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 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 use.
- To allow access by a new user, use xhost. For
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
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
when the server is started by
), and so it is not useful to anyone not authorized to connect to
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.
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
The X server (when running on a workstation) reads authorization information
from a file name passed on the command line with the -auth
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
- 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
- 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.
"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.