XSECURITY(1) X Version 11(Release 5) XSECURITY(1)
NAME
X Security - X display access control
SYNOPSIS
X provides mechanism for implementing many access control systems.
Release 5 includes four 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.
ACCESS SYSTEM DESCRIPTIONS
Host Access
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
environment 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.
MIT-MAGIC-COOKIE-1
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.
XDM-AUTHORIZATION-1
For sites in the US, Release 5 contains 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
10/89 Page 1
XSECURITY(1) X Version 11(Release 5) XSECURITY(1)
parts - a 56 bit DES encryption key and 64 bits of random data used
as the authenticator.
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 connections 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
requestor is authorized to connect by decrypting with the same DES
key and validating the authenticator and additional data. This
system is useful in many environments where host-based access
control is inappropriate and where network security cannot be
ensured.
SUN-DES-1
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 servers 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 use. To
allow access by a new user, use xhost. For example,
xhost keith@ joe@mit.edu
adds "keith" from the NIS domain of the local machine, and "joe" in
the "mit.edu" NIS domain. For keith or joe 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 SUN-DES-1 unix.expo.lcs.mit.edu:x.lcs.mit.edu
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.
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
identity 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.
Page 2 10/89
XSECURITY(1) X Version 11(Release 5) XSECURITY(1)
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 (FamilyWild, 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 setup time.
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 uses string values for the authorization data, and
displays them appropriately.
The X server (when running on a workstation) reads authorization
information from a file name passed on the command line with the -auth
option (see the Xserver 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 processes. This is especially useful when xinit
is used.
MIT-MAGIC-COOKIE-1
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 key.
XDM-AUTHORIZATION-1
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-COOKIE-1.
SUN-DES-1
This system needs a string representation of the principal which
identifies the associated X server. When xdm starts the X server,
it uses the root principal for the machine on which it is running
(unix.hostname@domain, e.g.
"unix.expire.lcs.mit.edu@x.lcs.mit.edu"). Putting the correct
principal name in the .Xauthority file causes Xlib to generate the
appropriate authorization information using the secure RPC library.
FILES
.Xauthority
10/89 Page 3
XSECURITY(1) X Version 11(Release 5) XSECURITY(1)
SEE ALSO
X(1), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)
Page 4 10/89