Microsoft is investigating reports of a vulnerability in a Windows ActiveX control that could allow an attacker to remotely take control of a computer, according to an advisory issued Friday. One security company rated the vulnerability critical, while Microsoft said it allows only limited attacks.The vulnerability, which is not patched yet, affects certain versions of Windows running Microsoft XML Core Services 4.0, a set of tools that allows programmers to use scripting languages to access XML documents.The affected versions are Windows 2000 Service Pack 4, Windows XP Service Pack 2, Windows Server 2003 and Microsoft Windows Server 2003 Service Pack 1.A user would have to visit a specially crafted website that triggers the XMLHTTP 4.0 ActiveX control, Microsoft said. The attacker would then have the same rights on the machines as the current logged-on user, and could gain complete control of the machine.Users can protect themselves by disabling the affected ActiveX control, although the workaround could stop some websites from functioning correctly. Microsoft describes how to disable the control in an advisory.The SANS Institute classified the flaw as a zero-day vulnerability, meaning the problem is public but not patched. The French Security Incident Response Team called it "critical."Microsoft issues patches for its software on the second Tuesday of the month. The speed at which a patch is issued depends on the risk of the vulnerability, and the company has issues patches out of cycle for widely exploited vulnerabilities.By Jeremy Kirk, IDG News Service (London Bureau)Keep checking in at our Security Feed for updated news coverage.