Diary of a reverse-engineer

Because we like to play with weird things.

Dissection of Quarkslab's 2014 Security Challenge

Introduction

As the blog was a bit silent for quite some time, I figured it would be cool to put together a post ; so here it is folks, dig in!

The French company Quarkslab recently released a security challenge to win a free entrance to attend the upcoming HITBSecConf conference in Kuala Lumpur from the 13th of October until the 16th.

The challenge has been written by Serge Guelton, a R&D engineer specialized in compilers/parallel computations. At the time of writing, already eight different people manage to solve the challenge, and one of the ticket seems to have been won by hackedd, so congrats to him!

According to the description of the challenge Python is heavily involved, which is a good thing for at least two reasons:

In this post I will describe how I tackled this problem, how I managed to solve it. And to make up for me being slow at solving it I tried to make it fairly detailed.

At first it was supposed to be quite short though, but well..I decided to analyze fully the challenge even if it wasn’t needed to find the key unfortunately, so it is a bit longer than expected :–).

Anyway, sit down, make yourself at home and let me pour you a cup of tea before we begin :–).

Finding the URL of the challenge

Very one-liner, much lambdas, such a pain

The first part of the challenge is to retrieve an url hidden in the following Python one-liner:

Very one-liner, much lambdas
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(lambda g, c, d: (lambda _: (_.__setitem__('$', ''.join([(_['chr'] if ('chr'
in _) else chr)((_['_'] if ('_' in _) else _)) for _['_'] in (_['s'] if ('s'
in _) else s)[::(-1)]])), _)[-1])( (lambda _: (lambda f, _: f(f, _))((lambda
__,_: ((lambda _: __(__, _))((lambda _: (_.__setitem__('i', ((_['i'] if ('i'
in _) else i) + 1)),_)[(-1)])((lambda _: (_.__setitem__('s',((_['s'] if ('s'
in _) else s) + [((_['l'] if ('l' in _) else l)[(_['i'] if ('i' in _) else i
)] ^ (_['c'] if ('c' in _) else c))])), _)[-1])(_))) if (((_['g'] if ('g' in
_) else g) % 4) and ((_['i'] if ('i' in _) else i)< (_['len'] if ('len' in _
) else len)((_['l'] if ('l' in _) else l)))) else _)), _) ) ( (lambda _: (_.
__setitem__('!', []), _.__setitem__('s', _['!']), _)[(-1)] ) ((lambda _: (_.
__setitem__('!', ((_['d'] if ('d' in _) else d) ^ (_['d'] if ('d' in _) else
d))), _.__setitem__('i', _['!']), _)[(-1)])((lambda _: (_.__setitem__('!', [
(_['j'] if ('j' in _) else j) for  _[ 'i'] in (_['zip'] if ('zip' in _) else
zip)((_['l0'] if ('l0' in _) else l0), (_['l1'] if ('l1' in _) else l1)) for
_['j'] in (_['i'] if ('i' in _) else i)]), _.__setitem__('l', _['!']), _)[-1
])((lambda _: (_.__setitem__('!', [1373, 1281, 1288, 1373, 1290, 1294, 1375,
1371,1289, 1281, 1280, 1293, 1289, 1280, 1373, 1294, 1289, 1280, 1372, 1288,
1375,1375, 1289, 1373, 1290, 1281, 1294, 1302, 1372, 1355, 1366, 1372, 1302,
1360, 1368, 1354, 1364, 1370, 1371, 1365, 1362, 1368, 1352, 1374, 1365, 1302
]), _.__setitem__('l1',_['!']), _)[-1])((lambda _: (_.__setitem__('!',[1375,
1368, 1294, 1293, 1373, 1295, 1290, 1373, 1290, 1293, 1280, 1368, 1368,1294,
1293, 1368, 1372, 1292, 1290, 1291, 1371, 1375, 1280, 1372, 1281, 1293,1373,
1371, 1354, 1370, 1356, 1354, 1355, 1370, 1357, 1357, 1302, 1366, 1303,1368,
1354, 1355, 1356, 1303, 1366, 1371]), _.__setitem__('l0', _['!']), _)[(-1)])
            ({ 'g': g, 'c': c, 'd': d, '$': None})))))))['$'])

I think that was the first time I was seeing obfuscated Python and believe me I did a really strange face when seeing that snippet. But well, with a bit of patience we should manage to get a better understanding of how it is working, let’s get to it!

Tidying up the last one..

Before doing that here are things we can directly observe just by looking closely at the snippet:

  • We know this function has three arguments ; we don’t know them at this point though
  • The snippet seems to reuse __setitem__ quite a lot ; it may mean two things for us:
    • The only standard Python object I know of with a __setitem__ function is dictionary,
    • The way the snippet looks like, it seems that once we will understand one of those __setitem__ call, we will understand them all
  • The following standard functions are used: chr, len, zip
    • That means manipulation of strings, integers and iterables
  • There are two noticeable operators: mod and xor

With all that information in our sleeve, the first thing I did was to try to clean it up, starting from the last lambda in the snippet. It gives something like:

Last lambda cleaned
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tab0 = [
    1375, 1368, 1294, 1293, 1373, 1295, 1290, 1373, 1290, 1293,
    1280, 1368, 1368, 1294, 1293, 1368, 1372, 1292, 1290, 1291,
    1371, 1375, 1280, 1372, 1281, 1293, 1373, 1371, 1354, 1370,
    1356, 1354, 1355, 1370, 1357, 1357, 1302, 1366, 1303, 1368,
    1354, 1355, 1356, 1303, 1366, 1371
]

z = lambda x: (
    x.__setitem__('!', tab0),
    x.__setitem__('l0', x['!']),
    x
)[-1]

That lambda takes a dictionary x, sets two items, generates a tuple with a reference to the dictionary at the end of the tuple ; finally the lambda is going to return that same dictionary. It also uses x[‘!’] as a temporary variable to then assign its value to x[‘l0’].

