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Internet users looking for a US Green Card are at risk of being conned by a fake advert into installing an adware-laden version of Firefox, security researchers have warned.

The ruse was spotted over the weekend after it began appearing in online ads peddling supposed US Green Card lotteries. Regardless of what make or version of browser surfers are using, punters served content from websites co-opted into the scam will be greeted with a warning that their browser is out-of-date, before they are encouraged into downloading and installing a version of Firefox.

"The browser build appears to be genuine, however it has been compromised with a multitude of add-ons, adware, toolbars and other malicious and irritating accompaniments that will find their way onto the user’s PC as part of the install," Christopher Boyd, a senior threat researcher at ThreatTrack Security, explains.

Boyd added that users would have to click on the rogue ad to get exposed to the attack, rather than getting hit simply by visiting a website featuring a dodgy ad stream.

"It looks like users would have to physically click the Green Card ad to be taken to the page - even if the ad in question redirected automatically, there is no "exploit install" with hidden additions," Boyd told El Reg. "The user would still have to download the executable and click through the installer prompts."

The Green Card entitles the bearer to permanent residency in the United States, and are highly prized by emigrants to the US and migrants already in America who do not have permanent resident status.

Digital detritus pushed through the ruse includes Delta Toolbar, Webcake, Optimizer Pro, QuickShare and an ad for “unlimited cloud storage”, according to a write-up of the scam featuring numerous screenshots on ThreatTrack's website. Any and all of these components have the effect of slowing down computers, littering desktops and generally making the online experience on infected PCs pretty wretched.

"If you attempt to install while offline, it won’t work as the install needs to download various components. However, should the install break it will abort but send you to a webpage promoting other offers and installs instead," Boyd explains.

The scam was first detected by security researchers at StopMalvertising, who have tied the scam back to malicious scripts hosted at c0n3.info.

StopMalvertising adds that the malicious installer downgrades a user's Firefox browser to version 13 as well installing several PUAs (Potentially Unwanted Applications, otherwise known as crapware). The current version of Firefox is version 23.

Version 13 came out more than a year ago in June 2012, since when numerous security updates have been applied to Mozilla's web browser software. Surfing the web with an obsolete browser opens the way for drive-by download attacks from compromised websites, so surfers who are hit by the adware are wide open to secondary infection from far more potent nasties such as banking trojans. ®

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