XSECURITY(7)                                             Miscellaneous Information Manual                                             XSECURITY(7)

NAME

       Xsecurity - X display access control

SYNOPSIS

       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 run-
              ning.  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  pre-
              sented  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  usu-
              ally  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 repre-
              sents 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-AUTHORIZA-
       TION-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  use-
       ful 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  princi-
              pal for the machine on which it is running (unix.hostname@domain, e.g., "unix.expire.lcs.mit.edu@our.domain.edu").  Putting the cor-
              rect 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 connec-
              tion  authentication  time, unlike the traditional host access control list which only contains numeric addresses and does not auto-
              matically 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" authentica-
              tion  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 1.6                                                      XSECURITY(7)