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Xsecurity - X display access control
X provides mechanism for implementing many access control systems. The
sample implementation includes six 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.
Server Interpreted Server-dependent methods of access control
Not all of these are available in all builds or implementations.
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
command. The list is stored in the server by network address,
not host names, so is not automatically updated if a host
changes address while the server is running. 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. See the GRANTING ACCESS
section of the Xserver man page for details on how this list is
initialized at server startup.
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.
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
.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 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
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 .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
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 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
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@example.com
adds "judy" from the Kerberos realm of the local machine, and
"gildea" from the "x.org" realm.
The Server Interpreted method provides two strings to the X
server for entry in the access control list. The first string
represents the type of entry, and the second string contains the
value of the entry. These strings are interpreted by the server
and different implementations and builds may support different
types of entries. The types supported in the sample
implementation are defined in the SERVER INTERPRETED ACCESS
TYPES section below. Entries of this type can be manipulated
via xhost. For example to add a Server Interpreted entry of
type localuser with a value of root, the command is xhost
THE AUTHORIZATION FILE
Except for Host Access control and Server Interpreted 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 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 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 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.
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
"firstname.lastname@example.org"). Putting the correct
principal name in the .Xauthority file causes Xlib to generate
the appropriate authorization information using the secure RPC
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.
SERVER INTERPRETED ACCESS TYPES
The sample implementation includes several Server Interpreted
IPv6 IPv6 literal addresses
hostname Network host name
localuser Local connection user id
localgroup Local connection group id
IPv6 A literal IPv6 address as defined in IETF RFC 3513.
The value must be a hostname as defined in IETF RFC 2396. Due to
Mobile IP and dynamic DNS, the name service is consulted at
connection authentication time, unlike the traditional host
access control list which only contains numeric addresses and
does not automatically update when a host’s address changes.
Note that this definition of hostname does not allow use of
literal IP addresses.
localuser & localgroup
On systems which can determine in a secure fashion the
credentials of a client process, the "localuser" and
"localgroup" authentication methods provide access based on
those credentials. The format of the values provided is
platform specific. For POSIX & UNIX platforms, if the value
starts with the character ’#’, the rest of the string is treated
as a decimal uid or gid, otherwise the string is defined as a
user name or group name.
If your system supports this method and you use it, be warned
that some programs that proxy connections and are setuid or
setgid may get authenticated as the uid or gid of the proxy
process. For instance, some versions of ssh will be
authenticated as the user root, no matter what user is running
the ssh client, so on systems with such software, adding access
for localuser:root may allow wider access than intended to the X
X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)