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Microsoft Outlook "mailto:" Parameter Passing Vulnerability

03.09.04

BACKGROUND

Microsoft Outlook provides an integrated solution for managing and
organizing e-mail messages, schedules, tasks, notes, contacts, and other
information. More information is available at
http://www.microsoft.com/outlook/.

DESCRIPTION

Insufficient filtering of parameters passed to Microsoft Corp.'s Outlook
e-mail client via the "mailto:" URI (RFC 2368) allows for remote script
execution within the "Local Machine" zone. When Outlook is installed, it
is enabled as the default e-mail handler. A "mailto:" URI will spawn
Outlook with the following command line switches:

    OUTLOOK.EXE -c IPM.Note /m "...

The problem manifests when the string '"' is interspersed within
the URI, thereby allowing an attacker to manipulate the command line
switches that Outlook is instantiated with. The following example URI:

    ... mailto:aa" /profile "xx" ...

causes Internet Explorer to start Outlook as such:

    OUTLOOK.EXE -c IPM.Note /m "aa" /profile "xx"

The provided URI can be of the form 'javascript:...', allowing an
attacker to execute arbitrary script code. In some implementations, this
script code executes under the context of the "Local Machine Zone". An
attacker does not need to socially engineer a target user into clicking
on a malicious link, as the process can be started automatically by
embedding the "mailto:" URI within an HTML IMG tag.

ANALYSIS

Successful exploitation allows an attacker to remotely execute arbitrary
code under the context of the "Local Machine" zone. Script code
executing with such privileges can be crafted to retrieve and execute
arbitrary third-party code, thereby leading to further compromise.

Microsoft had originally stated that only users who had set "Outlook
Today" as their default Outlook folder home page were affected. Further
analysis has revealed that users are not protected by changing their
default view. Additional manipulation allows an attacker to force
Outlook to start within the "Outlook Today" view with a specified URI.
This can be accomplished by first starting Outlook while specifying
"Outlook Today" as the starting view and then passing in a URI via the
'/recycle' command line switch. Another potential exploit vector may
involve the usage of the command line switch '/select
outlook:outlook%20today'.

iDEFENSE has proof of concept exploit code demonstrating the impact of
this vulnerability.

DETECTION

iDEFENSE has confirmed the existence of this vulnerability in Microsoft
Outlook 2002.

WORKAROUND

iDEFENSE is currently unaware of any valid workarounds.

VENDOR RESPONSE

Microsoft has released the following patches to address this issue:

Microsoft Office XP Service Pack 3
http://www.microsoft.com/downloads/details.aspx?FamilyId=85AF7BFD-6F69-4289-8BD1-EB966BCDFB5E&displaylang=en

CVE INFORMATION

The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has
assigned CAN-2004-0121 to this issue.

CREDIT

Jouko Pynnönen (http://iki.fi/jouko) is credited with this discovery.