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Microsoft Security Bulletin MS98-018 - Critical

Patch Available for Excel 'CALL Vulnerability'

Published: December 10, 1998

Version: 1.0

Originally Posted: December 10, 1998

Summary

Microsoft has released a patch that fixes a vulnerability in Microsoft® Excel® 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

Vulnerability Identifier: CVE-1999-1055

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 The Microsoft Product Security Notification Service for more information about this free customer service.

Microsoft has published the following Knowledge Base (KB) article on this issue:

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://office.microsoft.com/downloads/9798/Xl8p9pkg.aspx

More Information

Please see the following references for more information related to this issue.

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/contactussupport/?ws=support

Acknowledgments

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/technet/security

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 POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

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