In 2010, 25 percent of new worms have been specifically designed to spread through USB storage devices connected to computers, according to PandaLabs. These types of threats can copy themselves to any device capable of storing information such as cell phones, external hard drives, DVDs, flash memories and MP3/4 players.

This distribution technique is highly effective. With survey responses from more than 10,470 companies across 20 countries, it was revealed that approximately 48 percent of SMBs (with up to 1,000 computers) admit to having been infected by some type of malware over the last year. As further proof, 27 percent confirmed that the source of the infection was a USB device connected to a computer.

So far, these types of infections are still outnumbered by those that spread via email, but it is a growing trend. "There are now so many devices on the market that can be connected via USB to a computer: digital cameras, cell phones, MP3 or MP4 players," says Luis Corrons, Technical Director of PandaLabs. "This is clearly very convenient for users, but since all these devices have memory cards or internal memory, it is feasible that your cell phone could be carrying a virus without your knowledge."

How does it work?

There is an increasing amount of malware which, like the dangerous Conficker worm, spreads via removable devices and drives such as memory sticks, MP3 players and digital cameras.

The basic technique used is as follows: Windows uses the Autorun.inf file on these drives or devices to know which action to take whenever they are connected to a computer. This file, which is on the root directory of the device, offers the option to automatically run part of the content on the device when it connects to a computer.

By modifying Autorun.inf with specific commands, cyber-crooks can enable malware stored on the USB drive to run automatically when the device connects to a computer, thus immediately infecting the computer in question.

To prevent this, Panda Security has developed Panda USB Vaccine, a free product which offers a double layer of preventive protection, disabling the AutoRun feature on computers as well as on USB drives and other devices.