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static PyObject *
partial_setstate(partialobject *pto, PyObject *state)
{
 PyObject *fn, *fnargs, *kw, *dict;
 if (!PyArg_ParseTuple(state, "OOOO",
 &fn, &fnargs, &kw, &dict))
 return NULL;
 Py_XDECREF(pto->fn);
 Py_XDECREF(pto->args);
 Py_XDECREF(pto->kw);
 Py_XDECREF(pto->dict);
 pto->fn = fn;
 pto->args = fnargs; //we control pto->args here
`partial_setstate` performs no checks on the objects
it is passed as an argument.
static PyObject *
partial_repr(partialobject *pto)
{
 PyObject *result;
 PyObject *arglist;
 PyObject *tmp;
 Py_ssize_t i, n;
 arglist = PyUnicode_FromString("");
 if (arglist == NULL) {
 return NULL;
 }
 /* Pack positional arguments */
 assert (PyTuple_Check(pto->args)); //not compiled in release build
 n = PyTuple_GET_SIZE(pto->args);
 for (i = 0; i < n; i++) {
 tmp = PyUnicode_FromFormat("%U, %R", arglist,
 PyTuple_GET_ITEM(pto->args, i));
In partial_repr, `pto->args` is assumed to be a tuple and
unsafe functions `PyTuple_GET_SIZE` and `PyTuple_GET_ITEM`
are called on `pto->args`. This bug is particularly bad
because `PyUnicode_FromFormat` will call the object's repr
function. In this case, the attacker gains complete control
over the program counter.
vagrant@vagrant-ubuntu-wily-64:/vagrant/Python-3.5.1$ gdb -q ./python.exe
...
(gdb) r partialpoc.py
Starting program: /vagrant/Python-3.5.1/python.exe partialpoc.py
...
Program received signal SIGSEGV, Segmentation fault.
0x00000000004851f6 in PyObject_Repr (v=0x972c90) at Objects/object.c:482
482	 res = (*v->ob_type->tp_repr)(v);
(gdb) i r
rax 0x4141414141414141	4702111234474983745
rbx 0x972c90	9907344
rcx 0x52	82
rdx 0x7ffff7026718	140737337517848
rsi 0x0	0
rdi 0x972c90	9907344
rbp 0x6666666666666667	0x6666666666666667
rsp 0x7fffffffdb60	0x7fffffffdb60
r8 0x0	0
r9 0x6049a8	6310312
r10 0xffffffffffffffff	-1
r11 0xffffffffffffffff	-1
r12 0x7fffffffffffffff	9223372036854775807
r13 0x7fffffffdbe0	140737488346080
r14 0x6049a7	6310311
r15 0x0	0
rip 0x4851f6	0x4851f6 <PyObject_Repr+38>
eflags 0x10206	[ PF IF RF ]
cs 0x33	51
ss 0x2b	43
ds 0x0	0
es 0x0	0
fs 0x0	0
gs 0x0	0
(gdb) x/3i $pc
=> 0x4851f6 <PyObject_Repr+38>:	callq *%rax
 0x4851f8 <PyObject_Repr+40>:	test %rax,%rax
 0x4851fb <PyObject_Repr+43>:	mov %rax,%rbx
Please see the attached POC.

Merged with issue25943. This is the same bug.

I presume you meant Issue 25945, also about partial_setstate(). Issue 25943 is about the bsddb module.

Yes, thank you Martin.