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Security flaws in the help facility of most versions of Microsoft's Windows operating system could allow an attacker to take control of a user's PC, Microsoft warned late Wednesday.

All versions of the Windows operating system, from Windows 98 up, include a help function based on HTML. Much of its functionality is provided by an ActiveX control, a software component that can be called upon by a Web page.

An unchecked buffer in this control is what exposes the user's system, Microsoft says in security bulletin MS02-055.

The issue is rated "critical" by Microsoft because the help function can also be called by a Web site or HTML e-mail, not just by applications on a user's PC. An attacker who invoked the help facility and exploited the flaw could take virtually any action on a PC a user could take, Microsoft says.

Providing Protection

Users of Outlook 2002 or Outlook Express 6 are protected from the e-mail-based attack because those applications handle HTML e-mail messages in a different manner. Users who installed the Microsoft-recommended Outlook E-mail Security Update are also protected, Microsoft says.

A software patch is available to protect a system against a Web site attack. However, the user needs to have Internet Explorer version 5.01, 5.5, or 6 installed for it to work, Microsoft says.

The patch also eliminates two other flaws in the Windows help facility. These flaws relate to the way HTML Help files are handled. These files can contain shortcuts that can take any action on a system. Only trusted files should be allowed to use these shortcuts, but the flaws allow a way around this restriction, Microsoft says.

Microsoft rates these flaws "moderate" because staging an attack exploiting them would be tough. An attacker would have to know the location of a user's temporary Internet files folder, which should be impossible, according to Microsoft.

Recurring Problem?

Applying the patch does not provide a new ActiveX control for the help function, it updates the old control to remove the vulnerabilities, Microsoft says. This means that an attacker could remotely reinstall the vulnerable control, because it still has a valid digital signature from Microsoft.

This could be prevented by setting the so-called "Kill Bit" on the ActiveX control, disallowing Internet Explorer from ever invoking it. However, the Kill Bit is not set because it would make help facilities in many applications that link to the old control unavailable, Microsoft says.

Microsoft acknowledges that an attacker could reintroduce the vulnerable control and suggests that users disable the downloading of ActiveX controls in Internet Explorer.

Second Security Bulletin

In a separate security bulletin on Wednesday, Microsoft warned of two "moderate" risk flaws in the file decompression utility that is part of Windows Me, Windows XP, and the Windows 98 Plus Pack.

These flaws could allow an attacker to either run code on a user's system or place files in a location of his choice. However, the vulnerabilities cannot be exploited without user intervention and cannot be exploited remotely, Microsoft says in bulletin MS02-054.

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