=============================================
INTERNET SECURITY AUDITORS ALERT 2013-001
- Original release date: January 30th, 2013
- Last revised: March 25th, 2013
- Discovered by: Vicente Aguilera Diaz
- Severity: 4.3/10 (CVSSv2 Base Score)
=============================================

I. VULNERABILITY
-------------------------
CSRF vulnerability in LinkedIn

II. BACKGROUND
-------------------------
LinkedIn is a social networking service and website (www.linkedin.com)
for professionals. The site officially launched on May 5, 2003. As of
September 30, 2012 (the end of the third quarter), professionals are
signing up to join LinkedIn at a rate of approximately two new members
per second. Actually, Over 175 million professionals use LinkedIn to
exchange information, ideas and opportunities.

III. DESCRIPTION
-------------------------
CSRF (Cross-site Request Forgery) is an attack which forces an end
user to execute unwanted actions on a web application in which he/she
is currently authenticated. With a little help of social engineering
(like sending a link via email/chat), an attacker may force the users
of a web application to execute actions of the attacker's choosing. A
successful CSRF exploit can compromise end user data and operation in
case of normal user. If the targeted end user is the administrator
account, this can compromise the entire web application.

More info about CSRF:
https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)

LinkedIn is vulnerable to CSRF attacks in the "Add connections"
functionality. Specifically, in the "Send Invitation" request. The
only token for authenticate the user is a session cookie, and this
cookie is sent automatically by the browser in every request.

An attacker can create a page that includes requests to the "Send
Invitation" functionality of LinkedIn and add to his connections the
users who, being authenticated, visit the page of the attacker.

The attack is facilitated since the "Send Invitation" request can be
realized across the HTTP GET method instead of the POST method that is
realized habitually across the "Send Invitation" form.

IV. PROOF OF CONCEPT
-------------------------
Next, we show a typical request to the "Send Invitation" functionality:

POST /fetch/manual-invite-create HTTP/1.1
Host: www.linkedin.com
...

emailAddresses=<email>&subject=Invitation+to+connect+on+LinkedIn&csrfToken=ajax:1234567890123456789&sourceAlias=0_cB6j7zv7bfEcbTWXQyKwqELvCi7FWQRq-jJsq2WDImH

Some parameters are not used/validated by the application, so we can
remove these parameters from the request:
- csrfToken
- sourceAlias

Also, We can use HTTP GET method instead the HTTP POST method used at
this request. This makes it more easy the exploitation of the CSRF
vulnerability. So, finally, this HTTP request provoke the same result
that the original HTTP POST request:

GET
/fetch/manual-invite-create?emailAddresses=<email>&subject=Invitation+to+connect+on+LinkedIn

1. An attacker create a web page "csrf-exploit.html" that realize a
HTTP GET request to the "Send Invitation" functionality.

For example:
...
<img
src="http://www.linkedin.com/fetch/manual-invite-create?emailAddresses=<attacker_email>&subject="
width=0 height=0>
...

2. A user authenticated in LinkedIn visit the "csrf-exploit.html" page
controlled by the attacker.

For example, the attacker sends a mail to the victim (through the
messaging system that provides LinkedIn is better as it ensures that
the victim user is authenticated) and provokes that the victim visits
his page (using social engineering techniques).

3. The attacker receives an invitation request from the victim user,
so the attacker just accept this invitation and the user is added to
his connections/contacts.

V. BUSINESS IMPACT
-------------------------
A malicious user can access to the information they share users that
have been added to her contacts without his consent / knowledge.

VI. SYSTEMS AFFECTED
-------------------------
LinkedIn service.

VII. SOLUTION
-------------------------
Pending.

VIII. REFERENCES
-------------------------
http://www.linkedin.com
http://www.isecauditors.com

IX. CREDITS
-------------------------
This vulnerability has been discovered by
Vicente Aguilera Diaz vaguilera (at) isecauditors (dot) com).

X. REVISION HISTORY
-------------------------
January   16, 2013: Initial release
March     30, 2013: New update

XI. DISCLOSURE TIMELINE
-------------------------
January   16, 2013: Vulnerability acquired by
                    Internet Security Auditors.
March     10, 2013: Sent to Sec Team.
March     15, 2013: Notification about correction.
March     25, 2013: Sent to lists.

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

XIII. ABOUT
-------------------------
Internet Security Auditors is a Spain based leader in web application
testing, network security, penetration testing, security compliance
implementation and assessing. Our clients include some of the largest
companies in areas such as finance, telecommunications, insurance,
ITC, etc. We are vendor independent provider with a deep expertise
since 2001. Our efforts in R&D include vulnerability research, open
security project collaboration and whitepapers, presentations and
security events participation and promotion. For further information
regarding our security services, contact us.

XIV. FOLLOW US
-------------------------
You can follow Internet Security Auditors, news and security
advisories at:
https://www.facebook.com/ISecAuditors
https://twitter.com/ISecAuditors
http://www.linkedin.com/company/internet-security-auditors
http://www.youtube.com/