Title:
======
Skype Account Service - Session Token Bypass Vulnerability


Date:
=====
2012-11-15


References:
===========
http://www.vulnerability-lab.com/get_content.php?id=762
http://www.vulnerability-lab.com/get_content.php?id=739

MSRC ID: 13175


VL-ID:
=====
739


Common Vulnerability Scoring System:
====================================
6.2


Introduction:
=============
Skype is a proprietary voice-over-Internet Protocol service and software application originally created in 2003 by Swedish entrepreneur 
Niklas Zennström and his Danish partner Janus Friis. It has been owned by Microsoft since 2011. The service allows users to communicate 
with peers by voice, video, and instant messaging over the Internet. Phone calls may be placed to recipients on the traditional telephone 
networks. Calls to other users within the Skype service are free of charge, while calls to landline telephones and mobile phones are charged 
via a debit-based user account system. Skype has also become popular for its additional features, including file transfer, and videoconferencing. 
Competitors include SIP and H.323-based services, such as Linphone, as well as the Google Talk service, Mumble and Hall.com.

Skype has 663 million registered users as of September 2011. The network is operated by Microsoft, which has its Skype division headquarters 
in Luxembourg. Most of the development team and 44% of the overall employees of the division are situated in Tallinn and Tartu, Estonia.

Unlike most other VoIP services, Skype is a hybrid peer-to-peer and client–server system. It makes use of background processing on computers 
running Skype software. Skype`s original proposed name (Sky Peer-to-Peer) reflects this fact. Some network administrators have banned Skype 
on corporate, government, home, and education networks, citing reasons such as inappropriate usage of resources, excessive bandwidth usage, 
and security concerns.

(Copy of the Vendor Homepage: http://en.wikipedia.org/wiki/Skype)


Abstract:
=========
The Vulnerability Laboratory Research Team discovered a high severity web session vulnerability in Microsofts Skype Account Service Application.


Report-Timeline:
================
2012-10-31:  Researcher Notification & Coordination (VL - Benjamin Kunz Mejri)
2012-11-01:  Vendor Notification (MSRC)
2012-11-01:  Vendor Response/Feedback (MSRC)
2012-11-12:  Vendor Fix/Patch (Skype)
2012-11-15:  Public Disclosure (Vulnerability Laboratory)


Status:
========
Published


Affected Products:
==================
Microsoft
Product: Skype Application & Software Account Service v2012 Q3


Exploitation-Technique:
=======================
Remote


Severity:
=========
High


Details:
========
A session token protection bypass vulnerability is detected in Microsofts Skype WebServer Auth System Application.
The vulnerability allows remote attackers without privileged application account to bypass the token expire protection of 
the skype account application system.

The high severity application vulnerability is located in the recovery account function of the skype account service application. 
In the recovery notification mail function is a GET request as link which expires after 6hours. The bug is located in the reset 
function when processing to load the token_expire parameter with value 1 (invalid). Attackers can change the stable url by using 
a zero (0+ / +0) to bypass the checked token_expire value (parameter) to get a valid link with the expired session.

Successful exploitation of the remote vulnerability can result in session token_expire bypass to validate ago expired reset requests.


Vulnerable Service(s):
        [+] Microsoft > Skype > https://login.skype.com/

Vulnerable Section(s):
        [+] Account Application Service - https://login.skype.com/account

Vulnerable Module(s):
        [+] Recovery/Reset Function - https://login.skype.com/intl/de/account/password-reset-request

Vulnerable Module(s):
        [+] token_expired


Proof of Concept:
=================
The vulnerability can be exploited by remote attackers without required user inter action and without privileged user account.
For demonstration or reproduce ...

Reset Account: (Mail Link) 
https://api.skype.com/tracking/emails/click?m=7131000014602077951&go=account.changepassword&x=intcmp%3DT_140-_-H-_-311012-_-
account.changepassword%26token%3D621d8b0e773cc298a149a1b916118114%26application%3Daccount

which leads to the expired request with the already used session 6h+ ago ...

https://login.skype.com/intl/de/account/password-reset-request?token=621d8b0e773cc298a149a1b916118114&mode=&token_expired=1

It is only required to insert 0 (zero) in the GET/POST request make the session valid again. Replace token_expired=1 => token_expired=0

https://login.skype.com/intl/de/account/password-reset-request?token=621d8b0e773cc298a149a1b916118114&mode=&token_expired=(+)0

Video(PoC Demo):
        [+] http://www.youtube.com/watch?v=lrNj785QQlg


Solution:
=========
2012-11-12:  Vendor Fix/Patch (Skype)


Risk:
=====
The security risk of the token protection bypass vulnerability is estimated as high.


Credits:
========
Vulnerability Laboratory [Research Team] - Benjamin Kunz Mejri (bkm@vulnerability-lab.com)


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===========
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