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.
- 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. However, this key consists of two parts -
a 56 bit DES encryption key and 64 bits of random data used as the
- 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
- 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.
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
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 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
- 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, e.g.,
"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.
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
X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)