Mozilla developers are working on a new Firefox feature that will block the automated display of plug-in-based content like Flash videos, Java applets or PDF files, and will protect users from attacks that exploit vulnerabilities in browser plug-ins to install malware on their computers.

Known as "click to play," this feature has been present in the popular NoScript Firefox security extension for many years, as well as in other browsers like Google Chrome and Opera.

When click-to-play is enabled, the browser displays static images instead of the active content that requires plug-ins to be displayed. Users need to click on those images in order to authorize the loading of each plug-in-based element.

"A couple days ago I landed an initial implementation of 'click-to-play plugins' in desktop Firefox," Mozilla software engineer Jared Wein said in a blog post on Wednesday.

Wein's implementation is available for testing in the latest Firefox nightly build, but there's still work to be done. "I'm currently working on implementing the ability for plugin activation settings to be remembered on a per-site basis," he said.

Wein hopes to finish developing the feature before the deadline for code submissions in Firefox 14, but there's no guarantee that it will be included in that version.

Security experts agree that technically speaking, this feature is capable of preventing plug-in-based attacks and should be implemented in every browser. However, they point out that its effectiveness will ultimately depend on user behavior.

"Let's not forget that there has been a similar implementation in Internet Explorer (the Internet Explorer Information Bar) that did not really make any difference, since users would always allow blocked content to be executed," Bogdan Botezatu, a senior e-threat analyst at antivirus vendor BitDefender, said. "Similarly, users might allow click-to-play content as well, based on the mere assumption that they really have to have everything loaded on the page."

Botezatu believes that keeping click-to-play as an opt-in feature will significantly reduce its overall efficiency, because most non-technical users will leave it disabled. However, having it enabled by default in the browser would not be a good idea either, because it could result in the same users missing content they actually need, he said.

An alternative would be to have the browser automatically enable the feature when it detects that the required plug-in is outdated, and to display the content normally if the plug-in is up-to-date, Botezatu said. Firefox already checks for vulnerable plug-ins when it gets updated, so the mechanism to do this is already built into the browser.

Costin Raiu, director of the global research and analysis team at antivirus firm Kaspersky Lab, is also concerned about the impact of user behavior on the effectiveness of the click-to-play feature.

"You leave the choice to the user, who generally, when presented with a broken picture or a dialog which says click to see the movie, will click it," Raiu said. "Generally, best defenses work by taking this choice away from the user."

Raiu believes that Mozilla should build a click-to-play whitelist for popular websites like YouTube or Dailymotion, so users don't develop a reflex of approving plug-in-based content without giving it much thought.

On the downside, if the feature becomes popular, it is likely to be abused by attackers who could use rogue click-to-play images to trick users into downloading malicious files that masquerade as video codecs or browser plug-in updates.

This kind of abuse has happened in the past with the "What's New" page, which gets displayed after every Firefox update, the Safe Browsing alert page, which appears when users attempt to access known malicious websites, or the Firefox plug-in version check page, where users are informed about outdated plug-ins.

"I'm quite certain that the feature will be abused in social-engineering attacks, however, I don't think the risk outweighs the benefit," Raiu said. Botezatu feels the same way.

"Now is a good time to have a click-to-play feature, given the fact that web-based attacks have increased exponentially and - most importantly - are becoming cross-platform," Botezatu said, pointing to the recent attacks that exploited a vulnerability in the Java browser plug-in to infect over 600,000 Mac OS X computers with the Flashback malware.