New JavaScript spam wave distributes Locky ransomware

Antivirus firm ESET has observed a spike in detections of JS/Danger.ScriptAttachment, a malware downloader written in JavaScript that started on May 22 and peaked on May 25.

Many countries in Europe have been affected, with the highest detection rates being observed in Luxembourg (67 percent), the Czech Republic (60 percent), Austria (57 percent), the Netherlands (54 percent) and the U.K. (51 percent). The company's telemetry data also showed significant detection rates for this threat in Canada and the U.S.

JS/Danger.ScriptAttachment can download various malware programs, but recently it has been used to primarily distribute Locky, a widespread, malicious program that uses strong encryption to hold users' files hostage.

While Locky doesn't have any known flaws that would allow users to decrypt their files for free, security researchers from Bitdefender have developed a free tool that can prevent Locky infections in the first place. The tool makes the computer appear as if it's already infected by Locky by adding certain harmless flags, which tricks the malware into skipping it.

The use of JavaScript-based attachments to distribute Locky began earlier this year, prompting Microsoft to post an alert about it in April.

The attachments are usually .zip archive files that contain .js or .jse files inside. These files with will execute directly on Windows without the need of additional applications.

However, it is very uncommon for people to send legitimate applications written in JavaScript via email, so users should avoid opening this kind of files.