The dependence of Internet Explorer on other Windows components has allowed online attackers to work around the shored-up security of Microsoft's latest browser.
Last weekend, security researchers discovered a website using an previously unknown, or zero-day, vulnerability in a relatively unused ActiveX component of Windows to infect visitors' systems.
ActiveX components act as a common way to exchange data between various components of Windows. While the flawed ActiveX component - a part of Microsoft's XML Core Services 4.0 - is not shipped with Internet Explorer, attackers can use the browser to trigger the flaw and compromise any system on which the ActiveX control is installed.
The vulnerability underscores that the improvements in security in the latest version of Microsoft's browser, Internet Explorer 7, do not eliminate the threats of older components of Windows, said Gunter Ollmann, director of IBM Internet Security Systems' X-Force vulnerability research team.
"IE7 has made it a little bit harder for the attackers to do this," he said. "But once someone figures out how to bypass the security, they tell everyone else about it, and it it becomes the new standard."
The discovery of an exploit attacking a previously unknown vulnerability underscores the difficulty in closing down attacks on ActiveX components through the browser. ActiveX allows websites to add interactivity and greater functionality to a visitor's browsing experience. However, because the technology allows a website to affect changes on a visitor's PCs, the software components can also pose a danger.
ActiveX has been serious sore spot for Microsoft in its quest for hardening its operating system and applications against attacks. The technology evolved from the Object Linking and Embedding (OLE) features created in 1990 to allow Windows applications to exchange data. The general framework became the Component Object Model in 1993 - now known as the Distributed Component Object Model (DCOM) - while Microsoft renamed OLE 2.0 as ActiveX and pushed web developers to add more interactivity to their sites using the technology. As such, Internet Explorer became a common way of accessing a variety of the ActiveX components installed on a Windows computer.
Online criminals frequently use flaws in ActiveX to install malicious code on victims' PCs via their browsers. One tool - known as WebAttacker and sold from a Russian website for about $20 - has had great success in compromising the security of victims' computers.
In one case, a flaw in the Windows Data Access Components, an ActiveX control fixed in April by Microsoft, the attack tool successfully infected the visitor's PC between 12 per cent and 15 per cent, according to web security firm Websense.
Many of the ActiveX controls resident on consumers' PCs can no longer be accessed from Internet Explorer 7, under the browser's enhanced security model. However, the flawed XMLHTTP 4.0 ActiveX control, used in some corporate applications to access sites based on the extensible markup language (XML), escaped Microsoft's crackdown on dangerous components.
"There are loads of (ActiveX controls) that come with Windows and other applications that are disabled by default - this particular one is not," said Gary Schare, director of IE product management for Microsoft.
The ActiveX component made it onto Microsoft's list of preapproved controls, which are components that are frequently accessed by Windows users and that the company feels have had a solid level of security testing. The security audit obviously missed a vulnerability, but the threat model for reducing the risk of attack has worked well, Schare said.
"In general, the attack surface area for ActiveX has gone down dramatically," he said.