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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 com-
mand. 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
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 automati-
cally 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 appli-
cations 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
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 .Xau-
thority 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 authentica-
tor and additional data. This system is useful in many environ-
ments 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@ 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 suc-
cessfully 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
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 implementa-
tion 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 gen-
erate 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 dif-
ferent displays to share the same data file. A special connection fam-
ily (FamilyWild, value 65535) causes an entry to match every display,
allowing the entry to be used for all connections. Each entry addi-
tionally contains the authorization name and whatever private autho-
rization data is needed by that authorization type to generate the cor-
rect information at connection setup time.
The xauth program manipulates the .Xauthority file format. It under-
stands 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 dis-
plays them appropriately.
The X server (when running on a workstation) reads authorization infor-
mation 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 autho-
rization schemes listed above, the data needed by the server to ini-
tialize 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 gener-
ates 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.host-
name@domain, e.g., "firstname.lastname@example.org").
Putting the correct principal name in the .Xauthority file
causes Xlib to generate the appropriate authorization informa-
tion using the secure RPC library.
SERVER INTERPRETED ACCESS TYPES
The sample implementation includes several Server Interpreted mecha-
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 con-
nection 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
localuser & localgroup
On systems which can determine in a secure fashion the creden-
tials of a client process, the "localuser" and "localgroup"
authentication methods provide access based on those creden-
tials. The format of the values provided is platform specific.
For POSIX & UNIX platforms, if the value starts with the charac-
ter '#', the rest of the string is treated as a decimal uid or
gid, otherwise the string is defined as a user name or group
If your system supports this method and you use it, be warned
that some programs that proxy connections and are setuid or set-
gid may get authenticated as the uid or gid of the proxy
process. For instance, some versions of ssh will be authenti-
cated 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)
X Version 11 XSECURITY(7)