Advisory: IceWarp WebMail Server: Cross Site Scripting in Email View

During a penetration test, RedTeam Pentesting discovered that the IceWarp
WebMail Server is prone to Cross Site Scripting attacks in its email view.
This enables attackers to send emails with embedded JavaScript code,
for example, to steal users' session IDs.

Details

Product: IceWarp eMail Server / WebMail Server
Affected Versions: 9.4.1
Fixed Versions: 9.4.2
Vulnerability Type: Cross Site Scripting
Security Risk: high
Vendor URL: http://www.icewarp.com/
Vendor Status: notified, fixed version released
Advisory URL: http://www.redteam-pentesting.de/advisories/rt-sa-2009-001
Advisory Status: published
CVE: CVE-2009-1467
CVE URL: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2009-1467

Introduction

"Feature complete yet easy to use, WebMail Server Pro provides feature
rich Web 2.0 web-based access to email, calendars, contacts, files and
shared data from any computer with browser and internet connection,
without the usual configuration hassle. Thanks to advanced technologies
and application-like look and feel, Pro suggests it was born to become the
ultimate replacement of Outlook and similar desktop mail clients."

(from the vendor's homepage)

More Details

To prevent the execution of JavaScript and VBScript code in HTML emails
and to remove unwanted HTML tags, the IceWarp WebMail Server filters HTML
emails with the function cleanHTML() that is defined in the PHP file

html/webmail/server/inc/tools.php

This filtering function can be circumvented in various ways, to still
allow XSS to happen.

Tag Removal

Beginning in line 462 down to line 482, the cleanHTML() function removes
or changes a variety of keywords which are considered malicious. This
includes the removal of all attributes starting with "on" (e.g.
onmouseover, onload etc.) and the rewriting of the words "javascript" and
"vbscript" to "noscript".

Later, in line 485, the cleanHTML() function completely removes various HTML
tags from the email:

$string = preg_replace('#</(meta|xml|blink|link|embed|object|iframe|
frame|frameset|ilayer|layer|bgsound)[^>]>#i',"",$string);

By inserting one of these HTML tags, which gets removed by the filtering
function, between the keywords which get filtered before, the cleanHTML()
function will not recognize them anymore. Later, it will remove the HTML
tag and thereby make the keyword valid again.

Proof of Concept Tag Removal

An HTML email with the following content will open an alert box in
victims' browsers when they move the mouse over the "XSS" text of the
<div> tag:

<div o<xml>nmouseover="alert('XSS')";>XSS</div>

This circumvention should work in all recent browsers with JavaScript
support, during the penetration test Firefox 3.x was used.

Hex Encoding

Another way to circumvent the filtering is to use hex-encoded HTML
entities. The character "j" can e.g. be encoded as "j". The
cleanHTML() function decodes the given string once in line 459:

$string = html_entity_decode($string, ENT_COMPAT, "UTF-8");

By double encoding the input, all regular expressions used for filtering
will fail to recognize malicious content even after the first decoding.

The effect of this vulnerability is limited by the fact that HTML
encoded entities are recognized by browsers only if they are part of a
text node or attribute values.

Proof of Concept Hex Encoding

The "src" attribute value of the HTML tag

<img src=javascript:alert(RedTeam)>

can be double encoded as

<img src=&#x6a;&#x6
1;&#x76;&#x
61;&#x73;&#
x63;&#x72;&
#x69;&#x70;
&#x74;&#x3a
;&#x61;&#x6
c;&#x65;&#x
72;&#x74;&#
x28;&#x34;&
#x32;&#x29;>

This will be decoded by the html_entity_decode() function in line 459 to
the string

<img src=javascript
:alert(42)>

The value of the "src" attribute will not be recognized by the filtering
attempts following the decoding in the cleanHTML() function.

A message with such an image tag in the body will trigger the JavaScript
and open a popup box if the browser supports "javascript:" attribute
values in image elements' "src" attributes. This particular proof of
concept works with Internet Explorer 6, the newest Firefox will not
execute the JavaScript.

Workaround

Users can disable the HTML message view in their configuration, so emails
are only displayed as plaintext.

Fix

Upgrade to version 9.4.2.

Security Risk

The risk of this vulnerability is estimated as high. Attackers have the
possibility to execute arbitrary JavaScript code in users' browsers and
are thereby able to steal, for example, the users' session IDs. Users
also cannot disallow JavaScript, as it is needed by the application
itself to work properly.

History

2009-03-23 Vulnerabilities identified during a penetration test
2009-04-01 Meeting with customer and vendor
2009-04-28 CVE number assigned
2009-05-05 Vendor publishes fixed version
2009-05-05 Advisory released

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