Long story short, it basically takes a dictionary, updates it and returns it to the caller: clever trick to pass that same object across lambdas. We can also see that easily in Python directly:

lambda, dictionary & setitem
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In [8]: d = {}
In [9]: z(d)
Out[9]:
{'!': [1375,
  ...
 'l0': [1375,
  ...
}

That lambda is even called with a dictionary that will contain, among other things, the three user controlled variable: g, c, d. That dictionary seems to be some kind of storage used to keep track of all the variables that will be used across those lambdas.

lambda & the resulting dictionary
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# Returns { 'g' : g, 'c', 'd': d, '$':None, '!':tab0, 'l0':tab0}
last_res = (
    (
        lambda x: (
            x.__setitem__('!', tab0),
            x.__setitem__('l0', x['!']),
            x
        )[-1]
    )
    ({ 'g': g, 'c': c, 'd': d, '$': None})
)

..then the one before…

Now if we repeat that same operation with the one before the last lambda, we have the exact same pattern:

lambda before the last one
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tab1 = [
    1373, 1281, 1288, 1373, 1290, 1294, 1375, 1371, 1289, 1281,
    1280, 1293, 1289, 1280, 1373, 1294, 1289, 1280, 1372, 1288,
    1375, 1375, 1289, 1373, 1290, 1281, 1294, 1302, 1372, 1355,
    1366, 1372, 1302, 1360, 1368, 1354, 1364, 1370, 1371, 1365,
    1362, 1368, 1352, 1374, 1365, 1302
]

zz = lambda x: (
    x.__setitem__('!', tab1),
    x.__setitem__('l1', x['!']),
    x
)[-1]

Perfect, now let’s repeat the same operations over and over again. At some point, the whole thing becomes crystal clear (sort-of):

cleaned nested lambdas
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# Returns { 
  # 'g':g, 'c':c, 'd':d,
  # '!':[],
  # 's':[],
  # 'l':[j for i in zip(tab0, tab1) for j in i],
  # 'l1':tab1,
  # 'l0':tab0,
  # 'i': 0,
  # 'j': 1302,
  # '$':None
#}
res_after_all_operations = (
  (
    lambda x: (
        x.__setitem__('!', []),
        x.__setitem__('s', x['!']),
        x
    )[-1]
  )
  # ..
  (
    (
      lambda x: (
          x.__setitem__('!', ((x['d'] if ('d' in x) else d) ^ (x['d'] if ('d' in x) else d))),
          x.__setitem__('i', x['!']),
          x
      )[-1]
    )
    # ..
    (
      (
        lambda x: (
            x.__setitem__('!', [(x['j'] if ('j' in x) else j) for x[ 'i'] in (x['zip'] if ('zip' in x) else zip)((x['l0'] if ('l0' in x) else l0), (x['l1'] if ('l1' in x) else l1)) for x['j'] in (x['i'] if ('i' in x) else i)]),
            x.__setitem__('l', x['!']),
            x
        )[-1]
      )
      # Returns { 'g':g, 'c':c, 'd':d, '!':tab1, 'l1':tab1, 'l0':tab0, '$':None}
      (
        (
          lambda x: (
              x.__setitem__('!', tab1),
              x.__setitem__('l1', x['!']),
              x
          )[-1]
        )
        # Return { 'g' : g, 'c', 'd': d, '!':tab0, 'l0':tab0, '$':None }
        (
          (
            lambda x: (
                x.__setitem__('!', tab0),
                x.__setitem__('l0', x['!']),
                x
            )[-1]
          )
          ({ 'g': g, 'c': c, 'd': d, '$': None})
        )
      )
    )
  )
)

Putting it all together

After doing all of that, we know now the types of the three variables the function needs to work properly (and we don’t really need more to be honest):

  • g is an integer that will be mod 4
    • if the value is divisible by 4, the function returns nothing ; so we will need to have this variable sets to 1 for example
  • c is another integer that looks like a xor key ; if we look at the snippet, this variable is used to xor each byte of x[‘l’] (which is the table with tab0 and tab1)
    • this is the interesting parameter
  • d is another integer that we can also ignore: it’s only used to set x[‘i’] to zero by xoring x[’d’] by itself.

We don’t need anything else really now: no more lambdas, no more pain, no more tears. It is time to write what I call, an educated brute-forcer, to find the correct value of c:

bf_with_lambdas_cleaned.py
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import sys

def main(argc, argv):
    tab0 = [1375, 1368, 1294, 1293, 1373, 1295, 1290, 1373, 1290, 1293, 1280, 1368, 1368,1294, 1293, 1368, 1372, 1292, 1290, 1291, 1371, 1375, 1280, 1372, 1281, 1293,1373, 1371, 1354, 1370, 1356, 1354, 1355, 1370, 1357, 1357, 1302, 1366, 1303,1368, 1354, 1355, 1356, 1303, 1366, 1371]
    tab1 = [1373, 1281, 1288, 1373, 1290, 1294, 1375, 1371,1289, 1281, 1280, 1293, 1289, 1280, 1373, 1294, 1289, 1280, 1372, 1288, 1375,1375, 1289, 1373, 1290, 1281, 1294, 1302, 1372, 1355, 1366, 1372, 1302, 1360, 1368, 1354, 1364, 1370, 1371, 1365, 1362, 1368, 1352, 1374, 1365, 1302]

