XSecurity, Xsecurity - X display access control
X provides mechanism for implementing many access control systems. Release
6 includes five mechanisms:
Host Access Simple host-based access control.
MIT-MAGIC-COOKIE-1 Shared plain-text "cookies".
XDM-AUTHORIZATION-1 Secure DES based private-keys.
SUN-DES-1 Based on Sun's secure rpc system.
MIT-KERBEROS-5 Kerberos Version 5 user-to-user.
ACCESS SYSTEM DESCRIPTIONS
Any client on a host in the host access control list is allowed access
to the X server. This system can work reasonably well in an environ-
ment where everyone trusts everyone, or when only a single person can
log in to a given machine, and is easy to use when the list of hosts
used is small. This system does not work well when multiple people can
log in to a single machine and mutual trust does not exist. The list of
allowed hosts is stored in the X server and can be changed with the
xhost command. When using the more secure mechanisms listed below, the
host list is normally configured to be the empty list, so that only
authorized programs can connect to the display.
When using MIT-MAGIC-COOKIE-1, the client sends a 128 bit "cookie"
along with the connection setup information. If the cookie presented by
the client matches one that the X server has, the connection is allowed
access. The cookie is chosen so that it is hard to guess; xdm generates
such cookies automatically when this form of access control is used.
The user's copy of the cookie is usually stored in the .Xauthority file
in the home directory, although the environment variable XAUTHORITY can
be used to specify an alternate location. Xdm automatically passes a
cookie to the server for each new login session, and stores the cookie
in the user file at login.
The cookie is transmitted on the network without encryption, so there
is nothing to prevent a network snooper from obtaining the data and
using it to gain access to the X server. This system is useful in an
environment where many users are running applications on the same
machine and want to avoid interference from each other, with the caveat
that this control is only as good as the access control to the physical
network. In environments where network-level snooping is difficult,
this system can work reasonably well.
Sites in the United States can use a DES-based access control mechanism
called XDM-AUTHORIZATION-1. It is similar in usage to MIT-MAGIC-
COOKIE-1 in that a key is stored in the .Xauthority file and is shared
with the X server. However, this key consists of two parts -- a 56 bit
DES encryption key and 64 bits of random data used as the authentica-
When connecting to the X server, the application generates 192 bits of
data by combining the current time in seconds (since 00:00 1/1/1970
GMT) along with 48 bits of "identifier". For TCP/IP connections, the
identifier is the address plus port number; for local connections it is
the process ID and 32 bits to form a unique id (in case multiple con-
nections to the same server are made from a single process). This 192
bit packet is then encrypted using the DES key and sent to the X
server, which is able to verify if the requester is authorized to con-
nect by decrypting with the same DES key and validating the authentica-
tor and additional data. This system is useful in many environments
where host-based access control is inappropriate and where network
security cannot be ensured.
Recent versions of SunOS (and some other systems) have included a
secure public key remote procedure call system. This system is based
on the notion of a network principal; a user name and NIS domain pair.
Using this system, the X server can securely discover the actual user
name of the requesting process. It involves encrypting data with the X
server's public key, and so the identity of the user who started the X
server is needed for this; this identity is stored in the .Xauthority
file. By extending the semantics of "host address" to include this
notion of network principal, this form of access control is very easy
To allow access by a new user, use xhost. For example,
xhost keith@ ruthATmit.edu
adds "keith" from the NIS domain of the local machine, and "ruth" in
the "mit.edu" NIS domain. For keith or ruth to successfully connect to
the display, they must add the principal who started the server to
their .Xauthority file. For example:
xauth add expo.lcs.mit.edu:0 \
This system only works on machines which support Secure RPC, and only
for users which have set up the appropriate public/private key pairs on
their system. See the Secure RPC documentation for details. To access
the display from a remote host, you may have to do a keylogin on the
remote host first.
Kerberos is a network-based authentication scheme developed by MIT for
Project Athena. It allows mutually suspicious principals to authenti-
cate each other as long as each trusts a third party, Kerberos. Each
principal has a secret key known only to it and Kerberos. Principals
includes servers, such as an FTP server or X server, and human users,
whose key is their password. Users gain access to services by getting
Kerberos tickets for those services from a Kerberos server. Since the
X server has no place to store a secret key, it shares keys with the
user who logs in. X authentication thus uses the user-to-user scheme
of Kerberos version 5.
When you log in via xdm, xdm will use your password to obtain the ini-
tial Kerberos tickets. xdm stores the tickets in a credentials cache
file and sets the environment variable KRB5CCNAME to point to the file.
The credentials cache is destroyed when the session ends to reduce the
chance of the tickets being stolen before they expire.
Since Kerberos is a user-based authorization protocol, like the SUN-
DES-1 protocol, the owner of a display can enable and disable specific
users, or Kerberos principals. The xhost client is used to enable or
disable authorization. For example,
xhost krb5:judy krb5:gildeaATx.org
adds "judy" from the Kerberos realm of the local machine, and "gildea"
from the "x.org" realm.
THE AUTHORIZATION FILE
Except for Host Access control, each of these systems uses data stored in
the .Xauthority file to generate the correct authorization information to
pass along to the X server at connection setup. MIT-MAGIC-COOKIE-1 and
XDM-AUTHORIZATION-1 store secret data in the file; so anyone who can read
the file can gain access to the X server. SUN-DES-1 stores only the iden-
tity of the principal who started the server (unix.hostname@domain when the
server is started by xdm), and so it is not useful to anyone not authorized
to connect to the server.
Each entry in the .Xauthority file matches a certain connection family
(TCP/IP, DECnet or local connections) and X display name (hostname plus
display number). This allows multiple authorization entries for different
displays to share the same data file. A special connection family (Fami-
lyWild, value 65535) causes an entry to match every display, allowing the
entry to be used for all connections. Each entry additionally contains the
authorization name and whatever private authorization data is needed by
that authorization type to generate the correct information at connection
The xauth program manipulates the .Xauthority file format. It understands
the semantics of the connection families and address formats, displaying
them in an easy to understand format. It also understands that SUN-DES-1
and MIT-KERBEROS-5 use string values for the authorization data, and
displays them appropriately.
The X server (when running on a workstation) reads authorization informa-
tion from a file name passed on the command line with the -auth option (see
the Xserver(1X) manual page). The authorization entries in the file are
used to control access to the server. In each of the authorization schemes
listed above, the data needed by the server to initialize an authorization
scheme is identical to the data needed by the client to generate the
appropriate authorization information, so the same file can be used by both
This system uses 128 bits of data shared between the user and the X
server. Any collection of bits can be used. Xdm generates these keys
using a cryptographically secure pseudo random number generator, and so
the key to the next session cannot be computed from the current session
This system uses two pieces of information. First, 64 bits of random
data, second a 56 bit DES encryption key (again, random data) stored in
8 bytes, the last byte of which is ignored. Xdm generates these keys
using the same random number generator as is used for MIT-MAGIC-
This system needs a string representation of the principal which
identifies the associated X server. This information is used to encrypt
the client's authority information when it is sent to the X server.
When xdm starts the X server, it uses the root principal for the
machine on which it is running (unix.hostname@domain, for example,
"unix.expire.lcs.mit.eduATour.edu"). Putting the correct princi-
pal name in the .Xauthority file causes Xlib to generate the
appropriate authorization information using the secure RPC library.
Kerberos reads tickets from the cache pointed to by the KRB5CCNAME
environment variable, so does not use any data from the .Xauthority
file. An empty entry must still exist to tell clients that MIT-
KERBEROS-5 is available.
X(1X), xdm(1X), xauth(1X), xhost(1X), Xserver(1X)