Microsoft has released critical fixes for bugs in Office and the scanning engine used by the company's security products. The latter is of particular concern because it could, in theory, be very easily exploited by an attacker to run unauthorised software on a victim's PC.

The bug has to do with the way that Microsoft's Malware Protection Engine, used by Windows Live One Care and Windows Defender, processes PDF files. Windows Defender is available for free and ships with Microsoft's Vista operating system.

"An attacker could exploit the vulnerability by constructing a specially crafted PDF File that could potentially allow remote code execution when the target computer system receives, and the Microsoft Malware Protection Engine scans, the PDF file," Microsoft said in its bulletin on the patch, which was included as part of the February set of security updates.

This engine is also used by Microsoft's Antigen and Windows Forefront Security products, meaning that Exchange servers running these products could also be compromised.

Although there is not yet any known attack that exploits this flaw, it could become a serious problem for Windows users running Microsoft's security products, according to Amol Sarwate, vulnerability lab manager at Qualys "These attacks are very difficult to avoid," he said. "These products run automatically in the background."

Symantec rated this scanning engine flaw the most critical of the vulnerabilities just patched.

Microsoft actually released the patch for this flaw on 26 January, although it did not notify customers of this bug until Tuesday's bulletin, said Mark Griesi, a security programme manager with the company's Trustworthy Computing group.

Griesi said that there was no particular reason why Microsoft declined to notify customers of the update. But the fact that this was Microsoft's first security product patch since implementing its monthly system apparently contributed to the decision. "It was one of those first-time situations: 'Should we say something now or should we just wait,'" he said. "This time we decided to wait. We won't do it again."

In total, six of Microsoft's 12 updates are rated as "critical," the other six have the less-serious rating of "important."

Among the patches is a widely anticipated update for Microsoft Word, which fixes six bugs in Microsoft's word processing software. Hackers had been exploiting a number of these bugs in very targeted attacks over the past few months.

The updates fix 20 bugs in total, including all four of the Word flaws that hackers had been exploiting, Griesi said.

Other critical patches fix bugs in the ActiveX HTML Help control and Microsoft Data Access Components, both used by Windows, as well as flaws in Office and Internet Explorer.