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Dangers in Python's standard library

The Python programming language comes with "Batteries Included". A philosophy to have a large and immediately useful standard library. However, since the standard library comes with Python it is hard to refactor for older code depends on it. Because of this the standard library can in many cases be quite far behind what is available in the ecosystem as a whole.

Some of the Python standard library is downright dangerous. By dangerous I mean that special care has to be taken when using certain functions. This article highlights some of the more well known issues with the standard library but is by no means a comprehensive list of everything that can go wrong nor a claim that the Python standard library is bad.

This article is only about modules in the standard library, dangerous syntax, types, and other language abilities are saved for another day. As always pay attention and try to think 'what does this actually do and can it be used in a different way'.

Don't consider this to be a definitive list at any point in time. The initial listed modules are popularly used.

pickle

Pickle is a module in Python land to serialize 'arbitrary' objects. It is often used when someone needs an easy way to send an objects' state elsewhere.

The Pickle module has many pitfalls. One of them is the fact that Pickled data is meant to run on only the same Python version, and while it might sometimes work on different versions (Pickle has a notion of a protocol version) its interoperability leaves some things to be desired

More damning is the fact that loading Pickled data allows for arbitrary code execution. If you load Pickled data from sources you cannot trust, and really can you ever? It is woeful. This combined with the fact that a serialization format has interoperability issues should be enough to steer well clear of it.

See the following example of unpickling some data, causing it to print hello.

import pickle

pickle.loads(b'\x80\x03cbuiltins\nprint\nq\x00X\x05\x00\x00\x00helloq\x01\x85q\x02Rq\x03.')

# hello

If you do ever need to exchange data use a format that does not allow for any smart things. A good option is to use JSON, which is available in the standard library. You would need to write some code to explicitly convert your objects to a format you are happy with, and some code to explicitly convert some serialized data back to your objects.

There are libraries to help you with this such as marshmallow.

os.system, subprocess

os.system is inherently not safe with any user input and is likely not a function you want to ever use due to its working. You would want to use the subprocess.* functions and those come with a manual to use securely.

Seriously scrutinize any use of these classes of functions as they lead to mistakes, most of which are easy to turn against you.

shlex

shlex was a suggestion of mine in one of my previous articles. It is however commonly used improperly. Consider if the following is the splitting you want:

import shlex
import subprocess

user_input = shlex.split("foo;echo${IFS}hello")[0]
command = "echo {}".format(user_input)
subprocess.check_output(command, shell=True)

# b'foo\nhello\n'

The above shows a mismatch between what shlex thinks are separators for a shell and what the actual outcome is. Someone slightly familiar with shlex might assume that a shlex split always makes sure only a single argument is possible.

shlex can give a false sense of security if you are not absolutely certain you know how shells work and what is in your input. Since you can never be certain of the latter you should prefer to work around having to use it.

re

Regular expressions, you think you know them and now you have two problems. While a powerful language that likely doesn't lead to direct exploitation one does have to take care when writing these.

A possible attack is a denial of service by turning your own regular expressions against you. Note the timings below.

>>> timeit.timeit("import re;re.match('^(a+)+$', '{}!')".format("a" * 1))
1.4856852590019116
>>> timeit.timeit("import re;re.match('^(a+)+$', '{}!')".format("a" * 8))
40.852224354999635

xml

XML, or eXtensible Markup Language is a format commonly (or less commonly in current times) used to exchange data between different systems or for general data serialization. XML is extremely flexible with a lot of knobs, this has also led to a large amount of flaws possible in certain implementations.

This is well documented at the Python documentation website on xml.

The excellent defusedxml package written by Christian Heimes has an amazing README explaining all the issues, and has patches to make the standard Python libraries and some other libraries less vulnerable. Read the description on PyPI.

Any use of the built-in xml libraries should be scrutinized and where possible be replaced with lxml. lxml is a binding to libxml2 which comes with generally secure defaults and a network sandbox.

random

The default random module in Python will use a predictable random number generator. If you use it for anything that is supposed to be secret please use random.SecureRandom().

It is a good idea to always use SecureRandom unless you are certain you don't need it instead of assuming the reverse.