Microsoft has released a monster 12 patches to fix 16 vulnerabilities - eight of them critical - in Windows and Office.

It represents one of the largest single-day releases since Microsoft switched to a monthly patch approach in October 2003.

Fixes cover almost every supported version of Windows, including the recently updated Windows XP Service Pack 2, and patches for holes in everything from critical Windows components to the Internet Explorer and MSN Messenger.

These are among the most serious holes Microsoft addressed:

A critical vulnerability (MS05-009) in a component of Messenger that renders PNG image files used to display icons, such as smiley faces, in instant messaging conversations. If successfully exploited, the vulnerability could be hidden in a buddy icon and launched whenever MSN users load contact lists.

A critical vulnerability in the Server Message Block (SMB) protocol that affects Windows XP, Windows 2000 and Windows Server 2003 (MS05-011) that could be used to launch attacks on vulnerable Windows systems from Web pages. SMB is used to communicate between Windows machines and to share network resources such as printers and files.

A critical vulnerability in Microsoft's License Logging Service that affects Windows Sever 2003, Windows 2000 and Windows NT Server 4.0 (MS05-010). The service is a tool that helps customers manage software licenses for Microsoft server products. A remote attacker could use the vulnerability to cause the License Logging Service to fail, causing a denial of service attack on Windows Server 2003 systems or to install programs; view, change, or delete data; or to create new user accounts on Windows 2000 and NT Server 4.0 systems.

Four critical vulnerabilities in the Internet Explorer Web browser Versions 5 and 6 (MS05-014). The patch includes a fix for the "drag and drop" vulnerability that allows remote attackers to use Web-based attacks to place an executable file on a user's Windows system without the user receiving a dialog box asking for approval for the download.

With the exception of the Internet Explorer vulnerabilities, Microsoft does not know of any active attacks that try to exploit the vulnerabilities, which were all discovered by security researchers outside of the company, according to Stephen Toulouse, program manager at Microsoft's Security Response Center.

Microsoft recommends that companies assess their exposure to the vulnerabilities and apply all applicable software patches as soon as possible, he said.

Aware of the burden placed on enterprise security managers, Microsoft also released an Enterprise Scanning Tool to help detect vulnerable computers and supplement the Microsoft Baseline Security Analyser, according to information supplied by the company.

The company is also increasing the number of webcasts it holds to discuss deployment of the security updates, anticipating an increased need for help with the large patch release, Toulouse said.