It appears that a fake Adobe Flash browser plugin is hijacking on-screen web adverts to promote hardcore porn. This rogue add-on is even plugging racy adult-only teasers on web sites aimed at children. The software nasty, according to Jérôme Segura, a security researcher at antivirus firm Malwarebytes, has been named FlashPlayer11.safariextz. It poses as a "Flash Player update" and is largely distributed via X-rated web portals.

The file is downloaded from fplcdn.com, a domain registered at the end of last month; the registrant's details in the WHOIS database appear to be fishy, said Segura. The Safari extension is also compatible with Firefox, Chrome and Safari but not Internet Explorer, according to tests by Malwarebytes.

"In addition to injecting adverts within every single page you visit, this malicious extension is capable of ‘hijacking’ legitimate ads and replacing them with its own," Segura explained. "With such invasive adverts, cyber-crooks are likely to generate a lot of ‘views’ and even pay per clicks."

"If you believe you are seeing strange or inappropriate ads on the websites you regularly visit, it wouldn’t hurt checking the extensions installed in your browser and removing the offending ones," Segura advised, adding that netizens can avoid getting hit by the scam by taking care to install software updates from vendor’s official websites.

None of the antivirus vendors listed in VirusTotal detected the Safari component of the extension when Segura uploaded it. However, the executable used to hijack rival browsers was detected.

A write-up of the threat, together with screenshots, can be found in a blog post by Malwarebytes here.