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Xsecurity - X display access control
X provides mechanism for implementing many access control systems. The
sample implementation 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 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 com-
mand. 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 automati-
cally 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 appli-
cations 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
Sites who compile with DES support 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 .Xau-
thority 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/IPv4 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 authentica-
tor and additional data. This system is useful in many environ-
ments 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@ email@example.com
adds "keith" from the NIS domain of the local machine, and
"ruth" in the "mit.edu" NIS domain. For keith or ruth to suc-
cessfully 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 firstname.lastname@example.org
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 princi-
pals 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 pass-
word. 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 cre-
dentials 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 dis-
able specific users, or Kerberos principals. The xhost client
is used to enable or disable authorization. For example,
xhost krb5:judy krb5:email@example.com
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 informa-
tion 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 any-
one 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.
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 dif-
ferent displays to share the same data file. A special connection fam-
ily (FamilyWild, value 65535) causes an entry to match every display,
allowing the entry to be used for all connections. Each entry addi-
tionally contains the authorization name and whatever private autho-
rization data is needed by that authorization type to generate the cor-
rect information at connection setup time.
The xauth program manipulates the .Xauthority file format. It under-
stands 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 authoriza-
tion data, and displays them appropriately.
The X server (when running on a workstation) reads authorization infor-
mation 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 autho-
rization schemes listed above, the data needed by the server to ini-
tialize 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.
This system uses 128 bits of data shared between the user and
the X server. Any collection of bits can be used. Xdm gener-
ates 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.
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.
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.host-
name@domain, e.g., "firstname.lastname@example.org").
Putting the correct principal name in the .Xauthority file
causes Xlib to generate the appropriate authorization informa-
tion 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 entry with no data must still exist to
tell clients that MIT-KERBEROS-5 is available.
Unlike the .Xauthority file for clients, the authority file
passed by xdm to a local X server (with ``-auth filename'', see
xdm(1)) does contain the name of the credentials cache, since
the X server will not have the KRB5CCNAME environment variable
set. The data of the MIT-KERBEROS-5 entry is the credentials
cache name and has the form ``UU:FILE:filename'', where filename
is the name of the credentials cache file created by xdm. Note
again that this form is not used by clients.
X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)
4.5.0 XFree86 XSECURITY(7)