Throughout August, FireEye Labs has observed a few massive email campaigns distributing Locky ransomware. The campaigns have affected various industries, with the healthcare industry being hit the hardest based on our telemetry, as seen in Figure 1.
Figure 1. Top 10 affected industries
Numerous countries are affected, with the United States, Japan, and Republic of Korea topping the list, as seen in Figure 2.
Figure 2. Top affected countries
These detection spikes and change in tactics suggest that the cybercriminals are investing more to infect systems and maximize their profits. Additionally, we have observed that the delivery of Dridex via this distribution channel seems to have stopped, or nearly so, which could explain why we are seeing the Locky uptick.
Figure 3. Massive DOCM related campaigns on Aug. 9, Aug. 11 and Aug. 15, 2016
Our analysis showed high similarity in the macro code that was used in the Aug. 9, Aug. 11 and Aug. 15 campaigns. The following are the key comparisons:
- Each email campaign has a specific “one-off” campaign code that is used to download the Locky ransomware payload from the malicious malware server (see network pattern in Figure 4).
- The malicious URL embedded within macro code is encoded using the same encoding function, but with a different key for each campaign. Each character is encoded by multiplying its ASCII code with a specified key (an integer). Hence, its decoder would perform a division using the specified integer (see URL Decoder in Figure 4).
- The downloaded payload is encoded using 32 bytes rolling XOR key. A different key is used for each campaign. Rolling XOR is described as follows:
Plain [i] = Cipher [i] ^ Key [i % length of Key], where Plain is the computed plain text, Cipher is the cipher text, Key is the xor key, and i is the byte offset (see File Decoder in Figure 4).
Figure 4. Technical Overview
These latest campaigns are a reminder that users must be cautious when it comes to opening attachments in emails or they run the risk of becoming infected and possibly disrupting business operations.