Surge in Spam Campaign Delivering Locky Ransomware Downloaders

Surge in Spam Campaign Delivering Locky Ransomware Downloaders

FireEye Labs is detecting a significant spike in Locky ransomware
downloaders due to a pair of concurrent email spam campaigns impacting
users in over 50 countries. Some of the top affected countries are
depicted in Figure 1.


Figure 1. Affected countries

As seen in Figure 2, the steep spike starts on March 21, 2016, where
Locky is running campaigns that coincide with the new Dridex campaigns
that were discussed in the blog, “Stop
Scanning My Macro”.

Figure 2. Detection on spam delivered malware

Prior to Locky’s emergence in February 2016, Dridex was known to be
responsible for a relatively higher volume of email spam campaigns.
However, as shown in Figure 3, we can see that Locky is catching up
with Dridex’s spam activities. This is especially true for this week,
as we are seeing more Locky-related spam themes than Dridex. On top of
that, we also are seeing Dridex and Locky running campaigns on the
same day, which resulted in an abnormal detection spike.

Figure 3. Dridex versus Locky spam campaign over time

Locky Ransomware spam 

The new Locky spam campaign uses several themes such as “invoice
notice”, “attached image”, and “attached document themes”. See Figure
4 and Figure 5 for example campaign emails.

Figure 4. Urgent Invoice Campaign

Figure 5. Other Campaigns

The ZIP attachment as depicted in Figure 6 contains a malicious
JavaScript downloader that downloads and installs the Locky ransomware.

Figure 6. Zipped Content

In Figure 7, it is interesting to see that the recent Locky campaign
seems to prefer using a JavaScript-based downloader in comparison to
Microsoft Word and Excel macro-based downloaders, which were seen
being used in its early days.

Figure 7. Locky Downloader Mechanism

The preference for JavaScript downloaders could be due to the ease
to transform or obfuscate the script via automation to generate new
variants as depicted in Figure 8. As a result, the traditional
signature-based solution may not keep up with the variants where its
behavioral intent is the same. At the time of discovery, most of the
samples that we see are being detected by only one vendor, according
to VirusTotal.

Figure 8. Obfuscated JavaScript code

Conclusions

The volume of Locky ransomware downloaders observed is increasing
and it may potentially replace the Dridex downloader as the top
spammer. One of the latest victims of Locky is Methodist Hospital[1], where the victim was reportedly forced to pay
a ransom to retrieve their encrypted data. This suggests that the
cybercriminals are earning more from ransomware and this drives their
aggressive campaigns.

On top of that, JavaScript downloaders seem to be the preferred
medium for delivering its payload as it could be easily obfuscated to
create new variants.

Technical Analysis of a Locky Payload Sample

MD5 Sum: 3F118D0B888430AB9F58FC2589207988 (First seen on 2016-02-24
in VirusTotal)

Persistence Mechanism
  • The malware does not contain a persistence mechanism. An
    external tool or installer is required if the attacker desires
    persistence.
  • The malware contains the ability to install the
    following registry key for persistence; this functionality is
    disabled in this variant.

    • HKCU\Software\Microsoft\Windows\CurrentVersion\Run\Locky
      • <path_to_malware>
File System Artifacts
  • The malware encrypts files on the system and creates new files
    with the encrypted contents in the same directory with the following
    naming convention:

    • <system identifier><16 random
      hex digits>.locky
    • The <system identifier>
      value is the ASCII hexadecimal representation of the first eight
      bytes of the MD5 hash of the GUID of the system volume.
  • The malware drops a ransom note provided by the C2
    server in all directories with encrypted files and on the desktop of
    the current user:

    • _Locky_recover_instructions.txt
  • The malware drops an image on the desktop of the current
    user:

    • _Locky_recover_instructions.bmp
Registry Artifacts
  • The malware creates the registry key HKCU\Software\Locky.
    • id is set to a unique identifier generated for the
      compromised system.
    • pubkey is set to a binary buffer
      that contains a public RSA key.
    • paytext is set to a
      binary buffer containing the recovery instructions.
    • completed is set to 1.
  • The malware
    changes the desktop background to a bitmap containing the ransom
    instructions.

    • HKCU\Control Panel\Desktop\Wallpaper is set
      to:
      %CSIDL_DESKTOPDIRECTORY%\_Locky_recover_instructions.bmp

Network-Based Signatures

Command and Control (CnC)
  • The malware communicates with the following hard-coded hosts
    using HTTP over TCP port 80. The malware also uses a domain name
    generation algorithm as described below.

    • 188.138.88.184
    • 31.41.47.37
    • 5.34.183.136
    • 91.121.97.170
Beacon Packet
  • The malware beacon builds a HTTP POST request to /main.php as
    shown in Figure 9. The POST data is encoded using a custom
    algorithm.

Figure 9. HTTP POST request polling packet
(general packet structure)

Domain Generation Algorithm (DGA)

This sample contains a domain name generation algorithm that is
based on the current month, day and year. There are eight possible
domains per day and the domains change on the first of the month and
on even numbered days. Figure 10 contains Python code to generate the
eight possible domain names for the current date.


Figure 10. Locky Domain Generation Algorithm

[1] http://arstechnica.com/security/2016/03/kentucky-hospital-hit-by-ransomware-attack/

This content was originally published here.

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