NorthSec 2018 MarsAnalytica

Toshi Piazza

This is (yet another) posthumous writeup from NorthSec, on the MarsAnalytica challenge. It features a heavily (rop)fuscated binary which accepts a 19-character pin; if the pin is correct, it produces a flag, and otherwise prints an access denied message.

Unfortunately, running angr on the challenge doesn’t work in a reasonable amount of time; I instead opt to guide angr to the solution by stopping at the first symbolic branch:

p = angr.Project("./MarsAnalytica")
s = p.factory.entry_state(add_options=angr.options.unicorn)
sm = p.factory.simulation_manager(s)
sm.step(until=lambda lpg: len(lpg.active) > 1)

This code should stop at the first logical “check” of our input, and after 10 minutes or so the step function exits, and drops us into a repl:

$ python -i soln.py
>>>> sm
<SimulationManager with 2 active>
>>>> list(sm.active[1].guards)[-1]
<Bool 0x0#56 .. file_/dev/stdin_24_1_789_8[31;0] >s 0x20>

Here we see there are two active paths, the second of which has a path predicate that constrains a byte of our input to a value > 0x20. This is just an ascii check! Since the second active path is clearly the one we want, we can easily drop the first one and continue:

sm.drop(stash='active', filter_func=lambda s: s != sm.active[1])
sm.step(until=lambda lpg: len(lpg.active) > 1)

As we continue we see similar ascii checks for 0x20 and 0x7f on each byte of stdin. There are going to be 38 of these, and it would be tedious to comb through all of these by hand; we can blow past these by revising our script:

def constrain_stdin(st):
    for _ in xrange(19):
        k = st.posix.files[0].read_from(1)
        st.solver.add(k > 0x20)
        st.solver.add(k < 0x7f)
    st.posix.files[0].seek(0)
    st.posix.files[0].length = 19
p = angr.Project("./MarsAnalytica")
s = p.factory.entry_state(add_options=angr.options.unicorn)
constrain_stdin(s)
sm = p.factory.simulation_manager(s)
sm.step(until=lambda lpg: len(lpg.active) > 1)

Now, when we run angr once again we no longer stop at any of these ascii constraints since there’s only one sat branch for all of these conditions. Our next constraint looks something like the following:

$ python -i soln.py
>>>> sm
<SimulationManager with 2 active>
>>>> list(sm.active[0].guards)[-1]
<Bool 0x3fcf == __mul__(0x0#24 .. file_/dev/stdin_24_6_6_8, 0x0#24 ..  file_/dev/stdin_24_e_14_8, ((0x0#24 .. file_/dev/stdin_24_c_12_8 + (0xffffffff * 0x0#24 ..  file_/dev/stdin_24_a_10_8)) ^ 0x0#24 .. file_/dev/stdin_24_d_13_8))>

This looks much more interesting to us, and likely constitutes some constraints on our pin. We can hazard a guess and say that we want to get more constrained as opposed to less constrained, and take the first active branch. Continuing along, we notice a trend that the first active branch is always the branch which enforces the equality path constraint. We use this to our advantage to write a final automation script:

def constrain_stdin(st):
    for _ in xrange(19):
        k = st.posix.files[0].read_from(1)
        st.solver.add(k > 0x20)
        st.solver.add(k < 0x7f)
    st.posix.files[0].seek(0)
    st.posix.files[0].length = 19
p = angr.Project("./MarsAnalytica")
s = p.factory.entry_state(add_options=angr.options.unicorn)
constrain_stdin(s)
sm = p.factory.simulation_manager(s)

sm.step(until=lambda lpg: len(lpg.active) > 1)
while len(sm.deadended) == 0:
    sm.drop(stash='active', filter_func=lambda s: s != sm.active[0])
    print sm.one_active.state.posix.dumps(0)
    sm.step(until=lambda lpg: len(lpg.deadended) > 1 or len(lpg.active) > 1)
# output:
# @@@@@@!@@@G@}?7@@@@
# @}@@@@!!@@G@}?7?@@0
# 7}@@@w!!@]G@}?7?B(0
# 7}@1@w!!@]G&}?7?B(0
# 7}=1@w!!~]G&}?7?B(0
# 7@R1!w!0p]G&}?7aB("
# y?5}'q!a_),rrO7xgM{
# a>C|(@!\hB@qp5cpK1y
# {5O{*rc>z&(plA!`@&D
# +%>c9`cuth@Xk(7u`Fv
# e^?t6Yc>v-/iS'7p?%?
# {?@X+cc>h55MyA!s^DY
# q4Eo-eyMq-1dd0-leKx

This yields us a pin: q4Eo-eyMq-1dd0-leKx; we can verify it works by simply running the binary:

  __  __                                      _       _   _
 |  \/  |                   /\               | |     | | (_)
 | \  / | __ _ _ __ ___    /  \   _ __   __ _| |_   _| |_ _  ___ __ _
 | |\/| |/ _` | '__/ __|  / /\ \ | '_ \ / _` | | | | | __| |/ __/ _` |
 | |  | | (_| | |  \__ \ / ____ \| | | | (_| | | |_| | |_| | (_| (_| |
 |_|  |_|\__,_|_|  |___//_/    \_\_| |_|\__,_|_|\__, |\__|_|\___\__,_|
                                                 __/ |
                                                |___/        NSEC 2018


Citizen Access ID: q4Eo-eyMq-1dd0-leKx

[+] ACCESS GRANTED!

**  FLAG-l0rdoFb1Nq4EoeyMq1dd0leKx  **

[-] Session Terminated

Food for Thought

Initially I was hoping for a solution which employed Triton. Since it sports an associated pintool I assumed it would be a lot faster to run and to pick up path constraints. The technique of dumping the path predicate and solving for one particular branch was also inspired by Triton’s concolic execution.

Furthermore, Triton was used in a partial solution to the Tigress VM Challenges, another set of challenges which demonstrates virtualization obfuscation techniques. Unfortunately, I couldn’t get Triton to run on this binary but I’d love to see someone reconstruct the source using this same technique!