(erl0002100)

Michael Altfield Asks: Communicating with root child process launched with osascript "with administrator privileges"
How can I pass messages between a parent process that launches a child process as root using Apple Script and stdin/stdout?

I'm writing an anti-forensics GUI application that needs to be able to do things that require root permissions on MacOS. For example, shutting down the computer.

For security reasons, I do not want the user to have to launch the entire GUI application as root. Rather, I want to just spawn a child process with root permission and a very minimal set of functions.

Also for security reasons, I do not want the user to send my application its user password. That authentication should be handled by the OS, so only the OS has visibility into the user's credentials. I read that the best way to do this with Python on MacOS is to leverage osascript.

Unfortunately, for some reason, communication between the parent and child process breaks when I launch the child process using osascript. Why?

Example with sudo​

First, let's look at how it should work.

Here I'm just using sudo to launch the child process. Note I can't use sudo for my use-case because I'm using a GUI app. I'm merely showing it here to demonstrate how communication between the processes should work.

Parent (spawn_root.py)​

The parent python script launches the child script root_child.py as root using sudo.

Then it sends it a command soft-shutdown\n and waits for the response

Code:

#!/usr/bin/env python3
import subprocess, sys

from subprocess import PIPE, Popen
from getpass import getpass

proc = subprocess.Popen(
 [ 'sudo', sys.executable, 'root_child.py' ],
 stdin=subprocess.PIPE, stdout=subprocess.PIPE, text=True
)

print( "sending soft-shutdown command now" )
proc.stdin.write( "soft-shutdown\n" )
proc.stdin.flush()
print( proc.stdout.readline() )

proc.stdin.close()

Child (root_child.py)​

The child process enters an infinite loop listening for commands (in our actual application, the child process will wait in the background for the command from the parent; it won't usually just get the soft-shutdown command immediately).

Once it does get a command, it does some sanity checks. If it matches soft-shutdown, then it executes shutdown -h now with subprocess.Popen().

Code:

#!/usr/bin/env python3
import os, sys, re, subprocess

if __name__ == "__main__":

    # loop and listen for commands from the parent process
    while True:

        command = sys.stdin.readline().strip()

        # check sanity of recieved command. Be very suspicious
        if not re.match( "^[A-Za-z_-]+$", command ):
            sys.stdout.write( "ERROR: Bad Command Ignored\n" )
            sys.stdout.flush()
            continue

        if command == "soft-shutdown":
            try:
                proc = subprocess.Popen(
                 [ 'sudo', 'shutdown', '-h', 'now' ],
                 stdin=subprocess.PIPE, stdout=subprocess.PIPE, text=True
                )
                sys.stdout.write( "SUCCESS: I am root!\n" )
                sys.stdout.flush()
            except Exception as e:
                sys.stdout.write( "ERROR: I am not root :'(\n" )
                sys.stdout.flush()
                sys.exit(0)

            continue
        else:
            sys.stdout.write( "WARNING: Unknown Command Ignored\n" )
            sys.stdout.flush()
            continue

Example execution​

This works great. You can see in this example execution that the shutdown command runs without any exceptions thrown, and then the machine turns off.

Code:

user@host ~ % ./spawn_root.py  
sending soft-shutdown command now
SUCCESS: I am root!
...
user@host ~ % Connection to REDACTED closed by remote host.
Connection to REDACTED closed.
user@buskill:~$

Example with osascript​

Unfortunately, this does not work when you use osascript to get the user to authenticate in the GUI.

For example, if I change one line in the subprocess call in spawn_root.py from using sudo to using `osascript as follows

Parent (spawn_root.py)​

Code:

#!/usr/bin/env python3
import subprocess, sys

from subprocess import PIPE, Popen
from getpass import getpass

proc = subprocess.Popen(
 ['/usr/bin/osascript', '-e', 'do shell script "' +sys.executable+ ' root_child.py" with administrator privileges' ],
 stdin=subprocess.PIPE, stdout=subprocess.PIPE, text=True
)

print( "sending soft-shutdown command now" )
proc.stdin.write( "soft-shutdown\n" )
proc.stdin.flush()
print( proc.stdout.readline() )

proc.stdin.close()

Child (root_child.py)​

Code:

(no changes in this script, just use 'root_child.py' from above)

Example Execution​

This time, the parent gets stuck indefinitely when trying to communicate with the child.

Code:

user@host spawn_root_sudo_communication_test % diff simple/spawn_root.py simple_gui/spawn_root.py 
sending soft-shutdown command now

Why is it that I cannot communicate with a child process that was launched with osascript?

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