Python packages allow for arbitrary code execution at run time as well as install time. Code execution at run time makes sense because, well, that's what code does. But executing code at install time is a lesser known feature within the Python packaging ecosystem, and a potentially much more dangerous one.

To test it out let's download this repository:

$ git clone https://github.com/mschwager/0wned.git

Don't worry, there's nothing malicious going on, you can take a look at what's happening yourself.

Now let's install the package:

$ sudo python -m pip install 0wned/
$ cat /0wned
Created '/0wned' with user 'root' at 1536011622

During pip installation 0wned was able to successfully write to the root directory! This means that 0wned can do anything as the root or administrative user.

We can reduce the impact of this issue by installing packages with the --user flag:

$ python -m pip install --user 0wned/
$ cat ~/0wned
Created '/home/tempuser/0wned' with user 'tempuser' at 1536011624

You should always be wary of Python packages you're installing on your system, especially when using root/administrative privileges. There are a few ways to help mitigate these types of attacks:

• Install only binary distribution Python wheels using the --only-binary :all: flag. This avoids arbitrary code execution on installation (avoids setup.py).
• As mentioned above, install packages with the local user using the --user flag.
• Install packages in hash-checking mode using the --require-hashes flag. This will protect against remote tampering and ensure you're getting the package you intend to.
• Double check that you've spelled the package name correctly. There may be malicious packages typosquatting under a similar name.

You can hook almost any pip command by extending the correct setuptools module.

For example, 0wned takes advantage of the install class to do its thing:

from setuptools import setup
from setuptools.command.install import install

class PostInstallCommand(install):
    def run(self):
        # Insert code here
        install.run(self)

setup(
    ...
    cmdclass={
        'install': PostInstallCommand,
    },
    ...
)

And when pip install is run our custom PostInstallCommand class will be invoked.