The Vietnam Computer Emergency Response Team (VNCERT) on January 9 issued an alert noting that hackers using the virus are targeting "tens of millions" of computers in the country. 

The team, a subsidiary of the Ministry of Information and Communications, said CryptoLocker is an especially insidious form of Ransomeware malware, which was first detected in the wild in September 2013. 
The virus restricts access to infected computers and requires victims to pay a ransom of 100-300 USD or more within 72 hours in order to regain full access, it said. 

What makes CryptoLocker so hard to crack is the strong encryption method it uses to lock down the user data on the hard drive. 

If the ransom is not paid on time, the virus deletes the decryption key that is needed to decrypt all the files on the PC. 

The malware lands on PCs in the same way as other malware does, and a few sensible precautions can help minimise the chances of suffering a CrytoLocker attack. 

"Computer users should regularly copy their data to a backup hard disk. As soon as they recognise any signs of remote encryption, they should turn off the computer," said VNCERT's expert Ha Hai Thanh. 

Antivirus software will help, but the best defence is to avoid opening attachments and to back up your files, he noted. 

According to a recent report by VNCERT, at least 1 million Vietnamese computers have been exploited by foreign botnet networks to deliver more than 3.33 billion spam messages per day, and they could also be used to attack national information technology systems. 

VNCERT recently discovered that international networks saw the involvement of computers with Vietnamese IP addresses, including Zeus botnet, with 14,075 Vietnamese IP addresses, and Sality, Downadup and Trafficconverter, with a further 113,273. 

The networks' spyware not only destroyed information technology systems but also stole confidential documents from agencies and organisations.