setup.py arbitrary code execution

Most Python supply-chain attacks in the wild run through this. When you pip install a package from a source distribution (sdist), pip runs setup.py to build it. And setup.py is a plain Python script. It can download files, open shells, exfiltrate environment variables or install backdoors. All of that happens at install time, before your application ever imports the package.

# setup.py
from setuptools import setup
import os
os.system('echo "You are hacked" > /tmp/pwned.txt')
setup(name="totally-legit-package", version="1.0.0")

pip install ./totally-legit-package

The 2025 PyPI wave used look-alike domains to distribute packages with malicious setup.py files. TeamPCP went further in 2026. They stole maintainer credentials from CI/CD and published backdoored versions of legitimate packages directly. This is why the ecosystem keeps pushing toward wheels and isolated builds. Sdists aren't going away, though.