Multiple Vendor Multiple Product URI Handler Input Validation Vulnerability

iDefense Security Advisory 07.19.07
http://labs.idefense.com/intelligence/vulnerabilities/
Jul 19, 2007

I. BACKGROUND

Microsoft Internet Explorer and Mozilla Firefox are the two most popular
web browsers. Many people have both products installed since it is very
difficult to remove Internet Explorer from a Windows system.
Additionally, people are unlikely to remove Internet Explorer due to
the need to use it for some sites which will not work with other
browsers.

II. DESCRIPTION

Remote exploitation of an input handling vulnerability within multiple
browsers on the Microsoft Windows platform allows code execution as the
local user.

This vulnerability is due to interaction between programs. The most
commonly used Microsoft Windows URL protocol handling code doesn't
provide a way for the URI handling application to distinguish the end
of one argument from the start of another.

The problem is caused by the fact that browsers do not pct-encode
certain characters in some URIs, which does not comply with the
behavior that RFC3986 (also known as IETF STD 66) requires. As a
result, a specially constructed link could be interpreted as multiple
arguments by a URI protocol handler.

III. ANALYSIS

Exploitation of this vulnerability allows an attacker to execute
arbitrary commands as the current user. To exploit this vulnerability,
an attacker must persuade their target into visiting a website
containing a maliciously constructed link.

This vulnerability does not allow for code execution directly. Instead
it relies on the URI handling application to contain methods by which
code execution is obtainable. Even if no such methods exist, it may
still be possible to pass additional command line parameters that
execute unintended actions.

The target user must have an application installed which accepts command
line options after the URI passed to the protocol handler, such as
versions of Firefox before 2.0.0.5. When opening a URL, typically it is
started with a command line such as:

  [path/to/handler.exe] -url "%1"

In this case, the "%1" is replaced with the source URL. If the URL
contains a double-quote character followed by a space, the quoting will
be closed, and the rest of the source URL will be treated as new
arguments.

On June 14, 2007 Microsoft stated to us that this behavior is
documented, referencing (
http://msdn2.microsoft.com/en-us/library/aa767914.aspx). At this time,
the document contained an example handler for the 'note:' protocol to
explain how to create URL handlers. It contained this type of
vulnerability. The documentation did state that the "handler passes the
complete URL string to the application", but did not explicitly state
that multiple arguments could be injected, and that the URI would be
percent-decoded. If the example handler from the documentation was
added, calc.exe would be launched when opening a page that contained
the following HTML:

  <iframe src='note:"|calc.exe '>

As this document was written to inform developers how to construct these
handlers, it is very likely many applications which implement URL
handlers are also affected. Microsoft has updated the document around
July 17, 2007. It has replaced the 'note:' handler with a new handler
example, 'alert:', which launches a sample application that lists the
command line arguments the URL handler passed to it.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability using the
following products on the Windows XP SP2 operating system.

  Mozilla Organization; Firefox and Thunderbird 2.0.0.4
  Microsoft Corp.; Internet Explorer 7

Previous versions of each application may also be affected. Other
applications which access websites with an embedded browser control are
also affected.

While this vulnerability is due to various applications incorrectly
escaping URIs, the way code execution occurs is via helper
applications.

V. WORKAROUND

The following keys are examples of URL Protocol handlers. Removing the
'shell' sub-keys of dangerous URI handlers will reduce exposure to this
class of vulnerability. For example:

  HKEY_CLASSES_ROOT\FirefoxURL\shell
  HKEY_CLASSES_ROOT\Thunderbird.Url.mailto\shell
  HKEY_CLASSES_ROOT\Thunderbird.Url.news\shell
  HKEY_CLASSES_ROOT\mailto\shell
  HKEY_CLASSES_ROOT\news\shell
 
Some functionality will be reduced after removing these keys. Certain
applications regenerate their protocol handlers automatically; consider
applying a 'Deny' to 'Everyone' to the handler key (the key above
without the '\shell' component). To find other protocol handlers,
search the registry for the value name "URL Protocol".

The 'NoScript' add-on for Firefox will prevent Firefox being used to
execute arbitrary code with this vulnerability. This is a 3rd party
extension which allows the user to choose which sites can execute
JavaScript.

Turning off the rendering of HTML within mail applications will mitigate
exposure to this type of vulnerability via an e-mail attack. In
Thunderbird, under the 'View' menu, select 'Message Body As' and select
'Plain Text'.

VI. VENDOR RESPONSE

Microsoft stated this is "documented behavior" in June 2007, but in
mid-July 2007 updated the contents of the page describing how to
construct a URL handler. They now include a security note that this
type of exposure may occur, and describe in detail the steps taken. A
link to this page is shown in the sources and referenced in the
analysis.

Mozilla has changed its handling of URLs in Firefox 2.0.0.5. Thunderbird
2.0.0.5 is not yet available for download, but will reportedly also
change its handling. Directly downloadable vendor updates for this
report are accessible via the iDefense Intelligence Web portal (
https://ialert.iDefense.com) and the iDefense Intelligence XML Web
service. Information about non-directly downloadable vendor updates can
be found by clicking the URLs shown.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-3670 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

06/13/2007  Initial vendor notification
06/13/2007  Initial Microsoft response
06/13/2007  Initial Mozilla response
06/14/2007  Microsoft states defined behavior
07/17/2007  Microsoft updates MSDN article
07/17/2007  Mozilla releases Firefox 2.0.0.5
07/19/2007  Public disclosure

IX. CREDIT

This vulnerability was discovered by Greg MacManus of iDefense labs.

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X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

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