    func = (
        lambda g, c, d:
        (
            lambda x: (
                x.__setitem__('$', ''.join([(x['chr'] if ('chr' in x) else chr)((x['_'] if ('_' in x) else x)) for x['_'] in (x['s'] if ('s' in x) else s)[::-1]])),
                x
            )[-1]
        )
        (
            (
                lambda x:
                    (lambda f, x: f(f, x))
                (
                    (
                        lambda __, x:
                        (
                            (lambda x: __(__, x))
                            (
                                # i += 1
                                (
                                    lambda x: (
                                        x.__setitem__('i', ((x['i'] if ('i' in x) else i) + 1)),
                                        x
                                    )[-1]
                                )
                                (
                                    # s += [c ^ l[i]]
                                    (
                                        lambda x: (
                                            x.__setitem__('s', (
                                                    (x['s'] if ('s' in x) else s) +
                                                    [((x['l'] if ('l' in x) else l)[(x['i'] if ('i' in x) else i)] ^ (x['c'] if ('c' in x) else c))]
                                                )
                                            ),
                                            x
                                        )[-1]
                                    )
                                    (x)
                                )
                            )
                            # if ((x['g'] % 4) and (x['i'] < len(l))) else x
                            if (((x['g'] if ('g' in x) else g) % 4) and ((x['i'] if ('i' in x) else i)< (x['len'] if ('len' in x) else len)((x['l'] if ('l' in x) else l))))
                            else x
                        )
                    ),
                    x
                )
            )
            # Returns { 'g':g, 'c':c, 'd':d, '!':zip(tab1, tab0), 'l':zip(tab1, tab0), l1':tab1, 'l0':tab0, 'i': 0, 'j': 1302, '!':0, 's':[] }
            (
                (
                    lambda x: (
                        x.__setitem__('!', []),
                        x.__setitem__('s', x['!']),
                        x
                    )[-1]
                )
                # Returns { 'g':g, 'c':c, 'd':d, '!':zip(tab1, tab0), 'l':zip(tab1, tab0), l1':tab1, 'l0':tab0, 'i': 0, 'j': 1302, '!':0}
                (
                    (
                        lambda x: (
                            x.__setitem__('!', ((x['d'] if ('d' in x) else d) ^ (x['d'] if ('d' in x) else d))),
                            x.__setitem__('i', x['!']),
                            x
                        )[-1]
                    )
                    # Returns { 'g' : g, 'c', 'd': d, '!':zip(tab1, tab0), 'l':zip(tab1, tab0), l1':tab1, 'l0':tab0, 'i': (1371, 1302), 'j': 1302}
                    (
                        (
                            lambda x: (
                                x.__setitem__('!', [(x['j'] if ('j' in x) else j) for x[ 'i'] in (x['zip'] if ('zip' in x) else zip)((x['l0'] if ('l0' in x) else l0), (x['l1'] if ('l1' in x) else l1)) for x['j'] in (x['i'] if ('i' in x) else i)]),
                                x.__setitem__('l', x['!']),
                                x
                            )[-1]
                        )
                        # Returns { 'g' : g, 'c', 'd': d, '!':tab1, 'l1':tab1, 'l0':tab0}
                        (
                            (
                                lambda x: (
                                    x.__setitem__('!', tab1),
                                    x.__setitem__('l1', x['!']),
                                    x
                                )[-1]
                            )
                            # Return { 'g' : g, 'c', 'd': d, '!' : tab0, 'l0':tab0}
                            (
                                (
                                    lambda x: (
                                        x.__setitem__('!', tab0),
                                        x.__setitem__('l0', x['!']),
                                        x
                                    )[-1]
                                )
                                ({ 'g': g, 'c': c, 'd': d, '$': None})
                            )
                        )
                    )
                )
            )
        )['$']
    )

    for i in range(0x1000):
        try:
            ret = func(1, i, 0)
            if 'quarks' in ret:
                print ret
        except:
            pass
    return 1

if __name__ == '__main__':
    sys.exit(main(len(sys.argv), sys.argv))

And after running it, we are good to go:

w00tw00t
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D:\Codes\challenges\ql-python>bf_with_lambdas_cleaned.py
/blog.quarkslab.com/static/resources/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf

A custom ELF64 Python interpreter you shall debug

Recon

All right, here we are: we now have the real challenge. First, let’s see what kind of information we get for free:

recon
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overclok@wildout:~/chall/ql-py$ file b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf: ELF 64-bit LSB  executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs),
for GNU/Linux 2.6.26, not stripped
overclok@wildout:~/chall/ql-py$ ls -lah b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
-rwxrw-r-x 1 overclok overclok 7.9M Sep  8 21:03 b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf

The binary is quite big, not good for us. But on the other hand, the binary isn’t stripped so we might find useful debugging information at some point.

./b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
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overclok@wildout:~/chall/ql-py$ /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
Python 2.7.8+ (nvcs/newopcodes:a9bd62e4d5f2+, Sep  1 2014, 11:41:46)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

That does explain the size of the binary then: we basically have something that looks like a custom Python interpreter. Note that I also remembered reading Building an obfuscated Python interpreter: we need more opcodes on Quarkslab’s blog where Serge described how you could tweak the interpreter sources to add / change some opcodes either for optimization or obfuscation purposes.

Finding the interesting bits

The next step is to figure out what part of the binary is interesting, what functions have been modified, and where we find the problem we need to solve to get the flag. My idea for that was to use a binary-diffing tool between an original Python278 interpreter and the one we were given.

To do so I just grabbed Python278’s sources and compiled them by myself:

compiling Py278
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overclok@wildout:~/chall/ql-py$ wget https://www.python.org/ftp/python/2.7.8/Python-2.7.8.tgz && tar xzvf Python-2.7.8.tgz
overclok@wildout:~/chall/ql-py$ tar xzvf Python-2.7.8.tgz
overclok@wildout:~/chall/ql-py$ cd Python-2.7.8/ && ./configure && make
overclok@wildout:~/chall/ql-py/Python-2.7.8$ ls -lah ./python
-rwxrwxr-x 1 overclok overclok 8.0M Sep  5 00:13 ./python

The resulting binary has a similar size, so it should do the job even if I’m not using GCC 4.8.2 and the same compilation/optimization options. To perform the diffing I used IDA Pro and Patchdiff v2.0.10.

