Microsoft has now patched 15 bugs after officially shipping security updates following the fallout from the hack of DigiNotar.
Microsoft announced the patch last week before prematurely releasing information on the security updates without making the patches themselves available.
The company also dealt with more fallout from the June hack of DigiNotar by flipping the "kill switch" on SSL (secure socket layer) certificates issued by the Dutch certificate authority.
All of the updates and vulnerabilities were rated "important," the second-most-serious rating in the company's four-step system.
Two of the vulnerabilities are in Windows; five are in Excel, the spreadsheet included with Office; two involve non-application Office components; and six affect SharePoint and associated software, such as Groove and Office Web Apps.
Of the 15, two are "DLL load hijacking" vulnerabilities, a term that describes a class of bugs first revealed in August 2010. Microsoft has been patching its software to fix the problem - which can be exploited by tricking an application into loading a malicious file with the same name as a required dynamic link library, or DLL - since last November.
Apparently that job isn't finished: Microsoft has yet to close a 2010 advisory that warns users of DLL load hijacking bugs in the company's software.
The update users should deploy first is MS11-072, which includes bug fixes for all supported versions of Excel, including the newest Excel 2010 on Windows and Excel 2011 on the Mac, several security professionals said.
"The Excel one because of the attack vector, through malformed files," said Andrew Storms, director of security operations at nCircle Security, when asked which update should move to the top of the list.
Other experts agreed.
"Top priority should be given to MS11-072, which fixes an arbitrary code execution vulnerability in Excel," said Wolfgang Kandek, the chief technology officer for Qualys. "It affects all versions of Excel including the most recent 2010 version and to exploit this issue, attackers could create malicious Excel files, which, when opened on vulnerable hosts, can take control of the system."
"Excel-related email attachments and links have commonly been used in attacks on organisations and this one should be addressed," added Kurt Baumgartner, senior security researcher at Kaspersky Lab.
Other updates quashed bugs in WINS (Windows Internet Name Service), a component of Windows Server last fixed in May; and patched a cross-site scripting vulnerability in SharePoint Server 2010, Microsoft's collaboration software.
Alongside the five updates, Microsoft delivered yet another update to deal with the theft of more than 500 digital certificates from DigiNotar.
"We are releasing another update, adding six additional DigiNotar root certificates that are cross-signed...to the Untrusted Certificate Store," said Pete Voss, a spokesman for Microsoft's Trustworthy Computing group.
Cross-signed certificates are those that DigiNotar issued, but then had co-signed by another CA - in this case Entrust or GTE - to allow them to be used by Windows PCs or browsers which were not already equipped with a DigiNotar certificate.
Certificates issued by Entrust or GTE will not be affected by the today's update, said Storms.
Microsoft and others, including rival browser makers Google and Mozilla, and operating system competitor Apple, have scrambled for the last two weeks to block or ban DigiNotar certificates.
The ban expansion came a week after Microsoft blocked others on Windows XP, Vista and Windows 7.
September's security patches can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.