A third Excel flaw has been uncovered in a week, giving Excel users one more thing to worry about and opening one more door for hackers to get into corporate networks.
Unfortunately, this new flaw is more critical than the second flaw discovered earlier in the week. The second flaw required the user to click a specially-crafted hyperlink inside an Excel document, and the latest version of Excel even displayed a dialog box that had to be dismissed after clicking the link. This new flaw, however, only requires that the user open the Excel document.
These new flaws underscore the need for users to be very wary of unsolicited Office attachments. If you didn’t ask for it, it very well may be a malicious attachment—exercise caution.
In addition, the recent flaw in Microsoft Word and these flaws in Microsoft Excel have contributed to the placement of Microsoft Office on the SANS Top 20 list of vulnerabilities.
UPDATE: In following up on these reports, I came across a few additional links with more information on the vulnerabilities, including information that proof-of-concept code to exploit the second Excel vulnerability was available:
Notice of exploit code availability: http://www.eweek.com/article2/0,1759,1979409,00.asp
Secunia advisory on the second Excel flaw: http://secunia.com/advisories/20748/
MSRC response to the second Excel flaw: http://blogs.technet.com/msrc/archive/2006/06/20/437826.aspx
US-CERT Vulnerability Note on the second Excel flaw: http://www.kb.cert.org/vuls/id/394444