Session Token Remains Valid After Logout in IBM Lotus Domino Web Access

[dave.ferguson () fishnetsecurity com]

I. SYNOPSIS

Title: Session Token Remains Valid After Logout in IBM Lotus Domino Web Access 7.0.1
Release Date: 09/12/2006
Affected Application: IBM Lotus Domino Web Access 7.0.1
(versions prior to 7.0.1 were not tested but may still be vulnerable).

Nominal Severity: Low
Severity If Successfully Exploited: High
Impact: Attacker impersonates legitimate user
Mitigating Factors: Requires discovery of a valid LtpaToken to exploit.

Discovery: Dave Ferguson, Security Consultant, FishNet Security
Initial Notification of Vendor: 08/28/2006
Permanent Advisory Location:
http://www.fishnetsecurity.com/csirt/disclosure/ibm

II. EXECUTIVE SUMMARY

Vulnerability Overview:

In Lotus Domino Web Access (DWA) 7.0.1, the session token used to identify the user (called "LtpaToken") is not 
invalidated on the server upon user logout.  The cookie is removed from the browser, but the token continues to be 
recognized by the server until a configurable expiration time is reached.

Attack Overview:

The most likely attack scenario is session hijacking or session stealing.  Knowing a valid session token would allow a 
malicious person to access all functionality of the web application (except changing password, which requires knowledge 
of the current password).  Lotus DWA is a personal information management application that includes e-mail, calendar, 
and task management.  By hijacking (or stealing) a session, an attacker is able to impersonate a legitimate user, and 
can read the user's e-mail, send e-mail as the user, or change the user's preference settings.

III. TECHNICAL DETAIL

Vulnerability Details:

When a Lotus DWA user logs in, a cookie called "LtpaToken" is set into the browser and is used throughout the session 
to uniquely identify the user.  When a user logs out of DWA, the cookie is cleared from the browser, but this action 
has no effect on the server.  The token eventually expires on the server after some configurable amount of time.  A 
user who explicitly logs out of DWA may have a false sense of security.  The LtpaToken cookie in his browser is 
deleted, but the token is still valid from the server's perspective and can be used by an attacker if he can discover 
it.  Best practices in web application security would call for the LtpaToken to be invalidated/destroyed at logout 
time.  Note that the vulnerability described here was observed with Session authentication under the Domino Web Engine 
tab set to "Multiple Servers (SSO)".  The same behavior may occur with the "Single Server" configuration as well, but 
this was not tested.

The "LtpaToken" described here is a component in IBM's Lightweight Third-Party Authentication (LTPA) technology.  The 
LTPA technology was designed to be a defacto standard across the IBM product family. LTPA is used in both IBM WebSphere 
and Lotus Domino products and allows for single sign-on across physical servers.  For example, Domino can recognize and 
accept LTPA tokens created by WebSphere.  For more information, please see the IBM redpaper at 
http://www.redbooks.ibm.com/redpieces/pdfs/redp4104.pdf

IV. MITIGATING FACTORS

Keeping the LtpaToken confidential is critical to mitigating this issue.  An attacker must be able to discover a valid 
LtpaToken before it expires.  Because the LtpaToken is sent with each request, Lotus DWA should be deployed as a secure 
application.  This means an SSL certificate should be installed on the server so that encrypted (https) communication 
between the browser and the server occurs.

Cross-site scripting (XSS) is a common application-level attack that can be used to steal cookies such as LtpaToken.  
Running the application under SSL does not hinder XSS attacks.  Fortunately, Lotus Domino includes a module called 
Active Content Filter that is highly effective at removing potentially harmful scripts in e-mail messages.  Active 
Content Filtering should be turned on.

Finally, the overall risk level can be lowered by enabling an idle session timeout in addition to the absolute 
expiration time.  Ideally, from an application security perspective, the idle (inactivity) timeout would be much 
smaller than the absolute expiration.  Be aware that the increased security from having small timeout values may 
negatively affect end-user satisfaction in the application.

V. VENDOR RECOMMENDED ACTIONS

IBM recommends running Lotus DWA run under SSL and using a token expiration time of 30 minutes.

Please see IBM technote #1245589:
http://www-1.ibm.com/support/docview.wss?rs=463&uid=swg21245589

VI. CONTACT

You can reach the author of this advisory at: dave.ferguson[at]fishnetsecurity(dot)com