Patchdiff result
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---------------------------------------------------
PatchDiff Plugin v2.0.10
Copyright (c) 2010-2011, Nicolas Pouvesle
Copyright (C) 2007-2009, Tenable Network Security, Inc
---------------------------------------------------

Scanning for functions ...
parsing second idb...
parsing first idb...
diffing...
Identical functions:   2750
Matched functions:     176
Unmatched functions 1: 23
Unmatched functions 2: 85
done!

Once the tool has finished its analysis we just have to check the list of unmatched function names (around one hundred of them, so it’s pretty quick), and eventually we see that:

That function directly caught my eyes (you can even check it doesn’t exist in the Python278 source tree obviously :–)), and it appears this function is just setting up a Python module called do_not_run_me.

Let’s import it:

do_not_run_me module
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overclok@wildout:~/chall/ql-py$ /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
iPython 2.7.8+ (nvcs/newopcodes:a9bd62e4d5f2+, Sep  1 2014, 11:41:46)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import do_not_run_me
>>> print do_not_run_me.__doc__
None
>>> dir(do_not_run_me)
['__doc__', '__name__', '__package__', 'run_me']
>>> print do_not_run_me.run_me.__doc__
There are two kinds of people in the world: those who say there is no such thing as infinite recursion, and those who say ``There are two kinds of people in the world: those who say there is no such thing as infinite recursion, and those who say ...
>>> do_not_run_me.run_me('doar-e')
Segmentation fault

All right, we now have something to look at and we are going to do so from a low level point of view because that’s what I like ; so don’t expect big/magic hacks here :).

If you are not really familiar with Python’s VM structures I would advise you to read quickly through this article Deep Dive Into Python’s VM: Story of LOAD_CONST Bug, and you should be all set for the next parts.

do_not_run_me.run_me

The function is quite small, so it should be pretty quick to analyze:

  1. the first part makes sure that we pass a string as an argument when calling run_me,
  2. then a custom marshaled function is loaded, a function is created out of it, and called,
  3. after that it creates another function from the string we pass to the function (which explains the segfault just above),
  4. finally, a last function is created from another hardcoded marshaled string.

First marshaled function

To understand it we have to dump it first, to unmarshal it and to analyze the resulting code object:

unmarshaling the first function
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overclok@wildout:~/chall/ql-py$ gdb -q /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
Reading symbols from /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf...done.
gdb$ set disassembly-flavor intel
gdb$ disass run_me
Dump of assembler code for function run_me:
   0x0000000000513d90 <+0>:     push   rbp
   0x0000000000513d91 <+1>:     mov    rdi,rsi
   0x0000000000513d94 <+4>:     xor    eax,eax
   0x0000000000513d96 <+6>:     mov    esi,0x56c70b
   0x0000000000513d9b <+11>:    push   rbx
   0x0000000000513d9c <+12>:    sub    rsp,0x28
   0x0000000000513da0 <+16>:    lea    rcx,[rsp+0x10]
   0x0000000000513da5 <+21>:    mov    rdx,rsp

   ; Parses the arguments we gave, it expects a string object
   0x0000000000513da8 <+24>:    call   0x4cf430 <PyArg_ParseTuple>
   0x0000000000513dad <+29>:    xor    edx,edx
   0x0000000000513daf <+31>:    test   eax,eax
   0x0000000000513db1 <+33>:    je     0x513e5e <run_me+206>

   0x0000000000513db7 <+39>:    mov    rax,QWORD PTR [rip+0x2d4342]
   0x0000000000513dbe <+46>:    mov    esi,0x91
   0x0000000000513dc3 <+51>:    mov    edi,0x56c940
   0x0000000000513dc8 <+56>:    mov    rax,QWORD PTR [rax+0x10]
   0x0000000000513dcc <+60>:    mov    rbx,QWORD PTR [rax+0x30]

   ; Creates a code object from the marshaled string
   ; PyObject* PyMarshal_ReadObjectFromString(char *string, Py_ssize_t len)
   0x0000000000513dd0 <+64>:    call   0x4dc020 <PyMarshal_ReadObjectFromString>
   0x0000000000513dd5 <+69>:    mov    rdi,rax
   0x0000000000513dd8 <+72>:    mov    rsi,rbx

   ; Creates a function object from the marshaled string
   0x0000000000513ddb <+75>:    call   0x52c630 <PyFunction_New>
   0x0000000000513de0 <+80>:    xor    edi,edi
[...]
gdb$ r -c 'import do_not_run_me as v; v.run_me("")'
Starting program: /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf -c 'import do_not_run_me as v; v.run_me("")'
[...]

To start, we can set two software breakpoints @0x0000000000513dd0 and @0x0000000000513dd5 to inspect both the marshaled string and the resulting code object.

Just a little reminder though on the Linux/x64 ABI: “The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9”.

unmarshaled string inspection
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gdb$ p /x $rsi
$2 = 0x91
gdb$ x/145bx $rdi
0x56c940 <+00>:  0x63    0x00    0x00    0x00    0x00    0x01    0x00    0x00
0x56c948 <+08>:  0x00    0x02    0x00    0x00    0x00    0x43    0x00    0x00
0x56c950 <+16>:  0x00    0x73    0x14    0x00    0x00    0x00    0x64    0x01
0x56c958 <+24>:  0x00    0x87    0x00    0x00    0x7c    0x00    0x00    0x64
0x56c960 <+32>:  0x01    0x00    0x3c    0x61    0x00    0x00    0x7c    0x00
0x56c968 <+40>:  0x00    0x1b    0x28    0x02    0x00    0x00    0x00    0x4e
0x56c970 <+48>:  0x69    0x01    0x00    0x00    0x00    0x28    0x01    0x00
0x56c978 <+56>:  0x00    0x00    0x74    0x04    0x00    0x00    0x00    0x54
0x56c980 <+64>:  0x72    0x75    0x65    0x28    0x01    0x00    0x00    0x00
0x56c988 <+72>:  0x74    0x0e    0x00    0x00    0x00    0x52    0x6f    0x62
0x56c990 <+80>:  0x65    0x72    0x74    0x5f    0x46    0x6f    0x72    0x73
0x56c998 <+88>:  0x79    0x74    0x68    0x28    0x00    0x00    0x00    0x00
0x56c9a0 <+96>:  0x28    0x00    0x00    0x00    0x00    0x73    0x10    0x00
0x56c9a8 <+104>: 0x00    0x00    0x6f    0x62    0x66    0x75    0x73    0x63
0x56c9b0 <+112>: 0x61    0x74    0x65    0x2f    0x67    0x65    0x6e    0x2e
0x56c9b8 <+120>: 0x70    0x79    0x74    0x03    0x00    0x00    0x00    0x66
0x56c9c0 <+128>: 0x6f    0x6f    0x05    0x00    0x00    0x00    0x73    0x06
0x56c9c8 <+136>: 0x00    0x00    0x00    0x00    0x01    0x06    0x02    0x0a
0x56c9d0 <+144>: 0x01

