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…

• Eval really is dangerous (Ned Batchelder, June 2012)

• PyPy sandboxing

• For Linux, search for SECCOMP

Archives and absolute paths¶

• 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.

Archives and Zip Bomb (CVE-2019-9674)¶

Be careful of “Zip Bombs”: a very small archive can use a huge amount of memory and disk space once decompressed.

The zlib module allows to limit the maximum length: https://docs.python.org/dev/library/zlib.html#zlib.Decompress.decompress

For example, the OpenStack Nova was vulnerable of denial of service if a compressed virtual machine was a Zip Bomb: OSSA 2016-012 and CVE-2015-5162.

Turns out qemu image parser is not hardened against malicious input and can be abused to allocated an arbitrary amount of memory and/or dump a lot of information when used with “–output=json”.

Nova has been fixed using the prlimit command (with one implementation written in Python: prlimit.py) to limit the maximum memory of the process.

See:

• zipfile: Decompression pitfalls.

• bpo-36260: [security] CVE-2019-9674: Zip Bomb vulnerability

• CVE-2019-9674

• Wikipedia: Zip bomb

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:\Python27\ directory 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.