The National Crime Agency (NCA) has warned "small and midsized" businesses (SMBs) of a mass email spamming attack that's making PCs freeze and asking owners to cough up cash.

The email, which the NCA suspects has landed in "tens of millions" of UK computer users' inboxes, contains an attachment that claims to be sent from banks and other financial institutions (such as a voicemail, fax, invoice or details of a suspicious transaction being queried).

It is in fact a particularly nasty malware that installs a piece of ransomware known as Crytolocker when opened. After encrypting a PC's files, Crytolocker opens a window that displays a countdown and demands a payment of two Bitcoins in ransom (around £536 at the time of writing) for the decryption key.

Bitcoins are a virtual form of currency that have risen in popularity (and price) thanks to its low inflation rate and untraceable nature. However, they've also been used in a fair number of other illegal activities, including drug trading and other shady underground dealings.

Significant risk

The NCA warns that the scam is a "significant risk" and says that the anyone who is infected with the malware should report it to the National Cyber Crime Unit (NCCU), which is on the hunt for the perpetrators, via Action Fraud.

The agency has advised users not to click on suspicious attachments, update antivirus and operating system software, and disconnect any machines that come into contact with the malware.

Lee Miles, Deputy Head of the NCCU, said: "The NCA are actively pursuing organised crime groups committing this type of crime. We are working in cooperation with industry and international partners to identify and bring to justice those responsible and reduce risk to the public."