Microsoft has released ten security patches, including three deemed "critical," for bugs in a variety of the company's products. Released as part of the company's monthly updates, the critical patches repair flaws in Windows and Internet Explorer that could allow attackers to take complete control of a computer.

The bug in Internet Explorer could theoretically allow Web pages with malicious code stored in the form of PNG (Portable Network Graphics) graphics files to gain control of a user's system. Microsoft also found a similarly critical bug in the Windows HTML Help system, as well as a flaw in Microsoft's SMB (sever message block) file sharing protocol.

"There is the potential for an attacker to somehow create an automated attack that could result in some sort of virus or worm," said Stephen Toulouse, security program manage with Microsoft's Security Response Center.

All three of the critical flaws were generally unknown before Tuesday, and they affect all supported versions of Windows, Toulouse said. None of the ten bugs that were reported and patched has yet been exploited by attackers, he added.

Bulletins were also issued for important vulnerabilities in Web Client Service, Outlook Web Access for Exchange Server 5.5, Outlook Express, and Windows Interactive Training.

Moderate vulnerabilities were reported in Microsoft Agent, Telnet Client, and ISA Server 2000, Microsoft said.

Microsoft also re-released three patches, numbered MS05-019, MS02-035 and MS05-004. Some of these patches were re-released because they had stopped certain applications from running on Windows, Microsoft said.

Though it only merited a moderate rating, the Microsoft Agent bug is serious because it could allow attackers to gain control over pop-up messages on a user's desktop, said Russ Cooper, senior scientist at Cybertrust and editor of the NTBugtraq discussion list.

Agent is user interface software that is often preinstalled on computers in order to generate cartoon-like characters, similar to Microsoft Office's "Clippy" paperclip, which are used in some Web-based help systems.

If exploited, the Agent bug could trick users into downloading malicious code, by intercepting or spoofing the computer's pop-up security warnings, Cooper said. For example, it could turn an Internet Explorer security warning into a message that said, "I have now verified that this is, in fact, your bank," he said.

The vulnerability was rated moderate because Agent is not always automatically enabled and because the vulnerability does not directly allow an attacker to gain control of the system, according to Toulouse.

Tuesday's patches cover many more products than last month's security bulletin, which concerned only Microsoft Web View. This means that administrators will be spending more time testing and patching their systems this month, said Mitchell Ashley, chief technology officer with security vendor StillSecure. "This month's [bulletin] is really affecting a number of widespread products from Microsoft, both on the desktop and the server," he said.

Microsoft's next scheduled Security Bulletin will be released on 12 July.