Thursday, September 14, 2006

Python pickle and web framework security.

Some python web frame works are using pickle to store session data. Pickle is a well known poor choice for secure systems. However it seems to be more widely known by those writing network applications, than those making web frameworks.

Is your web framework using pickle for sessions despite the warnings in the python documentation about it being insecure?

By using sessions with pickle people who can write to the database servers session table can execute code on the app server. Or people who can get data into the session file/memcache data store can execute data.

This might be an issue if the database server is run by separate people than the app server. Or if the session table is compromised by an sql injection attack elsewhere.

There are some more secure ways of storing pickled data.

Pickle is deemed to be untrustworthy for data. In that it is not certain that code can not be snuck into the data that will be executed by pickle. So if some data from user input is put into the pickle, then it is possible that code could be run.

There are some people who know more about how to exploit pickle, however the warning in the python documentation is this:

""Warning:
The pickle module is not intended to be secure against erroneous or maliciously constructed data. Never unpickle data received from an untrusted or unauthenticated source."""


Cerealizer might be an alternative option...
http://home.gna.org/oomadness/en/cerealizer/index.html

Or maybe these other two.
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/415503
http://barnesc.blogspot.com/2006/01/rencode-reduced-length-encodings.html