[ZH2005-16SA] Insecure temporary file creation in Skype for Linux


Application: Skype for Linux
Version affected: <= 1.1.0.20
Vendor website : http://www.skype.com


Author: Giovanni Delvecchio
e-mail: badpenguin@zone-h.org



About Skype
========
Skype is a free program that uses the latest P2P technology to bring 
affordable and high-quality voice communications to people all over the 
world.It also provides a service of Instant Messaging.



Details
====
Each user has his own profile which can be personalized with a picture. When 
a user adds a picture for his profile, Skype creates in /tmp directory a 
file named "skype_profile.jpg" in an insecure manner, without checking if 
the file already exists and if it's a symbolic link.

-----------------------------------------------------------------------------------------------------------------------------------------
bad@badpenguin:~/skype-1.1.0.20$ strace -e trace=open skype
.
.
open("/home/bad/image.jpg", O_RDONLY|O_LARGEFILE) = 21 // picture chosen by 
user
open("/tmp/skype_profile.jpg", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 
23 // insecure temporary file creation (it should use O_EXCL or O_NOFOLLOW 
flag)
.
.
------------------------------------------------------------------------------------------------------------------------------------------


This could represent a security problem in a multi-user environment because 
usually /tmp directory is "world-writable".
Indeed, such problem could be exploited by a malicious local user via 
symlink attack to overwrite arbitrary files with the privileges of the user 
that running Skype.


Example:

ln -s file_to_overwrite /tmp/skype_profile.jpg

When the user will add a picture for his profile , the file symlinked by 
attacker will be overwritten with the file content that the user has chosen 
to update his profile.

In certain conditions a privilege escalation is possible.
An example of privilege escalation exploiting this type of vulnerability is 
the following:

from http://www.securityfocus.com/archive/82/327361/2003-06-29/2003-07-05/0 
:

"Starting release 9, Red Hat ships and uses pam_timestamp_check.so module
(accompanied by /sbin/pam_timestamp_check setuid helper), a part of the
new pam-0.75 (Pluggable Authentication Modules) package. PAM is a generic
centralized authentication and session management component that is also
shipped by an increasing number of other distributions, so it is
reasonable that the code is about to propagate to other distros.
The module mentioned implements a credential caching functionality, very
closely inspired on a tty ticketing system used in sudo.
The way the module works(and sudo), in essence, is that it gets current
pseudo-terminal name A, current user name B, and the user for which
credentials are cached, C (usually root for Red Hat applications, user
himself for sudo). Then the code checks for /var/run/sudo/B/A:C (or
/var/run/sudo/B/A if B == C), and if the file is recent (regardless of its 
content), the module returns success, and enables the user to skip the usual 
password
authentication.

Since there's no check for file origin, it should be more than obvious that 
suddenly, any insecure file creation problem in an application used by a 
superuser,it is possible to spoof a ticket in /var/run and bypass root 
password prompt and other checks, and perform administrative tasks, easily 
modifying system config, installing custom components (say, a rootshell), 
etc. All this by
crafting a single symlink that is later opened with O_CREAT with no O_EXCL
or O_NOFOLLOW."


Example:
----------------------------------------------------------------------------------------
#!/bin/sh

#get current terminal number from /dev/pts/xx
terminal_number=`tty | cut -f4 -d '/'`

user_ticket=$USER/$terminal_number:root
ln -s /var/run/sudo/$user_ticket /tmp/skype_profile.jpg
---------------------------------------------------------------------------------------------



Solution
=====
No fix available at the moment;
Grant only trusted users writing access to /tmp directory .



Timeline
=====
07 April 2005 - bug dicovered

08 April 2005 - Skype contacted by security@skype.net

14 April 2005 - 1th Response from Skype:
"Thank you for the email, we will pass it on to our developers.
Regards,
Andres"

25 May 2005 - Skype for Linux version 1.1.0.13 released, the problem is 
present again.

27 May 2005 - Skype re-contacted by security@skype.net

27 May 2005 - 2th Response from Skype:
"Giovanni, Thank you for the email again. I've spoken to our Linux 
developers and they assure me this will be fixed in the next version and 
they are considering posting an immediate advisory.
Again, your help is appreciated.
Regards,
Andres"

5 July 2005 - Skype for Linux version 1.1.0.20 released, but the bug hasn't 
been fixed.

15 July 205 - Public advisory



Author's Note
========
Although this type of vulnerability isn't a problem for a single desktop 
user, instead it could represent a problem in a multi user environment.
For example, Skype is used by many companies with some hundred of employees, 
and in such environment several users share an access on an host, locally or 
remotely . Any user,even with low privileges, could exploit this type 
vulnerabily to overwrite or create arbitrary files with the privileges of 
the user that running Skype, and in certain case conduct a privilege 
escalation.
A study made by CERT (Computer Emergency Response Team) called ITS (Insider 
Thread Study) shows like many times the incidents in companies are 
pepetrated by insider (eg. current or former employees ).
So, i think that in these conditions this bug may represent a security 
problem , and Skype should fix it quickly.



Reference
======
http://www.zone-h.org/advisories/read/id=7808

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