indicators. This is essential for developers wanting to quickly check if their contract is protected against reentrancy, or for auditors performing targeted security reviews.

HOW TO USE

Connect your EVM wallet (optional—the tool works read-only). Select the network from the dropdown. Enter the contract address (0x…42) you want to analyze. For a comprehensive check, paste the contract's Solidity source code in the source code field—this enables detection of reentrancy guard usage, state change patterns, and unsafe external calls. Click "check" to run the analysis. The tool fetches the bytecode and on-chain metadata, then performs pattern matching. The results display the contract's balance, transaction count, bytecode size, an overall status badge (Safe/Warning/Vulnerable), and a detailed list of findings with descriptions and location markers. Use the "clear" button to reset all fields.

TECHNICAL MECHANISM

The tool uses ethers.js v5.7.2 via the BGWallet provider. It fetches the contract bytecode with provider.getCode(address), balance with provider.getBalance(), and transaction count with provider.getTransactionCount(). The bytecode is scanned for the CALL opcode (0xF1) to detect external calls. For source code analysis, the tool performs pattern matching using regular expressions to identify reentrancy guards (ReentrancyGuard, nonReentrant), external calls (call, delegatecall, transfer, send), and state changes that occur after external calls. It also checks for unchecked low-level calls and transfers to msg.sender. Findings are categorized by severity—Vulnerable (critical), Warning (potential risk), or Safe (no issues detected)—and displayed with human-readable descriptions.

WHAT IT CANNOT SEE

This tool cannot detect reentrancy in contracts with complex or obfuscated logic beyond pattern matching—it relies on known patterns. It cannot detect reentrancy that does not follow common patterns (e.g., non-obvious state changes). It cannot verify that a reentrancy guard is correctly implemented—presence of a guard does not guarantee it is used correctly. It cannot provide a comprehensive security audit—this is a focused, heuristic scan. It cannot detect reentrancy in external dependencies or libraries. It cannot analyze contracts on non-EVM chains. The tool is limited by the accuracy of the pattern database and the completeness of the provided source code. A "Safe" result does not guarantee the contract is free from reentrancy.

PLEASE NOTE

Works exclusively on EVM-compatible chains: Ethereum Mainnet, Goerli, Sepolia, Polygon, Arbitrum, and Optimism. Solana, Tron, Starknet, SUI, TON, Aptos, Bitcoin, Cosmos, and XRP are not supported. This is a read-only, free security analysis tool—use it for initial contract inspection and to identify areas that warrant deeper review. The presence of a "Vulnerable" finding indicates a potential issue that should be investigated thoroughly. Always perform a full professional security audit before deploying or interacting with any contract.