[8693] in bugtraq
Microsoft Security Bulletin (MS98-018)
daemon@ATHENA.MIT.EDU (Aleph One)
Thu Dec 10 23:51:53 1998
Date: Thu, 10 Dec 1998 20:45:18 -0600
Reply-To: Bugtraq List <BUGTRAQ@NETSPACE.ORG>
From: Aleph One <aleph1@DFW.NET>
To: BUGTRAQ@NETSPACE.ORG
---------- Forwarded message ----------
Date: Thu, 10 Dec 1998 14:40:35 -0800
From: Microsoft Product Security <secnotif@MICROSOFT.COM>
To: MICROSOFT_SECURITY@ANNOUNCE.MICROSOFT.COM
Subject: Microsoft Security Bulletin (MS98-018)
The following is a Security Bulletin from the Microsoft Product Security
Notification Service.
Please do not reply to this message, as it was sent from an unattended
mailbox.
********************************
Microsoft Security Bulletin (MS98-018)
--------------------------------------
Patch Available for Excel "CALL Vulnerability"
Originally Posted: December 10, 1998
Summary
=======
Microsoft has released a patch that fixes a vulnerability in Microsoft (r)
Excel (r) that could allow certain types of executables to be run without a
warning to the user. A legitimate Excel function, CALL, allows executables
to be run from a worksheet. If the executable called by the function is of a
malicious nature, a worksheet containing this function could represent a
security risk to customers.
A fully supported patch that disables this functionality is available. As
detailed below in What Customers Should Do, Microsoft recommends that users
evaluate whether they are at risk from this attack and install the patch if
appropriate.
Issue
=====
CALL is an advanced function in Excel that allows a worksheet to call a
procedure in a dynamic link library (DLL) or code resource. It is a
legitimate function, and can be used in macros or as a worksheet function.
Excel generates a warning to the user before running macros, including those
containing the CALL function, and allows the user to decide whether or not
to run them. However, Excel does not generate a warning before executing
worksheet functions, and if used in this manner, CALL could be used to call
an external DLL without a warning to the user.
An attacker could exploit this functionality by embedding a CALL function
within an Excel spreadsheet and sending it to an unwary user. The attacker
would be able to control whether the CALL function fired when the victim
opened the spreadsheet or when another event occurred. It is important to
note that the CALL function does not perform any malicious action by itself,
and would serve only as an initiator for a malicious DLL.
While there have not been any reports of customers being adversely affected
by a worksheet using this construct, Microsoft is releasing this patch to
address the implied risks posed by these issues. Please note that the patch
eliminates the vulnerability by disabling the CALL function on a worksheet;
however, it does not disable the CALL function when used within macros. The
CALL function is used very infrequently as a worksheet function, and the
vast majority of customers can apply the patch without any loss in
functionality. However, customers who need to use CALL from within
worksheets may choose not to download and apply the patch.
Affected Software Versions
==========================
- Microsoft Excel 97
What Microsoft is Doing
=======================
On December 7, Microsoft released a patch that fixes the problem identified.
This patch is available for download from the sites listed below in What
Customers Should Do.
Microsoft has sent this security bulletin to customers
subscribing to the Microsoft Product Security Notification Service
(see http://www.microsoft.com/security/services/bulletin.asp
for more information about this free customer service).
Microsoft has published the following Knowledge Base (KB) article on this
issue:
- Microsoft Knowledge Base (KB) article Q196791,
XL97: Patch Available for Excel CALL Vulnerability,
http://support.microsoft.com/support/kb/articles/q196/7/91.asp
(Note: It might take 24 hours from the original posting of
this bulletin for the updated KB article to be visible in the
Web-based Knowledge Base.)
What Customers Should Do
========================
The patch for this vulnerability is fully supported. As discussed above in
Issue, the patch works by disabling the CALL worksheet function, but does
not disable the CALL function from within macros. Customers who need the
CALL worksheet function should evaluate the degree of risk that it poses to
their systems, and determine whether the best course of action is to apply
the patch or not.
Microsoft recommends that customers who do not have a need
to execute DLL procedures via worksheet functions apply the patch.
The patch is available via the Office Update web site at
http://officeupdate.microsoft.com/downloadDetails/xl97cfp.htm
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin 98-018, Patch Available for Excel
"Call Vulnerability" (the Web-posted version of this bulletin),
http://www.microsoft.com/security/bulletins/ms98-018.asp.
- Microsoft Knowledge Base (KB) article Q196791,
XL97: Patch Available for Excel CALL Vulnerability,
http://support.microsoft.com/support/kb/articles/q196/7/91.asp
Obtaining Support on this Issue
===============================
This is a supported patch. If you have problems installing
this patch or require technical assistance with this patch,
please contact Microsoft Technical Support. For information
on contacting Microsoft Technical Support, please see
http://support.microsoft.com/support/contact/default.asp
Acknowledgements
================
Microsoft wishes to acknowledge the contribution made by
Central Command, Inc. (http://www.avp.com), who reported
the problem to us.
Revisions
=========
- December 10, 1998: Bulletin Created
For additional security-related information about Microsoft products, please
visit http://www.microsoft.com/security
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