pythonshellcodectype

Python Ctype Segmentation Fault


This is the code to run shellcode using ctype. The shellcode runs "whoami" on a 64 bit linux. But this program gives me a "segmentation fault". But I m unable to figure out the error in it. The structure of the code is from: ctypes: Cast string to function?

#!/usr/bin/python

from ctypes import *

# /usr/bin/whoami
shellcode_data = ("\x6a\x3b\x58\x99\x48\xbb\x2f\x62\x69\x6e\x2f\x73\x68\x00\x53"
"\x48\x89\xe7\x68\x2d\x63\x00\x00\x48\x89\xe6\x52\xe8\x10\x00"
"\x00\x00\x2f\x75\x73\x72\x2f\x62\x69\x6e\x2f\x77\x68\x6f\x61"
"\x6d\x69\x00\x56\x57\x48\x89\xe6\x0f\x05");

shellcode = c_char_p(shellcode_data)
function = cast(shellcode, CFUNCTYPE(None))
function()

For 32bits architectures this will be the shell code:

shellcode_data = ("\x6a\x0b\x58\x99\x52\x66\x68\x2d\x63\x89\xe7\x68\x2f\x73\x68"
"\x00\x68\x2f\x62\x69\x6e\x89\xe3\x52\xe8\x10\x00\x00\x00\x2f"
"\x75\x73\x72\x2f\x62\x69\x6e\x2f\x77\x68\x6f\x61\x6d\x69\x00"
"\x57\x53\x89\xe1\xcd\x80");

Solution

  • The NX Bit prevents random data being executed on modern processors and OSs. To get around it, call mprotect. You should also define your shellcode as a binary instead of a character string, like this:

    #!/usr/bin/python
    import ctypes
    shellcode_data = (b"\x6a\x3b\x58\x99\x48\xbb\x2f\x62\x69\x6e\x2f\x73\x68\x00\x53"
        b"\x48\x89\xe7\x68\x2d\x63\x00\x00\x48\x89\xe6\x52\xe8\x10\x00"
        b"\x00\x00\x2f\x75\x73\x72\x2f\x62\x69\x6e\x2f\x77\x68\x6f\x61"
        b"\x6d\x69\x00\x56\x57\x48\x89\xe6\x0f\x05")
    
    shellcode = ctypes.create_string_buffer(shellcode_data)
    function = ctypes.cast(shellcode, ctypes.CFUNCTYPE(None))
    
    addr = ctypes.cast(function, ctypes.c_void_p).value
    libc = ctypes.CDLL('libc.so.6')
    pagesize = libc.getpagesize()
    addr_page = (addr // pagesize) * pagesize
    for page_start in range(addr_page, addr + len(shellcode_data), pagesize):
        assert libc.mprotect(page_start, pagesize, 0x7) == 0
    
    function()