Locky Ransomware Distributed Via DOCM Attachments in Latest Email Campaigns

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

From our trend analysis seen in Figure 3, Locky ransomware started
being delivered via DOCM format email attachments more extensively
beginning in August. This marks a change from the large campaigns we
observed in March,
where a JavaScript based downloader was generally being used to infect systems.

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:

  1. 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).
  2. 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).
  3. 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

The volume of Locky ransomware downloaders is increasing and the
tools and techniques being used in campaigns are constantly changing.
In this instance, we are seeing a shift from using a JavaScript based
downloader to infect victims to using the DOCM format. On top of that,
cybercrime trends have shown that attackers are distributing more
ransomware these days than banking trojans, as the former appears to
be more lucrative.

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.

*** This is a Security Bloggers Network syndicated blog from Threat Research Blog authored by Threat Research Blog. Read the original post at: http://www.fireeye.com/blog/threat-research/2016/08/locky_ransomwaredis.html