The Koler Android ransom Trojan is spreading in the US after turning itself into an SMS worm, it has been reported. It's a tactic that could allow a rare piece of mobile malware to escape the nether-world of sideloaded apps from dodgy porn sites.

Discovered circulating in the US by mobile security firm AdaptiveMobile, the worm version of Koler infects its first victims from porn sites but after that sends a URL to the contacts it finds in the victim's person’s address book in the hope of spreading further.

“Someone made a profile named -Luca Pelliciari- and he uploaded some of your photos! is that  you?,” it asks as an lure to get people to click on the link.

Users are redirected to a Dropbox page that asks users to install a photo viewing app, at which point Android’s default security setting forbidding third-party downloads should refuse the install as long as that hasn’t been deactivated.

For those unlucky enough to become infected, this will be a crash course in the inconvenience of ransomware.

Once installed, it blocks user‘s screen with a fake FBI page, which states the device has been blocked for containing child pornography and zoophilia,” said Adaptive Mobile’s Yicheng Zhou.

After locking the screen in ways that are difficult to overcome (though not difficult if the user boots into safe mode), victims are asked for a MoneyPak ransom.

The worm tactic is similar to another recent example of Android malware, the Selfmite worm discovered in June of this year but which stepped up its activity several weeks ago.

About three quarters of the latest Koler infections seen by the firm were in the US, which numbered into the hundreds after only one day of activity.  Smaller numbers were also being detected in parts of the Middle East.

By the standards of the PC world, Koler is small beer but it does prove that n Android worm is possible on some scale.

Long term, apart from the difficulty of getting itself installed, Koler faces the challenge of keeping its crude bitly/Dropbox command and control up and running – AdaptiveMobile said it had already asked for the embedded link and Dropbox download to be deactivated. The criminals can change location quite easily but this will inevitably undermine the worm’s power.