X-Windows Security

1. Motivation / introduction
2. How open X displays are found
3. The local-host problem
4. Snooping techniques - dumping windows
5. Snooping techniques - reading keyboard
6. Xterm - secure keyboard option
7. Trojan X programs [xlock and xdm]
8. X Security tools - xauth MIT-MAGIC-COOKIE
9. Concluding remarks

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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.

9. Concluding remarks

Thanks must go to Anthony Tyssen for sending me his accumulated info on X
security issues from varius usenet discussions. I hope someone has found
useful information in this text. It is released to the net.community with
the idea that it will help the user to understand the security problems
concerned with using X windows. Questions or remarks can be sent to the
following address:

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runeb / cF --- runeb@stud.cs.uit.no --- http://www.cs.uit.no/~runeb
