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Title: Flaw in Word Fields and Excel External Updates Could
Lead to Information Disclosure (Q330008)
Date: 16 October 2002
Software: Microsoft(r) Word and Microsoft(r) Excel
Impact: Information Disclosure
Max Risk: Moderate
Bulletin: MS02-059

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-059.asp.

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Issue:

Word and Excel provide a mechanism through which data from one
document can be inserted to and updated in another document. This
mechanism, known as field codes in Word and external updates in
Excel, can be automated to reduce the amount of manual effort
required by a user. An example of the use of Word field codes could
be the automatic insertion of a standard disclaimer paragraph in a
legal document. An example of the use of external updates in Excel
could be the automatic updating of a chart in one spreadsheet using
data in a different spreadsheet.

A vulnerability exists because it is possible to maliciously use
field codes and external updates to steal information from a user
without the user being aware. Certain events can trigger field code
and external update to be updated, such as saving a document or by
the user manually updating the links. Normally the user would be
aware of these updates occurring, however a specially crafted field
code or external update can be used to trigger an update without any
indication to the user. This could enable an attacker to create a
document that, when opened, would update itself to include the
contents of a file from the user's local computer.

In order for an attacker to take advantage of this vulnerability,
the attacker would need to perform the following steps:

-Craft a Word or Excel document that exploits the vulnerability
-Deliver it to the user, via email or some other method
-Entice the user to open the document
-Return the document to the attacker. (Microsoft is aware of one
case in which it would not be necessary for the user to do this.
There is one method through which the attacker's document could
post information directly to a web site, but it would only allow
the first line of the file to be sent)

Mitigating Factors:

• • The attacker would need to know the location of the file that he or
    she wanted to steal. If the correct filename were not presented,
    the attack would fail and an invalid field error message would be
    present in the document.
• • The user could always view the field codes or external updates. The
    field codes or external updates used in the attack can be revealed,
    as they are only hidden to prevent cluttering the document when it
    is being viewed or edited. A method of checking documents for
    additional undesired information is described in the Frequently
    Asked Questions below.
• • Although the attacker could take some steps to obscure the stolen
    information, the attacker would leave a clear audit trail. Since
    the field codes or external updates can be viewed, even if an attack
    is successful, the attacker would leave clear evidence in the
    document in the form of the stolen information and the malicious
    field codes used. This evidence could be used by law enforcement
    agencies if required
• • The vulnerability would not enable the attacker to delete, modify
    or add any files to the user's local system.
• • In virtually all circumstances, the attacker would need to entice
    the user into returning the document. No information would be
    revealed unless the user returned the document to the attacker.

Risk Rating:

• Internet systems: None
• Intranet systems: None
• Client systems: Moderate

Patch Availability:

• A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-059.asp
  for information on obtaining this patch.

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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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