Microsoft has released four sets of security patches for its products, fixing a critical flaw in Microsoft Jet Database Engine software that had been used in online attacks.

Security experts say that the Jet flaw ( MS08-028 ) should be patched first, since it has already been exploited. Microsoft had previously warned of this bug in a March 21 advisory.

Attackers could take advantage of this flaw to run unauthorized software on a victim's computer by tricking them into opening a Word document that has an embedded malicious Jet file or by previewing the file in Outlook 2003 or 2007. Jet files have the .mdb extension.

Microsoft fixed the vulnerability on Tuesday by repairing the Msjet40.dll file that is used to open Jet documents. "If the version of Msjet40.dll is lower than 4.0.9505.0, you have a vulnerable version of the Microsoft Jet Database Engine," Microsoft said in its advisory.

This month's four updates fix six software bugs.

Microsoft also patched two critical flaws in Word and a critical Publisher bug.

Finally, a pair of vulnerabilities in Microsoft's Malware Protection Engine could give attackers a way to crash security software such as Windows Live OneCare, Antigen, Windows Defender and the Forefront products.

Though these bugs are considered to be only a moderate risk, they should be taken seriously by system administrators, said Amol Sarwate, vulnerability lab manager at Qualys.

"If someone sends a malformed e-mail and that is processed by any of these antivirus and antispyware products, it would cause the product to crash," he said. "If you can crash security software which is supposed to protect you, then you are left with no protection at all."