Crash Course in X Windows Security

1. Motivation / introduction

X windows pose a security risk. Through a network, anyone can connect
to an open X display, read the keyboard, dump the screen and windows
and start applications on the unprotected display. Even if this is a
known fact throughout the computer security world, few attempts on
informing the user community of the security risks involved have been
made.  This article deals with some of the aspects of X windows
security. It is in no sense a complete guide to the subject, but
rather an introduction to a not-so-known field of computer
security. Knowledge of the basics of the X windows system is
necessary, I haven't bothered including an introductory section to
explain the fundamentals. I wrote some code during the research for
this article, but none of it is included herein.  If the lingual flow
of English seem mayhap strange and erroneous from byte to byte, this
is due to the fact that I'm Scandinavian.  Bare with it. :)

2. How open X displays are found

An open X display is in formal terms an X server that has its access
control disabled. Disabling access control is normally done with the
xhost command.

$ xhost +

allows connections from any host. A single host can be allowed
connection with the command

$ xhost + ZZZ.ZZZ.ZZZ.ZZZ

where Z is the IP address or host-name. Access control can be enabled
by issuing an

$ xhost -

command. In this case no host but the local-host can connect to the
display.  Period. It is as simple as that - if the display runs in
'xhost -' state, you are safe from programs that scans and attaches to
unprotected X displays.  You can check the access control of your
display by simply typing xhost from a shell. Sadly enough, most sites
run their X displays with access control disabled as default. They are
therefore easy prey for the various scanner programs circulating on
the net.

Anyone with a bit of knowledge about Xlib and sockets programming can
write an X scanner in a couple of hours. The task is normally
accomplished by probing the port that is reserved for X windows,
number 6000. If anything is alive at that port, the scanner calls
XOpenDisplay("IP-ADDRESS:0.0") that will return a pointer to the
display structure, if and only if the target display has its access
control disabled. If access control is enabled, XOpenDisplay returns 0
and reports that the display could not be opened.

E.g:

Xlib: connection to "display:0.0" refused by server
Xlib: Client is not authorized to connect to Server

The probing of port 6000 is necessary because of the fact that calling
XOpenDisplay() on a host that runs no X server will simply hang the
calling process. So much for unix programming conventions. :)

I wrote a program called xscan that could scan an entire subnet or
scan the entries in /etc/hosts for open X displays. My remark about
most sites running X displays with access control disabled, originates
from running xscan towards several sites on the internet.

3. The localhost problem

Running your display with access control enabled by using 'xhost -'
will guard you from XOpenDisplay attempts through port number
6000. But there is one way an eavesdropper can bypass this
protection. If he can log into your host, he can connect to the
display of the localhost. The trick is fairly simple. By issuing these
few lines, dumping the screen of the host 'target' is accomplished:

$ rlogin target
$ xwd -root -display localhost:0.0 > ~/snarfed.xwd
$ exit
$ xwud -in ~/snarfed.xwd

And voila, we have a screendump of the root window of the X server
target.

Of course, an intruder must have an account on your system and be able
to log into the host where the specific X server runs. On sites with a
lot of X terminals, this means that no X display is safe from those
with access. If you can run a process on a host, you can connect to
(any of) its X displays.

Every Xlib routine has the Display structure as it's first
argument. By successfully opening a display, you can manipulate it
with every Xlib call available. For an intruder, the most 'important'
ways of manipulating is grabbing windows and keystrokes.

4. Snooping techniques - dumping windows

The most natural way of snarfing a window from an X server is by using
the X11R5 utility xwd or X Window System dumping utility. To get a
grip of the program, here's a small excerpt from the man page

 DESCRIPTION
      Xwd is an X Window System window dumping utility.  Xwd allows Xusers
      to store window images in a specially formatted dump file.  This file
      can then be read by various other X utilities for redisplay, printing,
      editing, formatting, archiving, image processing, etc.  The target
      window is selected by clicking the pointer in the desired window.  The
      keyboard bell is rung once at the beginning of the dump and twice when
      the dump is completed.

Shortly, xwd is a tool for dumping X windows into a format readable by
another program, xwud. To keep the trend, here's an excerpt from the
man page of xwud:


 DESCRIPTION
      Xwud is an X Window System image undumping utility.  Xwud allows X
      users to display in a window an image saved in a specially formatted
      dump file, such as produced by xwd(1).

I will not go in detail of how to use these programs, as they are both
self-explanatory and easy to use. Both the entire root window, a
specified window (by name) can be dumped, or a specified screen.  As a
'security measure' xwd will beep the terminal it is dumping from, once
when xwd is started, and once when it is finished (regardless of the
xset b off command). But with the source code available, it is a
matter of small modification to compile a version of xwd that doesn't
beep or otherwise identifies itself - on the process list e.g.  If we
wanted to dump the root window or any other window from a host, we
could simply pick a window from the process list, which often gives
away the name of the window through the -name flag.  As before
mentioned, to dump the entire screen from a host:

$ xwd -root localhost:0.0 > file

the output can be directed to a file, and read with

$ xwud -in file

or just piped straight to the xwud command.

