Python Security

This page is an attempt to document security vulnerabilities in Python and the versions including the fix.

Pages

Python branches

  • (Latest update: 2017-03-28) Python 2.6, 3.0, 3.1, 3.2 don’t get security fixes anymore and so should be considered as vulnerable
  • Branches getting security fixes: 2.7, 3.3, 3.4 and 3.5
  • See Status of Python branches

Dangerous functions and modules

  • Python 2 input()
  • Python 2 execfile()
  • eval()
  • subprocess.Popen(shell=True)
  • str.format(), Python 3 str.format_map, and Python 2 unicode.format() all allow arbitrary attribute access on formatted values, and hence access to Python’s introspection features: Be Careful with Python’s New-Style String Format (Armin Ronacher, December 2016)
  • The pickle module executes arbitrary Python code: never use it with untrusted data.
  • archives:
    • tarfile: Never extract archives from untrusted sources without prior inspection. It is possible that files are created outside of path, e.g. members that have absolute filenames starting with “/” or filenames with two dots ”..”.
    • zipfile: Never extract archives from untrusted sources without prior inspection. It is possible that files are created outside of path, e.g. members that have absolute filenames starting with “/” or filenames with two dots ”..”. zipfile attempts to prevent that.

Security model

Bytecode

CPython doesn’t verify that bytecode is safe. If an attacker is able to execute arbitrary bytecode, we consider that the security of the bytecode is the least important issue: using bytecode, sensitive code can be imported and executed.

For example, the marshal doesn’t validate inputs.

Sandbox

Don’t try to build a sandbox inside CPython. The attack surface is too large. Python has many introspection features, see for example the inspect module. Python also many convenient features which executes code on demand. Examples:

  • the literal string '\N{Snowman}' imports the unicodedata module
  • the code to log a warning might be abused to execute code

The good design is to put CPython into a sandbox, not the opposite.

Ok, understood, but I want a sandbox in Python. Well...

RNG

The random module must not be used in security sensitive code, except of the random.SystemRandom class.

CPython Security Experts

  • Alex Gaynor
  • Antoine Pitrou
  • Christian Heimes
  • Donald Stufft

Windows

ASLR and DEP

ASLR and DEP protections enabled since Python 3.4 (and Python 2.7.11 if built using PCbuild/ directory).

Unsafe Python 2.7 default installation directory

Python 2.7 installer uses C:Python27directory by default. The created directory has the “Modify” access rights given to the “Authenticated Users” group. An attacker can modify the standard library or even modify python.exe. Python 3 installer now installs Python in “C:Program Files” by default to fix this issue. Override the default installation directory, or fix the directory permissions.

DLL injection

On Windows 8.1 and older, the installer is vulnerable to DLL injection: evil DLL written in the same download directory that the downloaded Python installer. See DLL Hijacking Just Won’t Die.

DLL injection using PATH

Inject a malicious DLL in a writable directory included in PATH. The “pip” step of the Python installer will run this DLL.

We consider that it is not an issue of Python (Python installer) itself.

Once you have write access to a directory on the system PATH (not the current user PATH) and the ability to write binaries that are not validated by the operating system before loading, there are many more interesting things you can do rather than wait for the Python installer to be run.

Misc

Python Security Response Team (PSRT)