Xsecurity

XSECURITY(7)           Miscellaneous Information Manual           XSECURITY(7)



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.

FILES
       .Xauthority

SEE ALSO
       X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)



X Version 11                    xorg-docs 1.7.1                   XSECURITY(7)