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Title:      Erroneous VeriSign-Issued Digital Certificates Pose 
            Spoofing Hazard
Date:       22 March 2001
Software:   All Microsoft customers should read the bulletin.
Impact:     Attacker could digitally sign code using the name 
            "Microsoft Corporation". 
Bulletin:   MS01-017

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-017.asp.
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Issue:
======
VeriSign, Inc., recently advised Microsoft that on January 30 and 31,
2001, it issued two VeriSign Class 3 code-signing digital
certificates to an individual who fraudulently claimed to be a
Microsoft employee. The common name assigned to both certificates is
"Microsoft Corporation". The ability to sign executable content using
keys that purport to belong to Microsoft would clearly be
advantageous to an attacker who wished to convince users to allow the
content to run. 
The certificates could be used to sign programs, ActiveX controls,
Office macros, and other executable content. Of these, signed ActiveX
controls and Office macros would pose the greatest risk, because the
attack scenarios involving them would be the most straightforward.
Both ActiveX controls and Word documents can be delivered via either
web pages or HTML mails. ActiveX controls can be automatically
invoked via script, and Word documents can be automatically opened
via script unless the user has applied the Office Document Open
Confirmation Tool. 

However, even though the certificates say they are owned by
Microsoft, they are not bona fide Microsoft certificates, and content
signed by them would not be trusted by default. Trust is defined on a
certificate-by-certificate basis, rather than on the basis of the
common name. As a result, a warning dialogue would be displayed
before any of the signed content could be executed, even if the user
had previously agreed to trust other certificates with the common
name "Microsoft Corporation". The danger, of course, is that even a
security-conscious user might agree to let the content execute, and
might agree to always trust the bogus certificates. 

VeriSign has revoked the certificates, and they are listed in
VeriSign's current Certificate Revocation List (CRL). However,
because VeriSign's code-signing certificates do not specify a CRL
Distribution Point (CDP), it is not possible for any browser's
CRL-checking mechanism to download the VeriSign CRL and use it.
Microsoft is developing an update that rectifies this problem. The
update package includes a CRL containing the two certificates, and an
installable revocation handler that consults the CRL on the local
machine, rather than attempting to use the CDP mechanism. 

Versions of the update are being prepared for all Microsoft platforms
released since 1995. However, because of the large number of
platforms that must be tested, the patches are not available at this
writing. Until the update is available, we urge customers to take
some or all of the following steps to protect themselves should they
encounter hostile code signed by one of the certificates. 
 - Visually inspect the certificates cited in all warning 
   dialogues. The two certificates at issue here were issued 
   on 29 and 30 January 2001, respectively. No bona fide 
   Microsoft certificates were issued on these dates. The 
   FAQ and Knowledge Base article Q293817 provide complete 
   details regarding both certificates. 
 - Install the Outlook Email Security Update 
   (http://www.officeupdate.com/2000/downloadDetails/Out2ksec.htm) 
   to prevent mail-borne programs from being launched, even via 
   signed components, and install the Office Document Open 
   Confirmation Tool 
   (http://officeupdate.microsoft.com/downloadDetails/confirm.htm)
   to force web pages to request permission before opening Office 
   documents. 
 - Consider temporarily removing the VeriSign Commercial Software 
   Publishers CA certificate from the Trusted Root Store. Knowledge
   Base article Q293819 provides details on how to do this. 

Mitigating Factors:
====================
 - The certificates are not trusted by default. As a result, 
   neither code nor ActiveX controls could be made to run without 
   displaying a warning dialogue. By viewing the certificate in 
   such dialogues, users can easily recognize the certificates. 
 - The certificates are not the bona fide Microsoft code-signing 
   certificates. Content signed by those keys can be distinguished 
   from bona fide Microsoft content. 

Patch Availability:
===================
 - A software update is under development and will be released 
   shortly. When it is available, we will update this bulletin 
   to provide information on how to obtain and use it. 


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THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED
"AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL
WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY
DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
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OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO
THE FOREGOING LIMITATION MAY NOT APPLY.



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