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May 10, 2025 • 3 min read

Smart Contract Security Auditing with Python: Finding Vulnerabilities Before Hackers Do

Smart Contract Security Audit Solidity Python Blockchain

TL;DR > One overlooked bug can drain $100 M+ in seconds. This guide shows how to combine manual review with Python automation to catch vulnerabilities early.

Top Vulnerabilities

Reentrancy

(bool success,) = msg.sender.call{{value: amount}}("");
require(success, "ETH transfer failed");
balances[msg.sender] -= amount; // <- too late!

Apply Checks‑Effects‑Interactions or ReentrancyGuard.

Access Control

function changeOwner(address _new) external {{
    owner = _new; // 🛑 anyone can own!
}}

Fix with:

modifier onlyOwner {{ require(msg.sender == owner); _; }}

Oracle Manipulation

Pump price via flash loan → drain protocol collateral. Use Chainlink TWAP, sanity bounds, or multi‑source oracles.

Python Tooling

Quick Regex Scan

import re, pathlib, sys
code = pathlib.Path(sys.argv[1]).read_text()
if re.search(r"\.call\{{?value", code):
    print("⚠️  External call spotted – review for reentrancy")

Slither Static Analysis

from slither import Slither
sl = Slither("MyContract.sol")
for result in sl.run_detectors():
    print(result)

Fuzz Test with Brownie

from brownie import accounts, reverts
def test_no_reentrancy(bank, attacker):
    with reverts():
        attacker.attack({"from": accounts[0]})

Attack Diagrams

Reentrancy

sequenceDiagram participant Attacker participant Bank Attacker->>Bank: withdraw() Bank-->>Attacker: send ETH Attacker-->>Bank: withdraw() (re‑enter) loop Repeat Bank-->>Attacker: send more end

Flash Loan Governance

sequenceDiagram participant Attacker participant LoanPool participant Gov Attacker->>LoanPool: flashBorrow() LoanPool-->>Attacker: tokens Attacker->>Gov: propose+vote Gov-->>Attacker: treasury funds Attacker->>LoanPool: repay

Case Studies

  • The DAO (2016): ~$60 M lost via reentrancy.
  • Beanstalk (2022): $182 M lost via flash‑loan governance.
  • Mango Markets (2022): $112 M oracle manipulation.

Need an Expert?

We turn audit reports into actionable fixes & provide end‑to‑end security hardening. Reach Out.

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