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NAME
Xsecurity - X display access control
SYNOPSIS
X provides mechanism for implementing many access control
systems. The sample implementation includes five mechan-
isms:
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
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 informa-
tion. 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 vari-
able 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 diffi-
cult, this system can work reasonably well.
XDM-AUTHORIZATION-1
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 authenticator.
When connecting to the X server, the application gen-
erates 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 sys-
tem. 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 .Xau-
thority 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 .Xau-
thority 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.
MIT-KERBEROS-5
Kerberos is a network-based authentication scheme
developed by MIT for Project Athena. It allows mutu-
ally 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 Ker-
beros. 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 princi-
pals. 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 .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.
Each entry in the .Xauthority file matches a certain connec-
tion 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 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 and MIT-
KERBEROS-5 use string values for the authorization data, and
displays them appropriately.
The X server (when running on a workstation) reads authori-
zation 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 informa-
tion, 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 cryptographi-
cally 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 prin-
cipal 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,
e.g., "unix.expire.lcs.mit.edu@our.domain.edu"). Put-
ting the correct principal name in the .Xauthority file
causes Xlib to generate the appropriate authorization
information using the secure RPC library.
MIT-KERBEROS-5
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.
FILES
.Xauthority
SEE ALSO
X11(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)
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