1XSECURITY(7) Miscellaneous Information Manual XSECURITY(7)
2
6 Xsecurity - X display access control
7
9 X provides mechanism for implementing many access control systems. The
10 sample implementation includes five mechanisms:
11 Host Access Simple host-based access control.
12 MIT-MAGIC-COOKIE-1 Shared plain-text "cookies".
13 XDM-AUTHORIZATION-1 Secure DES based private-keys.
14 SUN-DES-1 Based on Sun's secure rpc system.
15 Server Interpreted Server-dependent methods of access control
16 Not all of these are available in all builds or implementations.
17
19 Host Access
20 Any client on a host in the host access control list is allowed
21 access to the X server. This system can work reasonably well in
22 an environment where everyone trusts everyone, or when only a
23 single person can log in to a given machine, and is easy to use
24 when the list of hosts used is small. This system does not work
25 well when multiple people can log in to a single machine and
26 mutual trust does not exist. The list of allowed hosts is
27 stored in the X server and can be changed with the xhost com‐
28 mand. The list is stored in the server by network address, not
29 host names, so is not automatically updated if a host changes
30 address while the server is running. When using the more secure
31 mechanisms listed below, the host list is normally configured to
32 be the empty list, so that only authorized programs can connect
33 to the display. See the GRANTING ACCESS section of the Xserver
34 man page for details on how this list is initialized at server
35 startup.
36 MIT-MAGIC-COOKIE-1
38 When using MIT-MAGIC-COOKIE-1, the client sends a 128 bit
39 "cookie" along with the connection setup information. If the
40 cookie presented by the client matches one that the X server
41 has, the connection is allowed access. The cookie is chosen so
42 that it is hard to guess; xdm generates such cookies automati‐
43 cally when this form of access control is used. The user's copy
44 of the cookie is usually stored in the .Xauthority file in the
45 home directory, although the environment variable XAUTHORITY can
46 be used to specify an alternate location. Xdm automatically
47 passes a cookie to the server for each new login session, and
48 stores the cookie in the user file at login.
49 The cookie is transmitted on the network without encryption, so
51 there is nothing to prevent a network snooper from obtaining the
52 data and using it to gain access to the X server. This system
53 is useful in an environment where many users are running appli‐
54 cations on the same machine and want to avoid interference from
55 each other, with the caveat that this control is only as good as
56 the access control to the physical network. In environments
57 where network-level snooping is difficult, this system can work
58 reasonably well.
59 XDM-AUTHORIZATION-1
61 Sites who compile with DES support can use a DES-based access
62 control mechanism called XDM-AUTHORIZATION-1. It is similar in
63 usage to MIT-MAGIC-COOKIE-1 in that a key is stored in the .Xau‐
64 thority file and is shared with the X server. However, this key
65 consists of two parts - a 56 bit DES encryption key and 64 bits
66 of random data used as the authenticator.
67 When connecting to the X server, the application generates 192
69 bits of data by combining the current time in seconds (since
70 00:00 1/1/1970 GMT) along with 48 bits of "identifier". For
71 TCP/IPv4 connections, the identifier is the address plus port
72 number; for local connections it is the process ID and 32 bits
73 to form a unique id (in case multiple connections to the same
74 server are made from a single process). This 192 bit packet is
75 then encrypted using the DES key and sent to the X server, which
76 is able to verify if the requestor is authorized to connect by
77 decrypting with the same DES key and validating the authentica‐
78 tor and additional data. This system is useful in many environ‐
79 ments where host-based access control is inappropriate and where
80 network security cannot be ensured.
81 SUN-DES-1
83 Recent versions of SunOS (and some other systems) have included
84 a secure public key remote procedure call system. This system
85 is based on the notion of a network principal; a user name and
86 NIS domain pair. Using this system, the X server can securely
87 discover the actual user name of the requesting process. It
88 involves encrypting data with the X server's public key, and so
89 the identity of the user who started the X server is needed for
90 this; this identity is stored in the .Xauthority file. By
91 extending the semantics of "host address" to include this notion
92 of network principal, this form of access control is very easy
93 to use.
94 To allow access by a new user, use xhost. For example,
96 xhost keith@ ruth@mit.edu
97 adds "keith" from the NIS domain of the local machine, and
98 "ruth" in the "mit.edu" NIS domain. For keith or ruth to suc‐
99 cessfully connect to the display, they must add the principal
100 who started the server to their .Xauthority file. For example:
101 xauth add expo.lcs.mit.edu:0 SUN-DES-1 unix.expo.lcs.mit.edu@our.domain.edu
102 This system only works on machines which support Secure RPC, and
103 only for users which have set up the appropriate public/private
104 key pairs on their system. See the Secure RPC documentation for
105 details. To access the display from a remote host, you may have
106 to do a keylogin on the remote host first.
