[angr] Inquiry on applying Angr to finding backdoor of some firmwares.

Dongwoo Kim freefreek at gmail.com
Wed Mar 2 23:12:59 PST 2016


Hello,

I'm writing to get some information about Angr.

First of all, I'd like to say that you have made a great tool set for
binary analysis. I'm especially interested in finding backdoor in firmwares
like an example 'fauxware'. I have installed Angr and tested it with
fauxware. It works fine, which makes me excited.

However, I have faced some problems while applying it to some binaries that
have already known backdoors. I know that it is because I don't understand
how Angr works inside. I think I need some time to understand it.

Before I dig into it,  I'd like to ask you how to use Angr to find backdoor
of the attached binary which belongs to firmware of Access Point made by
Chinese company. It was reported in 2014. (
http://blog.trendmicro.com/trendlabs-security-intelligence/netis-routers-leave-wide-open-backdoor/
)

I extracted the problematic binary from the firmware. You can easily find
the backdoor "netcore" in call_mptlogin function, which allows for remote
login.

At first, I tried to apply Angr to this binary by using an example of
fauxware but I failed. I've attached solve.py that I modified as well.

I have browsed all the examples but I couldn't get information that I need
right now. I hope you figure out what problem is. I will be a good
practitioner and I will participate in developing some part of Angr if
possible. :)

I'm looking forward to seeing your reply.
Thank you for your time.

Dongwoo Kim
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#!/usr/bin/env python

import angr
import archinfo

# Look at fauxware.c! This is the source code for a "faux firmware" (@zardus
# really likes the puns) that's meant to be a simple representation of a
# firmware that can authenticate users but also has a backdoor - the backdoor
# is that anybody who provides the string "SOSNEAKY" as their password will be
# automatically authenticated.


def basic_symbolic_execution():
    # We can use this as a basic demonstration of using angr for symbolic
    # execution. First, we load the binary into an angr project.

    p = angr.Project('igdmptd', load_options={'auto_load_libs':False})

    # Now, we want to construct a representation of symbolic program state.
    # SimState objects are what angr manipulates when it symbolically executes
    # binary code.
    # The entry_state constructor generates a SimState that is a very generic
    # representation of the possible program states at the program's entry
    # point. There are more constructors, like blank_state, which constructs a
    # "blank slate" state that specifies as little concrete data as possible,
    # or full_init_state, which performs a slow and pedantic initialization of
    # program state as it would execute through the dynamic loader.

    state = p.factory.entry_state()
    #state = p.factory.blank_state(addr=0x4046a0)

    # States are relatively static objects, they don't do anything "smart".
    # You can read data into and out of them, but that's about it.
    # In order to actually perform symbolic execution, you need a Path.
    # Paths wrap states and are your interface for stepping them forward and
    # tracking their history.

    path = p.factory.path(state)

    # Now, in order to manage the symbolic execution process from a very high
    # level, we have a PathGroup. Path groups are just collections of paths
    # with various tags attached with a number of convenient interfaces for
    # managing them.

    pathgroup = p.factory.path_group(path)

    # Uncomment the following line to spawn an IPython shell when the program
    # gets to this point so you can poke around at the four objects we just
    # constructed. Use tab-autocomplete and IPython's nifty feature where if
    # you stick a question mark after the name of a function or method and hit
    # enter, you are shown the documentation string for it.

    #import IPython; IPython.embed()

    # Now, we begin execution. This will symbolically execute the program until
    # we reach a branch statement for which both branches are satisfiable.

    pathgroup.step(until=lambda lpg: len(lpg.active) > 1)

    # If you look at the C code, you see that the first "if" statement that the
    # program can come across is comparing the result of the strcmp with the
    # backdoor password. So, we have halted execution with two states, each of
    # which has taken a different arm of that conditional branch. If you drop
    # an IPython shell here and examine pathgroup.active[n].state.se.constraints
    # you will see the encoding of the condition that was added to the state to
    # constrain it to going down this path, instead of the other one. These are
    # the constraints that will eventually be passed to our constraint solver
    # (z3) to produce a set of concrete inputs satisfying them.

    # As a matter of fact, we'll do that now.

    input_0 = pathgroup.active[0].state.posix.dumps(0)
    input_1 = pathgroup.active[1].state.posix.dumps(0)

    # We have used a utility function on the state's posix plugin to perform a
    # quick and dirty concretization of the content in file descriptor zero,
    # stdin. One of these strings should contain the substring "SOSNEAKY"!

    #if 'netcore' in input_0:
    if 'Login successed!' in input_1:
        return input_1
    else:
        return input_0

def test():
    pass        # appease our CI infrastructure which expects this file to do something lmao

if __name__ == '__main__':
    print basic_symbolic_execution()

# You should be able to run this program and pipe its into fauxware in order to
# produce a "sucessfully authenticated" message
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