Over the past few weeks, researchers have observed a spike in the number of malicious spam emails using attached JavaScript files to distribute malware such as the infamous Locky ransomware.
In February, researchers at Trustwave discovered a spam campaign in which the operators behind the Dridex botnet had changed their distribution method and adopted JavaScript (.js) attachments instead of the usual Office documents with malicious macros. The payload in that campaign was Locky, and it became obvious that Dridex and the ransomware were connected.
Now, Proofpoint researchers warn of a spike in spam emails using .js attachments instead of documents, with hundreds of millions of messages being sent to users as part of this massive campaign. According to Proofpoint, the campaign relies on botnets to distribute huge volumes of malicious emails and has become the largest observed in recent years by the security firm.
The size of the campaign suggests that the malware operators are trying to capitalize on the novelty of .js attachments, since users are more likely to click on them than they would with .exe files. “Users have been trained to not click exe attachments, but many may not know what a “js” file is, or that it can be just as dangerous. The icon looks like a document which is somewhat confusing to users unfamiliar with this file type,” Bryan Burns, Vice President of Threat Research at Proofpoint, says.
The final payload in these spam email runs is almost exclusively Dridex or Locky, although the same type of malicious attachments has been used to distribute Teslacrypt and CryptoWall before. However, researchers say that the lures in the new campaign are similar to those used in previous runs that used Office document attachments.
Additionally, security firm ESET says that it has detected a massive increase in detections of the JS/Danger.ScriptAttachment malware. This malware is nothing but a dropper designed to download other malware on the infected systems, with crypto-ransomware families such as Locky accounting for most of these malicious payloads.
The malware is distributed as an email attachment and relies on social engineering tactics to determine the unsuspecting victim to execute it. ESET, which notes that ransomware is one of the most dangerous threats at the moment, reveals that the most affected countries in this distribution campaign are Luxembourg (67%), Czech Republic (60%), Austria (57%), Netherlands (54%) and the UK (51%).
Proofpoint researchers, on the other hand, say that the spam campaigns they have observed originated from botnets located worldwide, but that the largest sources of sending IP addresses are in India and Vietnam.
The large number of spam emails sent as part of these infection campaigns ensure that attackers managed to reach recipients that lack the necessary level of protection. Furthermore, since the attached files are not immediately recognizable as malicious, the chances of a successful infection are much higher.