And obviously you can’t use the Python marshal module to load & inspect the resulting object as the author seems to have removed the methods loads and dumps:

fuu
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overclok@wildout:~/chall/ql-py$ /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
Python 2.7.8+ (nvcs/newopcodes:a9bd62e4d5f2+, Sep  1 2014, 11:41:46)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import marshal
>>> dir(marshal)
['__doc__', '__name__', '__package__', 'version']

We could still try to run the marshaled string in our fresh compiled original Python though:

unmarshal in an original Python278
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>>> import marshal
>>> part_1 = marshal.loads('c\x00\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00C\x00\x00\x00s\x14\x00\x00\x00d\x01\x00\x87\x00\x00|\x00\x00d\x01\x00<a\x00\x00|\x00\x00\x1b(\x02\x00\x00\x00Ni\x01\x00\x00\x00(\x01\x00\x00\x00t\x04\x00\x00\x00True(\x01\x00\x00\x00t\x0e\x00\x00\x00Robert_Forsyth(\x00\x00\x00\x00(\x00\x00\x00\x00s\x10\x00\x00\x00obfuscate/gen.pyt\x03\x00\x00\x00foo\x05\x00\x00\x00s\x06\x00\x00\x00\x00\x01\x06\x02\n\x01')
>>> part_1.co_code
'd\x01\x00\x87\x00\x00|\x00\x00d\x01\x00<a\x00\x00|\x00\x00\x1b'
>>> part_1.co_varnames
('Robert_Forsyth',)
>>> part_1.co_names
('True',)

We can also go further by trying to create a function out of this code object, to call it and/or to disassemble it even:

fuu2
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>>> from types import FunctionType
>>> def a():
...     pass
...
>>> f = FunctionType(part_1, a.func_globals)
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "obfuscate/gen.py", line 8, in foo
UnboundLocalError: local variable 'Robert_Forsyth' referenced before assignment
>>> import dis
>>> dis.dis(f)
  6           0 LOAD_CONST               1 (1)
              3 LOAD_CLOSURE             0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/overclok/chall/ql-py/Python-2.7.8/Lib/dis.py", line 43, in dis
    disassemble(x)
  File "/home/overclok/chall/ql-py/Python-2.7.8/Lib/dis.py", line 107, in disassemble
    print '(' + free[oparg] + ')',
IndexError: tuple index out of range

Introducing dpy.py

All right, as expected this does not work at all: seems like the custom interpreter uses different opcodes which the original virtual CPU doesn’t know about. Anyway, let’s have a look at this object directly from memory because we like low level things (remember?):

inspecting the code object created
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gdb$ p *(PyObject*)$rax
$3 = {ob_refcnt = 0x1, ob_type = 0x7d3da0 <PyCode_Type>}

; Ok it is a code object, let's dump entirely the object now
gdb$ p *(PyCodeObject*)$rax
$4 = {
  ob_refcnt = 0x1,
  ob_type = 0x7d3da0 <PyCode_Type>,
  co_argcount = 0x0, co_nlocals = 0x1, co_stacksize = 0x2, co_flags = 0x43,
  co_code = 0x7ffff7f09df0,
  co_consts = 0x7ffff7ee2908,
  co_names = 0x7ffff7f8e390,
  co_varnames = 0x7ffff7f09ed0,
  co_freevars = 0x7ffff7fa7050, co_cellvars = 0x7ffff7fa7050,
  co_filename = 0x7ffff70a9b58,
  co_name = 0x7ffff7f102b0,
  co_firstlineno = 0x5,
  co_lnotab = 0x7ffff7e59900,
  co_zombieframe = 0x0,
  co_weakreflist = 0x0
}

Perfect, and you can do that for every single field of this structure:

  • to dump the bytecode,
  • the constants used,
  • the variable names,
  • etc.

Yes, this is annoying, very much so. That is exactly why there is dpy, a GDB Python command I wrote to dump Python objects in a much easy way directly from memory:

show-casing dpy
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gdb$ r
Starting program: /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
[...]
>>> a = { 1 : [1,2,3], 'two' : 31337, 3 : (1,'lul', [3,4,5])}
>>> print hex(id(a))
0x7ffff7ef1050
>>> ^C
Program received signal SIGINT, Interrupt.
gdb$ dpy 0x7ffff7ef1050
dict -> {1: [1, 2, 3], 3: (1, 'lul', [3, 4, 5]), 'two': 31337}

I need a disassembler now dad

But let’s get back to our second breakpoint now, and see what dpy gives us with the resulting code object:

dpy code object
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gdb$ dpy $rax
code -> {'co_code': 'd\x01\x00\x87\x00\x00|\x00\x00d\x01\x00<a\x00\x00|\x00\x00\x1b',
 'co_consts': (None, 1),
 'co_name': 'foo',
 'co_names': ('True',),
 'co_varnames': ('Robert_Forsyth',)}

