XSECURITY(7) Miscellaneous Information Manual XSECURITY(7)
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
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 run-
ning. 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 pre-
sented 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 usu-
ally 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
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.
The Server Interpreted method provides two strings to the X server for entry in the access control list. The first string repre-
sents 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-AUTHORIZA-
TION-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 uses 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 use-
ful 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
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 princi-
pal for the machine on which it is running (unix.hostname@domain, e.g., "firstname.lastname@example.org"). Putting the cor-
rect principal name in the .Xauthority file causes Xlib to generate the appropriate authorization information using the secure RPC
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
IPv6 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 connec-
tion authentication time, unlike the traditional host access control list which only contains numeric addresses and does not auto-
matically 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" authentica-
tion 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.
X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)
X Version 11 xorg-docs 1.6 XSECURITY(7)