Section: Environments, Tables, and Troff Macros (7)
Updated: xorg-docs 1.7Index
Return to Main Contents
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.
Server Interpreted Server-dependent methods of access control
Not all of these are available in all builds or implementations.
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. 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
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.
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 use.
To allow access by a new user, use xhost. For example,
xhost keith@ firstname.lastname@example.org
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 email@example.com
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.
- Server Interpreted
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 +si:localuser:root.
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
string values for the authorization data, and displays
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 library.
SERVER INTERPRETED ACCESS TYPES
The sample implementation includes several Server Interpreted mechanisms:
IPv6 IPv6 literal addresses
hostname Network host name
localuser Local connection user id
localgroup Local connection group id
A literal IPv6 address as defined in IETF RFC 3513. This allows adding
IPv6 addresses when the X server supports IPv6, but the xhost client was
compiled without IPv6 support.
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 display.
- ACCESS SYSTEM DESCRIPTIONS
- THE AUTHORIZATION FILE
- SERVER INTERPRETED ACCESS TYPES
- SEE ALSO