A determined woman, strong both physically and mentally, but with a soul that yearns to dominate the body and mind to someone who appeared stronger. 
Defcon 2015 Coding Skillz 1 Writeup
Posted on 8:59 AM by Tina
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.
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()
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