XSecurity(1X) XSecurity(1X)
NAME
XSecurity, Xsecurity - X display access control
DESCRIPTION
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 envi-
ronment 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 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 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 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
requester 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.
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 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@ ruth@mit.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 file. For example:
xauth add expo.lcs.mit.edu:0 SUN-DES-1 unix.expo.lcs.mit.edu@our.domain.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. 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 authenticate 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 initial 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:gildea@x.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 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.host-
name@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 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 (Fam-
ilyWild, 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 infor-
mation at connection setup time.
The xauth program manipulates the 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 information 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 processes. 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 cryptographi-
cally secure pseudo random number generator, and so the key to the next session cannot be computed from the current session key. This sys-
tem 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. 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.edu@our.domain.edu"). Putting the correct
principal name in the 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 file. An empty entry must
still exist to tell clients that MIT-KERBEROS-5 is available.
FILES
SEE ALSO
X(1X), xdm(1X), xauth(1X), xhost(1X), Xserver(1X)
XSecurity(1X)