Python xmlparse_setattro() Type Confusion

Vulnerabilities | Repro | Patch | Report

Python 3.4 and 3.5 suffer from a vulnerability caused by the behavior of the xmlparse_setattro() function. When called, the function uses the provided name argument in several conditional statements which assume that the name argument is a string.

However, if a name argument is provided that is not a string, this logic will make several calls to PyUnicode_CompareWithASCIIString that expect a string, yet receive some other type of object, leading to a type confusion vulnerability:

static int
xmlparse_setattro(xmlparseobject *self, PyObject *name, PyObject *v)
    /* Set attribute 'name' to value 'v'. v==NULL means delete */
    if (v == NULL) {
        PyErr_SetString(PyExc_RuntimeError, "Cannot delete attribute");
        return -1;
    if (PyUnicode_CompareWithASCIIString(name, "buffer_text") == 0) {

In some applications, it may be possible to exploit this behavior to achieve arbitrary code execution. The type confusion can be observed by running the following script:

from xml.parsers.expat import *
p = ParserCreate()
p.__setattr__(range(0xF), 0)

Which, depending on the arrangement of memory, may produce an exception such as this:

0:000> g
(d84.ce0): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=0086f904 ebx=0086f8fc ecx=0050005c edx=00b60138 esi=0050005e edi=00b60138
eip=61e9a967 esp=0086f8c8 ebp=0086f8e0 iopl=0 nv up ei ng nz na pe cy
cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010287
61e9a967 0fb701 movzx eax,word ptr [ecx] ds:002b:0050005c=????
0:000> k3
ChildEBP RetAddr
0086f8e0 61e9aa35 python35!find_maxchar_surrogates+0x37 [c:\build\cpython\objects\unicodeobject.c @ 1417]
0086f908 61eabcf3 python35!_PyUnicode_Ready+0x35 [c:\build\cpython\objects\unicodeobject.c @ 1466]
0086f918 61c547c3 python35!PyUnicode_CompareWithASCIIString+0x13
[c:\build\cpython\objects\unicodeobject.c @ 10784]

To fix this issue, it is recommended that xmlparse_setattro() be updated to validate that the name argument is a string and return out of the function early if it is not. A proposed patch is attached.

Credit: John Leitch (john@autosectools.com), Bryce Darling (darlingbryce@gmail.com)

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