Xterm windows are a different thing. You can not specify the name of
an xterm and then dump it. They are somehow blocked towards the
X_Getimage primitive used by xwd, so the following

$ xwd -name xterm

will result in an error. However, the entire root window (with Xterms
and all) can still be dumped and watched by xwud. Some protection.

5. Snooping techniques - reading keyboard

If you can connect to a display, you can also log and store every
keystroke that passes through the X server. A program circulating the
net, called xkey, does this trick. A kind of higher-level version of
the infamous ttysnoop.c. I wrote my own, who could read the keystrokes
of a specific window ID (not just every keystroke, as my version of
xkey).  The window ID's of a specific root-window, can be acquired
with a call to XQueryTree(), that will return the XWindowAttributes of
every window present. The window manager must be able to control every
window-ID and what keys are pressed down at what time.  By use of the
window-manager functions of Xlib, KeyPress events can be captured, and
KeySyms can be turned into characters by continuous calls to
XLookupString.

You can even send KeySym's to a Window. An intruder may therefore not
only snoop on your activity, he can also send keyboard events to
processes, like they were typed on the keyboard.  Reading/writing
keyboard events to an xterm window opens new horizons in process
manipulation from remote. Luckily, xterm has good protection
techniques for prohibiting access to the keyboard events.

6. Xterm - Secure keyboard option

A lot of passwords is typed in an xterm window. It is therefore
crucial that the user has full control over which processes can read
and write to an xterm.  The permission for the X server to send events
to an Xterm window, is set at compile time. The default is false,
meaning that all SendEvent requests from the X server to an xterm
window is discarded. You can overwrite the compile-time setting with a
standard resource definition in the .Xdefaults file:

xterm*allowSendEvents True

or by selecting Allow Sendevents on the Xterm Main Options
menu. (Accessed by pressing CTRL and the left mouse button But this is
_not_ recommended. Neither by me, nor the man page. ;) Read access is
a different thing.

Xterms mechanism for hindering other X clients to read the keyboard
during entering of sensitive data, passwords etc. is by using the
XGrabKeyboard() call. Only one process can grab the keyboard at any
one time. To activate the Secure Keyboard option, choose the Main
Options menu in your Xterm window (CTRL+Left mouse button) and select
Secure Keyboard.  If the colors of your xterm window inverts, the
keyboard is now Grabbed, and no other X client can read the KeySyms.

The versions of Xterm X11R5 without patch26 also contain a rather
nasty and very well known security hole that enables any user to
become root through clever use of symbolic links to the password
file. The Xterm process need to be setuid for this hole to be
exploitable.  Refer to the Cert Advisory:
CA-93:17.xterm.logging.vulnerability.

7. Trojan X clients - xlock and X based logins

Can you think of a more suitable program for installing a
password-grabbing trojan horse than xlock? I myself cannot. With a few
lines added to the getPassword routine in xlock.c, the password of
every user using the trojan version of xlock can be stashed away in a
file for later use by an intruder. The changes are so minimal, only a
couple of bytes will tell the real version from the trojan version.

If a user has a writable homedir and a ./ in her PATH environment
variable, she is vulnerable to this kind of attack. Getting the
password is achieved by placing a trojan version of Xlock in the users
homedir and waiting for an invocation.  The functionality of the
original Xlock is contained in the trojan version.  The trojan version
can even tidy up and destroy itself after one succesfull attempt, and
the user will not know that his password has been captured.

Xlock, like every password-prompting program, should be regarded with
suspicion if it shows up in places it should not be, like in your own
homedir.

Spoofed X based logins however are a bit more tricky for the intruder
to accomplish.  He must simulate the login screen of the login program
ran by XDM. The only way to ensure that you get the proper XDM login
program (if you want to be really paranoid) is to restart the
X-terminal, whatever key combination that will be for the terminal in
question.

8. X Security tools - xauth MIT-MAGIC-COOKIE 

To avoid unathorized connections to your X display, the command xauth
for encrypted X connections is widely used. When you login, xdm
creates a file .Xauthority in your homedir. This file is binary, and
readable only through the xauth command. If you issue the command

$ xauth list

you will get an output of:

your.display.ip:0  MIT-MAGIC-COOKIE-1  73773549724b76682f726d42544a684a

  display name     authorization type               key

The .Xauthority file sometimes contains information from older
sessions, but this is not important, as a new key is created at every
login session. To access a display with xauth active - you must have
the current access key.

If you want to open your display for connections from a particular
user, you must inform him of your key.
He must then issue the command

$ xauth add your.display.ip:0  MIT-MAGIC-COOKIE-1 73773549724b7668etc.

Now, only that user (including yourself) can connect to your display.
Xauthority is simple and powerful, and eliminates many of the security
problems with X.