Ubuntu Manpage: Xsecurity - X display access control

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NAME

       Xsecurity - X display access control

OVERVIEW

       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.

MIT-MAGIC-COOKIE-1
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.

XDM-AUTHORIZATION-1
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.

SUN-DES-1
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@ ruth@mit.edu
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 unix.expo.lcs.mit.edu@our.domain.edu
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 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.

MIT-MAGIC-COOKIE-1
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.

XDM-AUTHORIZATION-1
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.

SUN-DES-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., "unix.expire.lcs.mit.edu@our.domain.edu"). 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

       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.

       hostname
              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.

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