Windows users have been warned about the sudden return of one of the Internet’s most lethal families of malware, which encrypts data files on a victim’s PC before demanding a ransom for the unlock key.

In antivirus circles, GpCode is notorious but thankfully rare, having made new appearances in ever more sophisticated forms roughly once every 18 months or so since its criminal debut in 2004.

According to a new analysis by Kaspersky Lab researcher Vitaly Kamluk, who has documented previous outbreaks of the malware, the latest variant, Trojan-Ransom.Win32.GpCode.ax, works in a similarly nasty way to older versions.

The malware first encrypts a user’s data files – Kamluk doesn’t specify which types but previous versions have attacked all the obvious file extensions – using either RSA-1024 and AES-256, before demanding $120 (£77) using a direct bank transfer to hand the user back their own files.

“And remember: any harmful or bad words to our side will be a reason for ignoring your message and nothing will be done,” warns the ransom text displayed to the user.

The chances of getting an unlock file are probably slim even if payment is made, but Kamluk does offer a glimmer of hope for users swift enough to recognise that they have been infected.

“Pushing Reset/Power button on your desktop may save a significant amount of your valuable data! Please remember this and tell your friends that if you see a sudden popup of notepad with text like this,” says Kamluk. This presumably means that the malware takes it time to encrypt larger data stores, which gives the user a way of possibly stopping it before too much data has been made unrecoverable.

Can the encryption be broken using a powerful computer? For now, the answer is a resounding no. Infected users will need to resort to backups.

The story of GpCode is a curious one, starting with a significant attack in 2006 a later version of which prompted Kaspersky to ask for public help in cracking the encryption keys used. In 2008, it was reported that the malware had been traced to a single virus author living in Russia who was known to the authorities. Apparently, no action was taken.

What Kamluk does not say is whether he believes that the new version of GpCode is connected to the same author or is a new version based on the same ransom concept.

Separately, Kaspersky Lab has reported finding another piece of ransom malware, this time a less sophisticated example, Trojan-Ransom.Win32.Seftad.a, which demands a ransom from users after copying itself to the hard disk master boot record (MBR) and initiating a reboot.

Luckily, the demand for $100 via a website and claim that the hard drive has been encrypted is bluff. Users can either restore their MBR using one of a number of tools or visit the website and enter the password ‘aaaaaaciip’ to restore the original MBR. Unlike GpCode, no data files will have been touched by the infection.