Because we know the bytecode used by this interpreter is different than the original one, we have to figure out the equivalent between the instructions and their opcodes:

  1. Either we can reverse-engineer each handler of the virtual CPU,
  2. Either we can create functions in both interpreters, disassemble those (thanks to dpy) and match the equivalent opcodes

I guess we can mix both of them to be more efficient:

deducing equivalent opcodes
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Python 2.7.8 (default, Sep  5 2014, 00:13:07)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def assi(x):
...     x = 'hu'
...
>>> def add(x):
...     return x + 31337
...
>>> import dis
>>> dis.dis(assi)
  2           0 LOAD_CONST               1 ('hu')
              3 STORE_FAST               0 (x)
              6 LOAD_CONST               0 (None)
              9 RETURN_VALUE
>>> dis.dis(add)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 (31337)
              6 BINARY_ADD
              7 RETURN_VALUE
>>> assi.func_code.co_code
'd\x01\x00}\x00\x00d\x00\x00S'
>>> add.func_code.co_code
'|\x00\x00d\x01\x00\x17S'

# In the custom interpreter

gdb$ r
Starting program: /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Python 2.7.8+ (nvcs/newopcodes:a9bd62e4d5f2+, Sep  1 2014, 11:41:46)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def assi(x):
...     x = 'hu'
...
>>> def add(x):
...     return x + 31337
...
>>> print hex(id(assi))
0x7ffff7f0c578
>>> print hex(id(add))
0x7ffff7f0c5f0
>>> ^C
Program received signal SIGINT, Interrupt.
gdb$ dpy 0x7ffff7f0c578
function -> {'func_code': {'co_code': 'd\x01\x00\x87\x00\x00d\x00\x00\x1b',
               'co_consts': (None, 'hu'),
               'co_name': 'assi',
               'co_names': (),
               'co_varnames': ('x',)},
 'func_dict': None,
 'func_doc': None,
 'func_module': '__main__',
 'func_name': 'assi'}
gdb$ dpy 0x7ffff7f0c5f0
function -> {'func_code': {'co_code': '\x8f\x00\x00d\x01\x00=\x1b',
               'co_consts': (None, 31337),
               'co_name': 'add',
               'co_names': (),
               'co_varnames': ('x',)},
 'func_dict': None,
 'func_doc': None,
 'func_module': '__main__',
 'func_name': 'add'}

 # From here we have:
 # 0x64 -> LOAD_CONST
 # 0x87 -> STORE_FAST
 # 0x1b -> RETURN_VALUE
 # 0x8f -> LOAD_FAST
 # 0x3d -> BINARY_ADD

OK I think you got the idea, and if you don’t manage to find all of them you can just debug the virtual CPU by putting a software breakpoint @0x4b0960:

opcode fetching
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=> 0x4b0923 <PyEval_EvalFrameEx+867>:   movzx  eax,BYTE PTR [r13+0x0]

For the interested readers: there is at least one interesting opcode that you wouldn’t find in a normal Python interpreter, check what 0xA0 is doing especially when followed by 0x87 :–).

Back to the first marshaled function with all our tooling now

Thanks to our disassembler.py, we can now disassemble easily the first part:

disassembling
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PS D:\Codes\ql-chall-python-2014> python .\disassembler_ql_chall.py
  6           0 LOAD_CONST               1 (1)
              3 STORE_FAST               0 (Robert_Forsyth)

  8           6 LOAD_GLOBAL              0 (True)
              9 LOAD_CONST               1 (1)
             12 INPLACE_ADD
             13 STORE_GLOBAL             0 (True)

  9          16 LOAD_GLOBAL              0 (True)
             19 RETURN_VALUE
================================================================================

It seems the author has been really (too) kind with us: the function is really small and we can rewrite it in Python straightaway:

part_1
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def part1():
    global True
    Robert_Forsyth = 1
    True += 1

You can also make sure with dpy that the code of part1 is the exact same than the unmarshaled function we dumped earlier.

part_1 successfully decompiled
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>>> def part_1():
...  global True
...  Robert_Forsyth = 1
...  True += 1
...
>>> print hex(id(part_1))
0x7ffff7f0f578
>>> ^C
Program received signal SIGINT, Interrupt.
gdb$ dpy 0x7ffff7f0f578
function -> {'func_code': {'co_code': 'd\x01\x00\x87\x00\x00|\x00\x00d\x01\x00<a\x00\x00d\x00\x00\x1b',
               'co_consts': (None, 1),
               'co_name': 'part_1',
               'co_names': ('True',),
               'co_varnames': ('Robert_Forsyth',)},
 'func_dict': None,
 'func_doc': None,
 'func_module': '__main__',
 'func_name': 'part_1'}

Run my bytecode

The second part is also quite simple according to the following disassembly:

run my bytecode
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gdb$ disass run_me
Dump of assembler code for function run_me:
[...]
   ; Parses the arguments we gave, it expects a string object
   0x0000000000513da0 <+16>:    lea    rcx,[rsp+0x10]
   0x0000000000513da5 <+21>:    mov    rdx,rsp
   0x0000000000513da8 <+24>:    call   0x4cf430 <PyArg_ParseTuple>
   0x0000000000513dad <+29>:    xor    edx,edx
   0x0000000000513daf <+31>:    test   eax,eax
   0x0000000000513db1 <+33>:    je     0x513e5e <run_me+206>

   0x0000000000513db7 <+39>:    mov    rax,QWORD PTR [rip+0x2d4342]
   0x0000000000513dbe <+46>:    mov    esi,0x91
   0x0000000000513dc3 <+51>:    mov    edi,0x56c940
   0x0000000000513dc8 <+56>:    mov    rax,QWORD PTR [rax+0x10]
   0x0000000000513dcc <+60>:    mov    rbx,QWORD PTR [rax+0x30]

[...]
   ; Part1
[...]

