Defcon 2015 Coding Skillz 1 Writeup

Just connecting to the service, a 64bit cpu registers dump is received, and so does several binary code as you can see:

The registers represent an initial cpu state, and we have to reply with the registers result of the binary code execution. This must be automated becouse of the 10 seconds server socket timeout.

The exploit is quite simple, we have to set the cpu registers to this values, execute the code and get resulting registers.

In python we created two structures for the initial state and the ending state.

cpuRegs = {'rax':'','rbx':'','rcx':'','rdx':'','rsi':'','rdi':'','r8':'','r9':'','r10':'','r11':'','r12':'','r13':'','r14':'','r15':''}
finalRegs = {'rax':'','rbx':'','rcx':'','rdx':'','rsi':'','rdi':'','r8':'','r9':'','r10':'','r11':'','r12':'','r13':'','r14':'','r15':''}

We inject at the beginning several movs for setting the initial state:

for r in cpuRegs.keys():

    code.append('mov %s, %s' % (r, cpuRegs[r]))

The 64bit compilation of the movs and the binary code, but changing the last ret instruction by a sigtrap "int 3"

We compile with nasm in this way:

os.popen('nasm -f elf64 code.asm')

os.popen('ld -o code code.o ')

And use GDB to execute the code until the sigtrap, and then get the registers

fd = os.popen("gdb code -ex 'r' -ex 'i r' -ex 'quit'",'r')

for l in fd.readlines():

    for x in finalRegs.keys():

           ...

We just parse the registers and send the to the server in the same format, and got the key.

The code:

from libcookie import *

from asm import *

import os

import sys

host = 'catwestern_631d7907670909fc4df2defc13f2057c.quals.shallweplayaga.me'

port = 9999

cpuRegs = {'rax':'','rbx':'','rcx':'','rdx':'','rsi':'','rdi':'','r8':'','r9':'','r10':'','r11':'','r12':'','r13':'','r14':'','r15':''}

finalRegs = {'rax':'','rbx':'','rcx':'','rdx':'','rsi':'','rdi':'','r8':'','r9':'','r10':'','r11':'','r12':'','r13':'','r14':'','r15':''}

fregs = 15

s = Sock(TCP)

s.timeout = 999

s.connect(host,port)

data = s.readUntil('bytes:')

#data = s.read(sz)

#data = s.readAll()

sz = 0

for r in data.split('\n'):

    for rk in cpuRegs.keys():

        if r.startswith(rk):

            cpuRegs[rk] = r.split('=')[1]

    if 'bytes' in r:

        sz = int(r.split(' ')[3])

binary = data[-sz:]

code = []

print '[',binary,']'

print 'given size:',sz,'bin size:',len(binary)        

print cpuRegs

for r in cpuRegs.keys():

    code.append('mov %s, %s' % (r, cpuRegs[r]))

#print code

fd = open('code.asm','w')

fd.write('\n'.join(code)+'\n')

fd.close()

Capstone().dump('x86','64',binary,'code.asm')

print 'Compilando ...'

os.popen('nasm -f elf64 code.asm')

os.popen('ld -o code code.o ')

print 'Ejecutando ...'

fd = os.popen("gdb code -ex 'r' -ex 'i r' -ex 'quit'",'r')

for l in fd.readlines():

    for x in finalRegs.keys():

        if x in l:

            l = l.replace('\t',' ')

            try:

                i = 12

                spl = l.split(' ')

                if spl[i] == '':

                    i+=1

                print 'reg: ',x

                finalRegs[x] = l.split(' ')[i].split('\t')[0]

            except:

                print 'err: '+l

            fregs -= 1

            if fregs == 0:

                #print 'sending regs ...'

                #print finalRegs

                

                buff = []

                for k in finalRegs.keys():

                    buff.append('%s=%s' % (k,finalRegs[k]))

                print '\n'.join(buff)+'\n'

                print s.readAll()

                s.write('\n'.join(buff)+'\n\n\n')

                print 'waiting flag ....'

                print s.readAll()

                print '----- yeah? -----'

                s.close()

                

fd.close()

s.close()

More articles

1. Hacker Tools Mac
2. Hacking Tools For Games
3. Hacking Tools
4. Hack Tools For Windows
5. Hacking Tools 2019
6. Hacker Tools
7. Pentest Tools Kali Linux
8. Pentest Tools Find Subdomains
9. Hacking Tools For Beginners
10. Pentest Recon Tools
11. Hacker Tools 2020
12. Pentest Tools Free
13. Pentest Tools Find Subdomains
14. Blackhat Hacker Tools
15. Hack Tools Online
16. Tools Used For Hacking
17. Hacker Tools For Mac
18. New Hacker Tools
19. Hack Tools Online
20. Pentest Tools List
21. Hacker Tools Apk Download