Microsoft released a patch Wednesday that fixes 10 security flaws in various versions of the Web server component of Windows NT 4.0, Windows 2000, and Windows XP.

The most serious of the flaws could allow an attacker to take over a user's system, moving Microsoft to label the cumulative patch "critical" and to urge all customers hosting Web sites using the affected software to install the patch immediately.

Affected are Microsoft's Internet Information Server 4.0, IIS 5.0, and IIS 5.1, Microsoft says. IIS 4.0 is part of the Windows NT 4.0 Option Pack, IIS 5.0 ships as part of Windows 2000 Datacenter Server, Advanced Server, and Professional, and IIS 5.1 is part of Windows XP Professional, according to Microsoft.

Microsoft says it found two of the flaws itself, possibly as part of the effort to clean up its code. The software maker thanks various security vendors and experts for reporting the other vulnerabilities.
Two of the vendors that discovered security holes, eEye Digital Security and @stake, issued advisories later Wednesday about the problems they found.

The flaw discovered by eEye would allow an attacker to remotely overflow the buffer of the Active Server Pages Internet Services Application Programming Interface filter in IIS 4.0 to execute code of the attacker's choice on the target system, eEye says in its advisory. The ASP ISAPI filter is installed by default on Windows NT/2000 servers and is used for interactive, dynamic Web content, eEye says.

The second advisory, issued by security consultancy @stake, concerns a buffer overflow in the component of IIS 4.0 and 5.0 that handles .htr files. The vulnerability can be remotely exploited to cause code execution, @stake says.

A flaw in the script-mapping function of the ISAPI that handles .htr files--files used for Web-based password administration--opened this hole, @stake says.

The cumulative patch, besides plugging the new holes, also fixes all of the vulnerabilities patched for IIS 4.0 since Windows NT 4.0 Service Pack 6a and all vulnerabilities patched to date for IIS 5.0 and 5.1. System administrators are cautioned to read the caveats section in Microsoft security bulletin MS02-018 before applying the patch.