Malware writers have used Crossrider, a cross-browser extension development framework, to build a click-fraud worm that spreads on Facebook, security researchers from antivirus firm Kaspersky Lab said on Monday.

Crossrider is a legitimate Javascript framework that implements a unified API (application programming interface) for building Mozilla Firefox, Google Chrome and Internet Explorer extensions.

The API allows developers to write code that will run inside different browsers and, by extension, on different OSes. The framework is still in beta testing and its creators plan on adding support for Safari soon.

"It is quite rare to analyze a malicious file written in the form of a cross-platform browser plugin. It is, however, even rarer to come across plugins created using cross-browser engines," said Kaspersky Lab malware expert Sergey Golovanov in a blog post on Monday.

The new piece of malware is called LilyJade and is being sold on underground forums for US$1,000. Its creator claims that it can infect browsers running on Linux or Mac systems and that since it doesn't have any executable files, no antivirus program is designed to look for it.

The malware's purpose appears to be click fraud. It is capable of spoofing rogue advertisement modules on Yahoo, YouTube, Bing/MSN, AOL, Google and Facebook, Golovanov said. When users view or click on these ads, the malware's creators earn money through affiliate programs.

In order to spread, the malware leverages its control over infected browsers to piggyback on active Facebook sessions and send spam messages on behalf of authenticated Facebook users.

The links included in LilyJade's Facebook spam messages direct users to compromised websites that load the Nuclear Pack exploit kit into a hidden iframe, Golovanov said.

Exploit kits like Nuclear Pack attempt to exploit vulnerabilities in outdated software -- usually browser plug-ins like Java, Flash Player or Adobe Reader -- in order to infect computers with malware.

The concept of malware running inside the browser as an extension is not new, but it seems to be increasingly popular with malware writers. Last week, the Wikimedia Foundation warned users that seeing commercial ads on Wikipedia is most likely the result of their browsers being infected with malicious extensions.

Social networking worms also appear to be making a comeback. On Friday, Symantec reported about a new variant of a worm called W32.Wergimog, which spreads by sending spam messages on Facebook, Hi5, Hyves, Linkedin, MySpace, Omegle and Twitter.

On Thursday, researchers from Trend Micro reported about a different worm that spreads through several social networks and instant messaging applications.