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Multiple Vendor Multiple Product URI Handler Input Validation
Vulnerability
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SUMMARY
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. Remote
exploitation of an input handling vulnerability within multiple browsers
on the Microsoft Windows platform allows code execution as the local user.
DETAILS
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.
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>
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.
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.
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'.
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.
CVE Information:
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-3670>
CVE-2007-3670
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
ADDITIONAL INFORMATION
The information has been provided by Greg MacManus.
The original article can be found at:
<http://labs.idefense.com/intelligence/vulnerabilities/display.php?id=565>
http://labs.idefense.com/intelligence/vulnerabilities/display.php?id=565
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