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 [Toc] [Back] 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. 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@ [email protected]
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
[email protected]
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:[email protected]
adds "judy" from the Kerberos realm of the local
machine, and "gildea" from the "x.org" realm.
THE AUTHORIZATION FILE [Toc] [Back] 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.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 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 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
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.hostname@domain, for example,
"[email protected]"). 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.
SEE ALSO
X(1X), xdm(1X), xauth(1X), xhost(1X), Xserver(1X)
XSecurity(1X)
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