Skip to content

tutorial.md

Latest commit

 

History

History
224 lines (167 loc) · 5.85 KB

tutorial.md

File metadata and controls

224 lines (167 loc) · 5.85 KB
Raw

🛡️ Sanctifier Tutorial: Fix the Broken Contract

Welcome to the Sanctifier interactive tutorial. In this guide, you'll audit a deliberately vulnerable smart contract, identify the security flaws, and apply fixes using Sanctifier's analysis tools.

By the end you will:

  • Identify 3 common Solidity vulnerabilities
  • Use Sanctifier to surface them automatically
  • Apply and verify the fixes

Prerequisites

  • Node.js ≥ 18
  • Sanctifier CLI installed (npm install -g @hypersafed/sanctifier)
  • Basic Solidity knowledge

Step 1 — Clone the Tutorial Contract

Create a new file contracts/VulnerableVault.sol with the following code. Do not fix anything yet — the bugs are intentional.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title VulnerableVault
/// @notice DO NOT USE IN PRODUCTION — intentionally broken for tutorial purposes
contract VulnerableVault {
    mapping(address => uint256) public balances;
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    // 🐛 Bug 1: No reentrancy guard
    function withdraw(uint256 amount) external {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
        balances[msg.sender] -= amount; // state updated AFTER external call
    }

    // 🐛 Bug 2: Anyone can call this — missing access control
    function emergencyDrain() external {
        payable(msg.sender).transfer(address(this).balance);
    }

    // 🐛 Bug 3: Timestamp dependence — miners can manipulate block.timestamp
    function isWithdrawalWindow() public view returns (bool) {
        return block.timestamp % 2 == 0;
    }

    function deposit() external payable {
        balances[msg.sender] += msg.value;
    }
}

Step 2 — Run Sanctifier

Point Sanctifier at the contract:

sanctifier analyze contracts/VulnerableVault.sol

You should see output similar to:

[CRITICAL] Reentrancy: withdraw() — state mutated after external call (line 17)
[CRITICAL] Missing access control: emergencyDrain() — no onlyOwner modifier (line 23)
[MEDIUM]   Timestamp dependence: isWithdrawalWindow() — block.timestamp is miner-manipulable (line 28)

3 issues found. 0 passed.

Step 3 — Fix the Bugs

Now apply the fixes one at a time.

Fix 1 — Reentrancy (Checks-Effects-Interactions)

Move the state update before the external call:

function withdraw(uint256 amount) external {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount; // ✅ state updated BEFORE external call
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
}

Why? A malicious contract can re-enter withdraw() before the balance is deducted, draining the vault. This is how the 2016 DAO hack worked — $60M lost.

Fix 2 — Access Control

Add an onlyOwner modifier:

modifier onlyOwner() {
    require(msg.sender == owner, "Not authorised");
    _;
}

function emergencyDrain() external onlyOwner { // ✅ restricted
    payable(msg.sender).transfer(address(this).balance);
}

Why? Without access control, any address can drain the vault instantly. Always restrict privileged functions.

Fix 3 — Remove Timestamp Dependence

Replace block.timestamp logic with an explicit admin-controlled window:

bool public withdrawalOpen;

function setWithdrawalWindow(bool open) external onlyOwner {
    withdrawalOpen = open;
}

function isWithdrawalWindow() public view returns (bool) {
    return withdrawalOpen; // ✅ no miner manipulation possible
}

Why? Miners can adjust block.timestamp by up to ~15 seconds, making time-based conditions exploitable in certain scenarios.

Step 4 — Verify with Sanctifier

Run the analyzer again against the fixed contract:

sanctifier analyze contracts/VulnerableVault.sol

Expected output:

✅ No issues found. All checks passed.

Step 5 — The Fixed Contract

Here is the complete fixed version for reference:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title SecureVault
/// @notice Fixed version of VulnerableVault — tutorial reference
contract SecureVault {
    mapping(address => uint256) public balances;
    address public owner;
    bool public withdrawalOpen;

    constructor() {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Not authorised");
        _;
    }

    function deposit() external payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint256 amount) external {
        require(isWithdrawalWindow(), "Withdrawal window closed");
        require(balances[msg.sender] >= amount, "Insufficient balance");
        balances[msg.sender] -= amount;
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }

    function emergencyDrain() external onlyOwner {
        payable(msg.sender).transfer(address(this).balance);
    }

    function setWithdrawalWindow(bool open) external onlyOwner {
        withdrawalOpen = open;
    }

    function isWithdrawalWindow() public view returns (bool) {
        return withdrawalOpen;
    }
}

What You Learned

Vulnerability Pattern Fix
Reentrancy External call before state update Checks-Effects-Interactions
Missing access control No modifier on privileged fn onlyOwner modifier
Timestamp dependence block.timestamp in logic Admin-controlled state flag

Next Steps

  • Run Sanctifier on your own contracts: sanctifier analyze <path>
  • Explore the full rule reference
  • Join the discussion on GitHub