Software Engineering Institute

Microsoft Excel contains a vulnerability that could be exploited when Excel opens a specially crafted document. The vulnerability is caused by incorrect handling of malformed STYLE records. This vulnerability affects both Windows and Mac versions of Excel

By convincing a user to open a specially crafted Excel document, an attacker could execute arbitrary code with the privileges of the user running Excel. If the user is logged in with administrative privileges, the attacker could take complete control of a vulnerable system. This vulnerability may also cause Excel to crash.

Apply an update

This vulnerability is addressed in Microsoft Security Bulletin MS06-059.

Do not open untrusted Excel documents

Do not open unfamiliar or unexpected Excel or other Office documents, particularly those hosted on web sites or delivered as email attachments. Please see Cyber Security Tip ST04-010.

Disable automatic opening of Excel documents

By default, Excel 97 and Excel 2000 will configure Internet Explorer to automatically open Excel documents. This feature can be disabled by using the Office Document Open Confirmation Tool. Mozilla Firefox users should disable automatic opening of files, as specified in the Securing Your Web Browser document.

Do not rely on file name extension filtering

In most cases, Windows will call Excel to open a document even if the document has an unknown file extension. For example, if spreadsheet.qwer contains the correct file header information, Windows will open presentation.qwer with Excel. Filtering for common extensions (e.g., .xls and .xlt) will not detect all Excel documents. Additionally, a Excel file with no file extension will also open with the Excel application.