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Cryptolocker copycat ransomware emerges – but an antidote is possible

Security upstart claims it's found a file-gobbling nasty with weak encryption

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Hot on the tail of devilish Cryptolocker comes a copycat software nasty that holds victim's files to ransom – but the newcomer's encryption is potentially breakable, we're told.

Security startup IntelCrawler claims a "large-scale distribution" of the new so-called Locker malware began earlier this month.

Locker, once it has infected a PC, copies and encrypts a victim's documents, adding a ".perfect" extension, and then deletes the original data. The trojan also places a contact.txt file in each directory containing contact details of the malware author – usually a throwaway mobile phone number or an email address.

Victims are warned that if they harass or threaten the extortionist, the decryption key to unlock the files will be deleted, revealing the mindset of the scumbags behind the scam.

IntelCrawler contacted a crook listed in the contact file, and was told someone would have to pay up $150 to a Perfect Money or QIWI VISA Virtual Card number to receive the decryption key needed to restore the information on a Locker-infected machines.

In order to decrypt, you need to provide an identifying code written in the “contact.txt” file, as well as the hostname of the compromised computer.

"It seems to be the hackers just compare the list of infected IP addresses of the users together with their hostnames," according to IntelCrawler.

Locker is a rank amateur effort compared to the CryptoLocker crew, who run their scam using a network of command-and-control servers and use a combination of 256-bit AES and 2048-bit RSA crypto to hold data to ransom (the master key being held in the crims' servers).

But despite its less-advanced design, Locker has already managed to attack Windows-powered computers in the US, we're told – including Washington DC, Texas and Missouri – plus PCs in the Netherlands, Turkey, Germany and Russia. Locker also, we're told, avoids infecting machines running tools used by security researchers, a tactic undoubtedly aimed at ensuring the malware stays under the radar for as long as possible.

The software nasty spreads mostly by drive-by downloads from compromised websites. Executables disguised as MP3 files are another vector of infection.

The Locker malware uses the TurboPower LockBox library, a cryptographic toolkit for Delphi: specifically, it uses AES-CTR for encrypting the contents of files on infected devices. But shortcomings in the programming will apparently make it possible for researchers to develop skeleton keys capable of unscrambling files on compromised kit. IntelCrawler's researchers are working on a universal antidote.

"We have found a decryption method and universal strings [keys] for decryption on any infected client," Andrey Komarov, IntelCrawler's chief exec, told El Reg.

Komarov added that detection of the malware by antivirus packages is low, with only Avira able to detect the pathogen as of Thursday evening. ®

Internet Security Threat Report 2014

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