Details about the cross-platform browser exploitation technique known as "clickjacking" have started to emerge. Among the more alarming ways it can be used: covertly watching and listening to people who have microphones and Webcams attached to their computers.

"Web pages know what Web sites you've been to ..., where you're logged in, what you watch on YouTube, and now they can literally 'see' and 'hear' you," warned Jeremiah Grossman, founder and CTO of WhiteHat Security, in a blog post.

On Tuesday, Flash developer Guy Aharonovsky published a proof-of-concept exploit to show how clickjacking can be used to spy on people. "I've written a quick and dirty Javascript game [to] exploit just that, and demonstrate how an attacker can get a hold of the user's camera and microphone," he said in a blog post. "This can be used, for example, with platforms [like Ustream.tv or Justin.tv] or to stream to a private server to create a malicious surveillance platform."

Grossman and Robert "RSnake" Hansen, founder and CEO of SecTheory, discovered the clickjacking technique, called "UI redressing" by some, and planned to discuss it at the 2008 Open Web Application Security Project USA NYC security conference last month. But the pair decided to delay disclosure to allow affected vendors time to address the issue.

Unfortunately, this hasn't proven to be easy because clickjacking encompasses a range of attack techniques and affects a variety of software applications, including the major Web browsers and a number of plug-ins like Adobe Flash.

Clickjacking, as Grossman previously described the attack, "gives an attacker the ability to trick a user into clicking on something only barely or momentarily noticeable. Therefore, if a user clicks on a Web page, they may actually be clicking on content from another page."

Adobe on Tuesday issued a security advisory that describes a way to mitigate the risk faced by those with Adobe Flash Player installed -- almost everyone online. The advisory applies to Adobe Flash Player 9.0.124.0 and earlier.

Adobe recommends setting the "Always deny" button in the Global Privacy Settings panel of the Adobe Flash Player settings. The company is also working on an update to its Flash Player software that will address the vulnerability.

While this step protects against clickjacking attacks that use Adobe Flash, many other software applications remain vulnerable. In a blog post, Hansen outlines the status of eight different issues, only some of which have been resolved. He stresses that there are multiple variants of clickjacking, some of which require cross-domain access, some of which use iframes, some of which require JavaScript, and some of which involve page overlays.

How to defend against clickjacking? "Put tape over your camera, disable your microphone, install NoScript, and/or disable your plug-ins," advises Grossman, even as he concedes few users will be willing to lose access to YouTube and Flash games as a result.

Nevertheless, U.S. CERT said much the same thing in an advisory late last month.

The latest version of NoScript, a Firefox browser plug-in that blocks Flash, Java, and JavaScript, includes a new anti-clickjacking feature called ClearClick. It reveals transparent or concealed windows so the user can see attempts to co-opt clicks for malicious purposes.