Snippets

XOR Hex Flare-On 2017 Challenge 3 Solution

Created by XOR Hex last modified
#!/usr/bin/env python2

'''
Author: @XOR_Hex
#flareon4
Challenge 3

' Three parts
' 0) Dump the ASM from IDA, Start @ 0x40107C and the length is 0x79 (see box @ loc_401029)
'      bytes = idaapi.get_many_bytes(0x40107C, 0x79)
'      with open('/home/firefly/buffer.asm', 'wb') as f:
'            f.write(bytes)
' 1) Figure out what byte the code needs to be xor'ed with.  Last word must match 0xfb5eL (loop at 401039, comparison check: 40105E)
'    Code to be xor'ed and add'ed is located at 40107C and the length is 0x79 (see box @ loc_401029)
'    To determine the input byte the modified (xord + add) bytes have to be passed through sub_4011E6 before the comparison check can be done
' 2) After the check passes the state memory from angr needs to be interpreted as code, so use capstone
'    The op_str from capstone contains what looks like ASCII characters...
' 3) Take the capstone output and interpret the ASCII codes to get the flag
'''

import angr
import sys

p = angr.Project('greek_to_me.exe', load_options={'auto_load_libs': False})
f2 = None

# Interate through all of the possible byte values to find the correct "user" input to de-mask the flag
for buf in xrange(0x100):

    print("Trying buf = {0}".format(buf))

    # Variable to store the bits written to disk using IDA
    asm = None
    # Store the output from the first de-obfuscation routine
    b2 = []
    # Read in bytes written to file from IDA
    with open('greek_to_me_buffer.asm', 'rb') as f:
        asm = f.read()

    # Re-implement loc_401039
    dl = buf 
    for byte in asm:
        bl = ord(byte)
        bl = bl ^ dl
        bl = bl & 0xff
        bl = bl + 0x22
        bl = bl & 0xff
        b2.append(bl)


    # Set up angr to "run" sub_4011E6 
    s = p.factory.blank_state(addr=0x4011E6)
    s.mem[s.regs.esp+4:].dword = 1    # Angr memory location to hold the xor'ed and add'ed bytes
    s.mem[s.regs.esp+8:].dword = 0x79 # Length of ASM
    
    # Copy bytes output from loc_401039 into address 0x1 so Angr can run it
    asm = ''.join(map(lambda x: chr(x), b2))
    s.memory.store(1, s.se.BVV(int(asm.encode('hex'), 16), 0x79 * 8 ))

    # Create a simulation manager...
    #import pdb; pdb.set_trace()
    simgr = p.factory.simulation_manager(s)

    # Tell Angr where to go, though there is only one way through this function, 
    # we just need to stop after ax is set
    simgr.explore(find=0x401268)
    
    # Once ax is set, check to see if the value in ax matches the comparison value
    for found in simgr.found:
        #import pdb; pdb.set_trace()
        print('        ax = %s' % hex(found.state.solver.eval(found.state.regs.ax)))
        # Comparison check
        if hex(found.state.solver.eval(found.state.regs.ax)) == '0xfb5eL':
            # Upon success, dump the asm
            code = ("%x" % found.state.solver.eval_upto(found.state.memory.load(1, 0x79), 1)[0]).decode('hex')
            print('\n        Winner is: {0}\n\n'.format(buf))
            print('          %s' % code)
            bl = None
            dl = None
            flag = []
            # Using capstone, interpret the ASM
            from capstone import *
            md = Cs(CS_ARCH_X86, CS_MODE_32)
            for i in md.disasm(code, 0x1000):
                flag_char = None
                # The if statements do the work of interpreting the ASCII codes to their value counterpart
                if i.op_str.split(',')[0].startswith("byte ptr"):
                   flag_char = chr(long(i.op_str.split(',')[1], 16)) 
                if i.op_str.split(',')[0].startswith('bl'):
                    bl = chr(long(i.op_str.split(',')[1], 16))  
                if i.op_str.split(',')[0].startswith('dl'):
                    dl = chr(long(i.op_str.split(',')[1], 16))  
                if i.op_str.split(',')[1].strip() == 'dl':
                    flag_char = dl
                if i.op_str.split(',')[1].strip() == 'bl':
                    flag_char = bl 
                
                if (flag_char):
                    flag.append(flag_char.strip())

                print("          0x%x\t%s\t%s\t%s" %(i.address, i.mnemonic, i.op_str, flag_char))

            print('\n\n')
            print(''.join(flag))

            sys.exit(0)

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