107 Server Interpreted
109 The Server Interpreted method provides two strings to the X
110 server for entry in the access control list. The first string
111 represents the type of entry, and the second string contains the
112 value of the entry. These strings are interpreted by the server
113 and different implementations and builds may support different
114 types of entries. The types supported in the sample implementa‐
115 tion are defined in the SERVER INTERPRETED ACCESS TYPES section
116 below. Entries of this type can be manipulated via xhost. For
117 example to add a Server Interpreted entry of type localuser with
118 a value of root, the command is xhost +si:localuser:root.
119
121 Except for Host Access control and Server Interpreted Access Control,
122 each of these systems uses data stored in the .Xauthority file to gen‐
123 erate the correct authorization information to pass along to the X
124 server at connection setup. MIT-MAGIC-COOKIE-1 and XDM-AUTHORIZATION-1
125 store secret data in the file; so anyone who can read the file can gain
126 access to the X server. SUN-DES-1 stores only the identity of the
127 principal who started the server (unix.hostname@domain when the server
128 is started by xdm), and so it is not useful to anyone not authorized to
129 connect to the server.
130 Each entry in the .Xauthority file matches a certain connection family
132 (TCP/IP, DECnet or local connections) and X display name (hostname plus
133 display number). This allows multiple authorization entries for dif‐
134 ferent displays to share the same data file. A special connection fam‐
135 ily (FamilyWild, value 65535) causes an entry to match every display,
136 allowing the entry to be used for all connections. Each entry addi‐
137 tionally contains the authorization name and whatever private autho‐
138 rization data is needed by that authorization type to generate the cor‐
139 rect information at connection setup time.
140 The xauth program manipulates the .Xauthority file format. It under‐
142 stands the semantics of the connection families and address formats,
143 displaying them in an easy to understand format. It also understands
144 that SUN-DES-1 uses string values for the authorization data, and dis‐
145 plays them appropriately.
146 The X server (when running on a workstation) reads authorization infor‐
148 mation from a file name passed on the command line with the -auth
149 option (see the Xserver manual page). The authorization entries in the
150 file are used to control access to the server. In each of the autho‐
151 rization schemes listed above, the data needed by the server to ini‐
152 tialize an authorization scheme is identical to the data needed by the
153 client to generate the appropriate authorization information, so the
154 same file can be used by both processes. This is especially useful
155 when xinit is used.
156 MIT-MAGIC-COOKIE-1
158 This system uses 128 bits of data shared between the user and
159 the X server. Any collection of bits can be used. Xdm gener‐
160 ates these keys using a cryptographically secure pseudo random
161 number generator, and so the key to the next session cannot be
162 computed from the current session key.
163 XDM-AUTHORIZATION-1
165 This system uses two pieces of information. First, 64 bits of
166 random data, second a 56 bit DES encryption key (again, random
167 data) stored in 8 bytes, the last byte of which is ignored. Xdm
168 generates these keys using the same random number generator as
169 is used for MIT-MAGIC-COOKIE-1.
170 SUN-DES-1
172 This system needs a string representation of the principal which
173 identifies the associated X server. This information is used to
174 encrypt the client's authority information when it is sent to
175 the X server. When xdm starts the X server, it uses the root
176 principal for the machine on which it is running (unix.host‐
177 name@domain, e.g., "unix.expire.lcs.mit.edu@our.domain.edu").
178 Putting the correct principal name in the .Xauthority file
179 causes Xlib to generate the appropriate authorization informa‐
180 tion using the secure RPC library.
181
183 The sample implementation includes several Server Interpreted mecha‐
184 nisms:
185 IPv6 IPv6 literal addresses
186 hostname Network host name
187 localuser Local connection user id
188 localgroup Local connection group id
189 IPv6 A literal IPv6 address as defined in IETF RFC 3513. This
191 allows adding IPv6 addresses when the X server supports IPv6,
192 but the xhost client was compiled without IPv6 support.
193 hostname
195 The value must be a hostname as defined in IETF RFC 2396. Due to
196 Mobile IP and dynamic DNS, the name service is consulted at con‐
197 nection authentication time, unlike the traditional host access
198 control list which only contains numeric addresses and does not
199 automatically update when a host's address changes. Note that
200 this definition of hostname does not allow use of literal IP
201 addresses.
202 localuser & localgroup
204 On systems which can determine in a secure fashion the creden‐
205 tials of a client process, the "localuser" and "localgroup"
206 authentication methods provide access based on those creden‐
207 tials. The format of the values provided is platform specific.
208 For POSIX & UNIX platforms, if the value starts with the charac‐
209 ter '#', the rest of the string is treated as a decimal uid or
210 gid, otherwise the string is defined as a user name or group
211 name.
212 If your system supports this method and you use it, be warned
214 that some programs that proxy connections and are setuid or set‐
215 gid may get authenticated as the uid or gid of the proxy
216 process. For instance, some versions of ssh will be authenti‐
217 cated as the user root, no matter what user is running the ssh
218 client, so on systems with such software, adding access for
219 localuser:root may allow wider access than intended to the X
220 display.
221
223 .Xauthority
224
226 X(7), xdm(1), xauth(1), xhost(1), xinit(1), Xserver(1)
227X Version 11 xorg-docs 1.7.1 XSECURITY(7)