A popular free security tool for the Firefox browser has been upgraded to block one of the most dangerous and troubling security problems facing the Web today.

NoScript is a small application that integrates into Firefox. It blocks scripts in programming languages such as JavaScript and Java from executing on untrusted Web pages. The scripts could be used to launch an attack on a PC.

The latest release of NoScript, version 1.8.2.1, will stop so-called "clickjacking," where a person browsing the Web clicks on a malicious, invisible link without realizing it, said Giorgio Maone, an Italian security researcher who wrote and maintains the program.

Clickjacking has been known for several years but is drawing attention again after two security researchers, Robert Hansen and Jeremiah Grossman, warned last month of new scenarios that could compromise a person's privacy or even worse, steal money from a bank account.

Unfortunately, clickjacking is possible due to a fundamental design feature in HTML that allows Web sites to embed content from other Web pages, Maone said. Nearly all Web browsers are vulnerable to a clickjacking attack.

"It's a very hard thing to fix because it's part of the very fabric of the Web and the browser," Maone said.

The embedded content can be invisible but a person can still unknowingly interact with it. A clickjacking attack takes advantage of that by tricking a user into clicking on a button that appears to do some function but actually does something entirely different.

Clickjacking can also be accomplished by manipulating the plug-ins of other applications, such as Adobe's Flash program and Microsoft's Silverlight. For example, researchers in recent days have shown it's possible for a clickjacking attack to turn on a person's Web camera and microphone without their knowledge.

In an advisory on Tuesday, Adobe said it will issue a patch for Flash by the end of the month.