The Locky ransomware has switched back to the distribution method it used when it first emerged in the wild, namely Office documents with malicious macros, FireEye researchers warn.
Spotted for the first time in February, Locky stood up because it could encrypt unmapped network shares and because it was immediately associated with the actor behind the Dridex botnet. Both threats were using macros for distribution and both were receiving the same improvements to the code-hiding mechanism.
According to FireEye Labs researchers, however, the ransomware is using malicious macros once again to target individuals and organizations in a broad range of industries around the world. The same as before, spam emails are used in massive runs that hit the healthcare industry the hardest, the security researchers reveal. The telecoms and transportation sectors were also among the most affected.
The campaigns hit targets worldwide, yet the United States, Japan, and Republic of Korea are the top affected countries, FireEye Labs says. Thailand, Singapore, Germany, Hong Kong, Malaysia, Taiwan, and Saudi Arabia round up the top ten most targeted countries.
The malicious documents are DOCM files that contain macros which, once enabled, install the ransomware onto the victim’s computer. As Cisco researchers explained a couple of weeks back, cybercriminals often pack malware inside DOCM documents and then rename them to DOCX, a format that doesn’t support macros. However, Office knows that the documents are DOCM and open them as such, thus allowing macros to run, if the user chooses to enable them.
FireEye researchers noticed that Locky’s operators switched back to macros in DOCM files in the beginning of August. They also say that the distribution of Dridex via this channel has essentially come to a stop, which has resulted in an increase in Locky infections. However, what caught researchers’ attention were three massive campaigns spotted on Aug. 9, Aug. 11 and Aug. 15.
While Locky continued to use other distribution methods during the past couple of weeks, these campaigns relied heavily on DOCM files, showing that the ransomware’s operators “are investing more to infect systems and maximize their profits,” FireEye researchers say.
These campaigns showed similarities in the used macro code, researchers say: each of them used a specific “one-off” campaign code for downloading Locky from the server; the malicious URL embedded within macro code is encoded using the same encoding function, but with a different key for each campaign; and the downloaded payload is encoded using 32 bytes rolling XOR key (the key is different for each campaign).