Xsecurity(5)

NAME

       Xsecurity - X display access control

SYNOPSIS

       X provides mechanism for implementing many access control systems.  The
       sample implementation includes six 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.
           MIT-KERBEROS-5                Kerberos Version 5 user-to-user.
           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 com-
mand. 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 automati-
cally 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 appli-
cations 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 .Xau-
thority 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 authentica-
tor and additional data. This system is useful in many environ-
ments 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 suc-
cessfully 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.

MIT-KERBEROS-5
Kerberos is a network-based authentication scheme developed by
MIT for Project Athena. It allows mutually suspicious princi-
pals to authenticate each other as long as each trusts a third
party, Kerberos. Each principal has a secret key known only to
it and Kerberos. Principals includes servers, such as an FTP
server or X server, and human users, whose key is their pass-
word. Users gain access to services by getting Kerberos tickets
for those services from a Kerberos server. Since the X server
has no place to store a secret key, it shares keys with the user
who logs in. X authentication thus uses the user-to-user scheme
of Kerberos version 5.

When you log in via xdm, xdm will use your password to obtain
the initial Kerberos tickets. xdm stores the tickets in a cre-
dentials cache file and sets the environment variable KRB5CCNAME
to point to the file. The credentials cache is destroyed when
the session ends to reduce the chance of the tickets being
stolen before they expire.

Since Kerberos is a user-based authorization protocol, like the
SUN-DES-1 protocol, the owner of a display can enable and dis-
able specific users, or Kerberos principals. The xhost client
is used to enable or disable authorization. For example,
xhost krb5:judy krb5:gildea@x.org
adds "judy" from the Kerberos realm of the local machine, and
"gildea" from the "x.org" realm.

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 implementa-
tion 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  gen-
       erate  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 dif-
       ferent displays to share the same data file.  A special connection fam-
       ily  (FamilyWild,  value 65535) causes an entry to match every display,
       allowing the entry to be used for all connections.   Each  entry  addi-
       tionally  contains  the  authorization name and whatever private autho-
       rization data is needed by that authorization type to generate the cor-
       rect information at connection setup time.

       The  xauth  program manipulates the .Xauthority file format.  It under-
       stands the semantics of the connection families  and  address  formats,
       displaying  them  in an easy to understand format.  It also understands
       that SUN-DES-1 and MIT-KERBEROS-5 use string values for the  authoriza-
       tion data, and displays them appropriately.

       The X server (when running on a workstation) reads authorization infor-
       mation 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  autho-
       rization  schemes  listed  above, the data needed by the server to ini-
       tialize 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  gener-
              ates  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.host-
              name@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  informa-
              tion using the secure RPC library.

       MIT-KERBEROS-5
              Kerberos  reads  tickets  from  the  cache  pointed  to  by  the
              KRB5CCNAME environment variable, so does not use any  data  from
              the .Xauthority file.  An entry with no data must still exist to
              tell clients that MIT-KERBEROS-5 is available.

              Unlike the .Xauthority file  for  clients,  the  authority  file
              passed  by xdm to a local X server (with ``-auth filename'', see
              xdm(1)) does contain the name of the  credentials  cache,  since
              the  X  server will not have the KRB5CCNAME environment variable
              set.  The data of the MIT-KERBEROS-5 entry  is  the  credentials
              cache name and has the form ``UU:FILE:filename'', where filename
              is the name of the credentials cache file created by xdm.   Note
              again that this form is not used by clients.

SERVER INTERPRETED ACCESS TYPES

       The  sample  implementation  includes several Server Interpreted mecha-
       nisms:
           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.

       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 con-
              nection 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  creden-
              tials  of  a  client  process,  the "localuser" and "localgroup"
              authentication methods provide access  based  on  those  creden-
              tials.   The format of the values provided is platform specific.
              For POSIX & UNIX platforms, if the value starts with the charac-
              ter  '#',  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 set-
              gid  may  get  authenticated  as  the  uid  or  gid of the proxy
              process.  For instance, some versions of ssh will  be  authenti-
              cated  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