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.

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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