   0x0000000000513df7 <+103>:   mov    rsi,QWORD PTR [rsp+0x10]
   0x0000000000513dfc <+108>:   mov    rdi,QWORD PTR [rsp]
   ; Uses the string passed as argument to run_me as a marshaled object
   ; PyObject* PyMarshal_ReadObjectFromString(char *string, Py_ssize_t len)
   0x0000000000513e00 <+112>:   call   0x4dc020 <PyMarshal_ReadObjectFromString>

   0x0000000000513e05 <+117>:   mov    rsi,rbx
   0x0000000000513e08 <+120>:   mov    rdi,rax

   ; Creates a function out of it
   0x0000000000513e0b <+123>:   call   0x52c630 <PyFunction_New>
   0x0000000000513e10 <+128>:   xor    edi,edi
   0x0000000000513e12 <+130>:   mov    rbp,rax
   0x0000000000513e15 <+133>:   call   0x478f80 <PyTuple_New>

   ; Calls it
   ; PyObject* PyObject_Call(PyObject *callable_object, PyObject *args, PyObject *kw)
   0x0000000000513e1a <+138>:   xor    edx,edx
   0x0000000000513e1c <+140>:   mov    rdi,rbp
   0x0000000000513e1f <+143>:   mov    rsi,rax
   0x0000000000513e22 <+146>:   call   0x422b40 <PyObject_Call>

Basically, the string you pass to run_me is treated as a marshaled function: it explains why you get segmentation faults when you call the function with random strings. We can just jump over that part of the function because we don’t really need it so far: set $eip=0x513e27 and job done!

Second & last marshaled function

By the way I hope you are still reading — hold tight, we are nearly done! Let’s dump the function object with dpy:

Second part inspection with dpy
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-----------------------------------------------------------------------------------------------------------------------[regs]
  RAX: 0x00007FFFF7FA7050  RBX: 0x00007FFFF7F0F758  RBP: 0x00000000007B0270  RSP: 0x00007FFFFFFFE040  o d I t s Z a P c
  RDI: 0x00007FFFF7F0F758  RSI: 0x00007FFFF7FA7050  RDX: 0x0000000000000000  RCX: 0x0000000000000828  RIP: 0x0000000000513E56
  R8 : 0x0000000000880728  R9 : 0x00007FFFF7F8D908  R10: 0x00007FFFF7FA7050  R11: 0x00007FFFF7FA7050  R12: 0x00007FFFF7FD0F48
  R13: 0x00000000007EF0A0  R14: 0x00007FFFF7F3CB00  R15: 0x00007FFFF7F07ED0
  CS: 0033  DS: 0000  ES: 0000  FS: 0000  GS: 0000  SS: 002B
-----------------------------------------------------------------------------------------------------------------------[code]
=> 0x513e56 <run_me+198>:       call   0x422b40 <PyObject_Call>
-----------------------------------------------------------------------------------------------------------------------------
gdb$ dpy $rdi
function -> {'func_code': {'co_code': '\\x7c\\x00\\x00\\x64\\x01\\x00\\x6b\\x03\\x00\\x72\\x19\\x00\\x7c\\x00\\x00\\x64\\x02\\x00\\x55\\x61\\x00\\x00\\x6e\\x6e\\x00\\x7c\\x01\\x00\\x6a\\x02\\x00\\x64\\x03\\x00\\x6a\\x03\\x00\\x64\\x04\\x00\\x77\\x00\\x00\\xa0\\x05\\x00\\xc8\\x06\\x00\\xa0\\x07\\x00\\xb2\\x08\\x00\\xa0\\x09\\x00\\xea\\x0a\\x00\\xa0\\x0b\\x00\\x91\\x08\\x00\\xa0\\x0c\\x00\\x9e\\x0b\\x00\\xa0\\x0d\\x00\\xd4\\x08\\x00\\xa0\\x0e\\x00\\xd5\\x0f\\x00\\xa0\\x10\\x00\\xdd\\x11\\x00\\xa0\\x07\\x00\\xcc\\x08\\x00\\xa0\\x12\\x00\\x78\\x0b\\x00\\xa0\\x13\\x00\\x87\\x0f\\x00\\xa0\\x14\\x00\\x5b\\x15\\x00\\xa0\\x16\\x00\\x97\\x17\\x00\\x67\\x1a\\x00\\x53\\x86\\x01\\x00\\x86\\x01\\x00\\x86\\x01\\x00\\x54\\x64\\x00\\x00\\x1b',
   'co_consts': (None,
     3,
     1,
     '',
     {'co_code': '\\x8f\\x00\\x00\\x5d\\x15\\x00\\x87\\x01\\x00\\x7c\\x00\\x00\\x8f\\x01\\x00\\x64\\x00\\x00\\x4e\\x86\\x01\\x00\\x59\\x54\\x71\\x03\\x00\\x64\\x01\\x00\\x1b',
      'co_consts': (13, None),
      'co_name': '<genexpr>',
      'co_names': ('chr',),
      'co_varnames': ('.0', '_')},
     75,
     98,
     127,
     45,
     89,
     101,
     104,
     67,
     122,
     65,
     120,
     99,
     108,
     95,
     125,
     111,
     97,
     100,
     110),
   'co_name': 'foo',
   'co_names': ('True', 'quarkslab', 'append', 'join'),
   'co_varnames': ()},
 'func_dict': None,
 'func_doc': None,
 'func_module': '__main__',
 'func_name': 'foo'}

Even before studying / disassembling the code, we see some interesting things: chr, quarkslab, append, join, etc. It definitely feels like that function is generating the flag we are looking for.

Seeing append, join and another code object (in co_consts) suggests that a generator is used to populate the variable quarkslab. We also can guess that the bunch of bytes we are seeing may be the flag encoded/encrypted — anyway we can infer too much information to me just by dumping/looking at the object.

