May 10, 2025 • 3 min read
Smart Contract Security Auditing with Python: Finding Vulnerabilities Before Hackers Do
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.
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