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; } assert(PyUnicode_Check(name)); 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 python35!find_maxchar_surrogates+0x37: 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 (johnleitch@outlook.com), Bryce Darling (darlingbryce@gmail.com)

New changeset ff2c4f281720 by Serhiy Storchaka in branch '3.4': Issue #25019: Fixed a crash caused by setting non-string key of expat parser. https://hg.python.org/cpython/rev/ff2c4f281720 New changeset 6006231dcaae by Serhiy Storchaka in branch '3.5': Issue #25019: Fixed a crash caused by setting non-string key of expat parser. https://hg.python.org/cpython/rev/6006231dcaae New changeset edf25acae637 by Serhiy Storchaka in branch 'default': Issue #25019: Fixed a crash caused by setting non-string key of expat parser. https://hg.python.org/cpython/rev/edf25acae637

Thank you for your contribution John. The committed patch slightly differs from the proposed patch. Error message now is the same as in setattr() and general __setattr__(). Tests are moved to existing test class for testing of attribute setting. Improved tests for valid attributes.