Let’s use our magic disassembler.py to see those codes objects:

part2 & its generator disassembled
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 19     >>    0 LOAD_GLOBAL              0 (True)
              3 LOAD_CONST               1 (3)
              6 COMPARE_OP               3 (!=)
              9 POP_JUMP_IF_FALSE       25

 20          12 LOAD_GLOBAL              0 (True)
             15 LOAD_CONST               2 (1)
             18 INPLACE_SUBTRACT
             19 STORE_GLOBAL             0 (True)
             22 JUMP_FORWARD           110 (to 135)

 22     >>   25 LOAD_GLOBAL              1 (quarkslab)
             28 LOAD_ATTR                2 (append)
             31 LOAD_CONST               3 ('')
             34 LOAD_ATTR                3 (join)
             37 LOAD_CONST               4 (<code object <genexpr> at 023A84A0, file "obfuscate/gen.py", line 22>)
             40 MAKE_FUNCTION            0
             43 LOAD_CONST2              5 (75)
             46 LOAD_CONST3              6 (98)
             49 LOAD_CONST2              7 (127)
             52 LOAD_CONST5              8 (45)
             55 LOAD_CONST2              9 (89)
             58 LOAD_CONST4             10 (101)
             61 LOAD_CONST2             11 (104)
             64 LOAD_CONST6              8 (45)
             67 LOAD_CONST2             12 (67)
             70 LOAD_CONST7             11 (104)
             73 LOAD_CONST2             13 (122)
             76 LOAD_CONST8              8 (45)
             79 LOAD_CONST2             14 (65)
             82 LOAD_CONST10            15 (120)
             85 LOAD_CONST2             16 (99)
             88 LOAD_CONST9             17 (108)
             91 LOAD_CONST2              7 (127)
             94 LOAD_CONST11             8 (45)
             97 LOAD_CONST2             18 (95)
            100 LOAD_CONST12            11 (104)
            103 LOAD_CONST2             19 (125)
            106 LOAD_CONST16            15 (120)
            109 LOAD_CONST2             20 (111)
            112 LOAD_CONST14            21 (97)
            115 LOAD_CONST2             22 (100)
            118 LOAD_CONST15            23 (110)
            121 BUILD_LIST              26
            124 GET_ITER
            125 CALL_FUNCTION            1
            128 CALL_FUNCTION            1
            131 CALL_FUNCTION            1
            134 POP_TOP
        >>  135 LOAD_CONST               0 (None)
            138 RETURN_VALUE
================================================================================
 22           0 LOAD_FAST                0 (.0)
        >>    3 FOR_ITER                21 (to 27)
              6 LOAD_CONST16             1 (None)
              9 LOAD_GLOBAL              0 (chr)
             12 LOAD_FAST                1 (_)
             15 LOAD_CONST               0 (13)
             18 BINARY_XOR
             19 CALL_FUNCTION            1
             22 YIELD_VALUE
             23 POP_TOP
             24 JUMP_ABSOLUTE            3
        >>   27 LOAD_CONST               1 (None)
             30 RETURN_VALUE

Great, that definitely sounds like what we described earlier.

I need a decompiler dad

Now because we really like to hack things, I decided to patch a Python decompiler to support the opcodes defined in this challenge in order to fully decompile the codes we saw so far.

I won’t bother you with how I managed to do it though ; long story short: it is built it on top of fupy.py which is a readable hackable Python 2.7 decompiler written by the awesome Guillaume Delugre — Cheers to my mate @Myst3rie for telling about this project!

So here is decompiler.py working on the two code objects of the challenge:

decompiiiiiilation
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PS D:\Codes\ql-chall-python-2014> python .\decompiler_ql_chall.py
PART1 ====================
Robert_Forsyth = 1
True = True + 1

PART2 ====================
if True != 3:
    True = True - 1
else:
    quarkslab.append(''.join(chr(_ ^ 13) for _ in [75, 98, 127, 45, 89, 101, 104, 45, 67, 104, 122, 45, 65, 120, 99, 108, 127, 45, 95, 104, 125, 120, 111, 97, 100, 110]))

Brilliant — time to get a flag now :–). Here are the things we need to do:

  1. Set True to 2 (so that it’s equal to 3 in the part 2)
  2. Declare a list named quarkslab
  3. Jump over the middle part of the function where it will run the bytecode you gave as argument (or give a valid marshaled string that won’t crash the interpreter)
  4. Profit!
win
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overclok@wildout:~/chall/ql-py$ /usr/bin/b7d8438de09fffb12e3950e7ad4970a4a998403bdf3763dd4178adf
Python 2.7.8+ (nvcs/newopcodes:a9bd62e4d5f2+, Sep  1 2014, 11:41:46)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> True = 2
>>> quarkslab = list()
>>> import do_not_run_me as v
>>> v.run_me("c\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00C\x00\x00\x00s\x04\x00\x00\x00d\x00\x00\x1B(\x01\x00\x00\x00N(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00(\x00\x00\x00\x00s\x07\x00\x00\x00rstdinrt\x01\x00\x00\x00a\x01\x00\x00\x00s\x02\x00\x00\x00\x00\x01")
>>> quarkslab
['For The New Lunar Republic']

Conclusion

This was definitely entertaining, so thanks to Serge and Quarkslab for putting this challenge together! I feel like it would have been cooler to force people to write a disassembler or/and a decompiler to study the code of run_me though ; because as I mentioned at the very beginning of the article you don’t really need any tool to guess/know roughly where the flag is, and how to get it. I still did write all those little scripts because it was fun and cool that’s all!

Anyway, the codes I talked about are available on my github as usual if you want to have a look at them. You can also have look at wildfire.py if you like weird/wild/whatever Python beasts!

That’s all for today guys, I hope it wasn’t too long and that you did enjoy the read.

By the way, we still think it would be cool to have more people posting on that blog, so if you are interested feel free to contact us!