Cyber security firm ESET is warning people against a wave of infected emails after discovering an unusual high record of a malicious downloader called Nemucod in several countries, including New Zealand and Australia.

Since March 11, there was a rapid increase in the number of infected emails containing a malicious attachment, which downloads and installs ransomware onto an infected device. When opened, it encrypts victims' files on their PCs and requires a ransom for decryption.

According to ESET, Nemucod is spread via emails, which contain attached zipped files. Emails are written in a very trustworthy way, claiming to be invoices, notices of appearance in court or other official documents.

Attackers encourage users to open the malicious attachment that contains a JavaScript file, which after it is opened, downloads and installs Nemucod to the victim's PC.

“Nemucod currently downloads mainly ransomware, for example TeslaCrypt or Locky. These encrypt the data on the victim's computer and demand ransom,” says Peter Staník, security evangelist at ESET.

Both TeslaCrypt and Locky ransomware use encryption standards similar to those used by financial institutions when securing online payments, Stanki says.

In New Zealand, the Nemucod malware accounted for 40% of all malware detected by ESET protection systems.

In Australia, the Nemucod malware accounted for 33% of all malware detected by ESET protection systems.

How to protect against this threat:

• Do not open attachments sent to you in emails from unknown senders.
• Warn colleagues who most frequently receive emails from external sources – for instance financial departments or human resources.
• Regularly backup your data. In case of infection, this will help you recover all your data. An external disc or other storage should not remain connected to your computer in order to avoid infection by filecoder.
• Regularly install updates of your OS and other software you use. If you still use Windows XP, seriously consider moving to another, supported operating system of Windows.
• Security software must also be used with all updates installed, ideally with the latest version. IT security vendors are packing new versions of their software with additional security features.
• Users of ESET solutions are protected when ESET LiveGrid® Reputation System  is turned on. This technology protects users' devices against ransomware by actively blocking their processes.