XSecurity, Xsecurity - X display access control
X provides mechanism for implementing many access control systems. Release 6 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.|
|MIT-KERBEROS-5||Kerberos Version 5 user-to-user.|
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. 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. 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.
Except for Host 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 ([email protected] 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 and MIT-KERBEROS-5 use 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
option (see the
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 system uses 128 bits of data shared between the user
and the X server. Any collection of bits can be used.
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.
these keys using the same random number generator as is used for MIT-MAGIC-COOKIE-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
starts the X server, it uses the root principal for the machine
on which it is running ([email protected],
for example, "[email protected]"). Putting the correct
principal name in the
file causes Xlib to
generate the appropriate authorization information using the secure RPC library.
Kerberos reads tickets from the cache pointed to by the
so does not use any data from the
An empty entry must still exist to tell clients that MIT-KERBEROS-5 is available.
X(1X), xdm(1X), xauth(1X), xhost(1X), Xserver(1X)