Microsoft researchers have observed a sharp increase in Crowti ransomware infections during October in the latest indication that spam email campaigns continue to be surprisingly effective in delivering malicious payloads on consumer and enterprise systems.

Crowti is a malware that encrypts all data files on desktops and mobile computers it infects. It then demands a ransom in bitcoins to unencrypt the files and make data accessible again. Crowti is similar to other, better-known types of ransomware such as CryptoLocker. It is distributed primarily through malicious attachments in spam email messages, though security researchers have seen it bundled in exploit kits as well.

Microsoft’s Malware Protection Center first reported Crowti in June. In the months immediately following its discovery, infection rates hovered between 1,500 and 2,500 systems per month and even dropped to below 500 per month for a brief period. However, in October, security researchers observed a sharp increase in Crowti infections as the result of new malware distribution campaigns. In mid-October, Crowti infections in Microsoft’s customer base topped 4,000 systems, with more than 71 percent of them located in the United States.

“Crowti is being distributed via spam campaigns with email attachments designed to entice the receiver to open them,” Microsoft said.

Many of the attachments have names that appear fairly innocuous and are designed to entice unwary users to open them. The commonly used attachment names that researchers discovered include names that suggest the attachment contains important phone numbers, is an incoming fax report or is an invoice of some sort.

Researchers also discovered Crowti being distributed via several exploit kits, including RIG, Nuclear and RedKit V2.

“Crowti’s primary payload is to encrypt the files on your PC,” Microsoft warned. “It usually brands itself with the name CryptoDefense or CryptoWall” after encrypting the files on a system. A message then instructs the victim to click on some provided links that direct the victim to a Tor Web page for paying up the ransom using bitcoins.

As with other ransomware, there is no guarantee that victims who ante up will regain access to their encrypted data or get their PC back to its original state.

“We do not recommend paying the ransom,” Microsoft said.

Crowti can be particularly damaging in an enterprise setting because it can be used to take vital corporate or customer data hostage.

“In most cases, ransomware such as Crowti can encrypt files and leave them inaccessible,” Microsoft warned, urging businesses and consumers to pay attention to some basic, long-standing security precautions.

In addition to not clicking on suspicious emails and attachments, users should make sure their systems and software are properly updated with all the latest security patches. Many of the exploits being used by Crowti take advantage of vulnerabilities in browser plug-ins such as Java and Flash. The flaws have all been patched long ago, so users who have installed them should be safe.