Threat Research

New Trickbot Plugin Harvests Email Addresses from SQL Servers, ScreenLocker Module Not for Ransom

Just a week after publishing our discovery of Trickbot’s networkDLL, the FortiGuard Labs monitoring system has found a new module called squlDll that is being actively distributed to the banking trojan’s victims. 

Fig.1 Command received from Trickbot C2 to execute the new squlDLL module

This new component is one of Trickbot’s several modules used for scraping email addresses from the machines of its victims. It targets Microsoft SQL Servers using legitimate libraries, similar to what we observed in its mailcollection module. Moreover, it is able to scrape credentials from the victim by force enabling WDigest authentication and utilizing the popular Mimikatz tool.

As usual with Trickbot, the new module is comprised of embedded binaries that we’ve seen in previous modules.

Fig.2 The squlDLL module embedding structure

One of the familiar modules is screenLocker, which was previously assumed to be an underdeveloped lock-for-ransom capability. However, our analysis now suggests a different and simpler purpose for it. 

Harvesting Emails from SQL Servers

Sub-modules are written and executed in the user’s %TEMP% directory and then deleted after their functions are completed. As in previous Trickbot modules, every time a sub-module is executed by the loader, a shared memory is allocated by the main loader that the sub-module can access and use to store collected data. The main loader then retrieves the data through the shared memory.

sqlFinder

The first sub-module to be executed is sqlFinder, whose main function is to list all the network-visible instances of Microsoft SQL Servers from the compromised machine. To achieve this, it uses the legitimate SQL Server Database Management Objects (SQLDMO.DLL) COM interface to list all available SQL server instances on the victim’s network. 

Fig.3 Listing available SQL servers

mailCollector

Another sub-module is executed that gathers email addresses from the collected SQL server instances.

It uses the ADO COM object interface to talk to the database. At first, it attempts to connect to all available SQL servers using Windows Authentication mode. If the user that is currently logged in on the infected machine has permission to connect to the SQL server, the connection attempt succeeds. 

Fig.4 Connecing to SQL servers using Windows Authentication Mode

After successfully connecting to a SQL server instance, the next thing that it does is to search for target databases. All SQL Server System Databases – such as “master”, “tempdb”, “model” and “msdb” – are skipped. From the target databases found, it queries all the tables using its information schema and then tries to collect emails.

Fig.5 Enumerating databases and tables

In the search_email function, it tries to find email addresses by executing an SQL query using the “LIKE” operator to capture all table field data that matches the wildcard string format, %%@%%.%%”.

Fig.6 Collecting emails from tables

Gathering Credentials using Mimikatz

In an attempt to extract credentials from the victim system, it uses the popular Mimikatz tool. One of the methods it uses is to target WDigest credentials that are stored in the LSA memory in plain text. When Microsoft released Windows 8.1 they introduced a way to mitigate such an attack by adding a switch in the form of the registry entry

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\WDigest\UseLogonCredential

Basically, the storing of WDigest credentials in memory can be disabled by setting the value of this entry to ‘0’. Otherwise, it can be set to ‘1’ as some services rely on WDigest authentication. In Windows 8.1 and newer versions, this feature is disabled by default. In 2014 Microsoft released a security update to integrate this setting into some older versions of Windows.

To make sure that the tool succeeds in obtaining plain text credentials, this malware module makes sure that WDigest authentication is enabled by modifying/adding the registry entry.

Fig.7 Modifying the registry entry to enable WDigest authentication

However, the user must log in after the feature is enabled in order for the credentials to be stored in the memory. This is where the screenLocker module comes into play.

A number of researchers had previously assumed that this module was a screen lock ransom in the making. However, based on its usage here, it has a much simpler purpose. It initiates a routine to lock the screen of the victim to in order to somehow force the victim into entering their login credentials in order to log back in. This function is only executed on Windows 8/Server 2012 or newer versions.

Fig.8 OS check and lock screen routine
Fig.9 ScreenLocker module’s main function locks the user’s screen

An embedded Mimikatz library is then decrypted and directly loaded in memory, which reduces the malware’s physical file trace in the victim’s system. The library’s export getUserPasswordPairs is then called to start gathering credentials.

Fig.10 Function to load and execute Mimikatz library in memory to gather credentials

Conclusion

Trickbot is being actively updated in the wild, and this is just one of the several modules that we’ve found being pushed out to victims in the past few weeks. Most of these modules are focused on gathering credentials and scraping email addresses using different techniques – more addresses means more targets.

As always, FortiGuard Labs will be on the lookout for any major developments of this botnet.

-= FortiGuard Lion Team =-

Solutions

Fortinet users are protected with the following solutions:

  • Trickbot components are detected by FortiGuard Antivirus
  • FortiSandbox rates Trickbot execution as “High Risk”
  • Malicious download URLs and C2s are blocked by our FortiGuard Web Filtering Service.

IOC

File Samples

b001d11ab1fc2b5ec52a79f6cfe5379a504bc6a79610e2a7dc6a04e74dd5df17 (main trickbot) - W32/Trickbot.KAD!tr

079dc54026cc88aa72ca3111e1a7416a354ed95e6d0e8adaa0e48f6965d90256 (squlDLL module) - W32/Trickbot.KAD!tr

696888c3131da282866fc339b78bd6d96b168a6ecfd78c3050fd40b7ebe484f2 (squlDLL module) - W32/TrickBot.AK!tr

7ea10d92fcd48f3a35743643acd815496a6eb97a8ad1ed15c294034a09f9a579 (squlDLL module) - W32/TrickBot.AK!tr

9d1393c11c4ead9d78d57ba0a04fa4ca1d66eba50dc0e5f99b43772ef0ad9995(squlDLL module) - W32/TrickBot.AK!tr

93b057124ba35a9db9da7c04dfc2ccbbeddb4a0658e0376d1f9a11a2eabe32c4(mailCollector) - W32/Trickbot.KAD!tr

5299d604945baf69032681ce110add6014501556eaafbeadc1e4e5d84b526541(sqlFinder) - W32/Trickbot.KAD!tr

78f9394590eae9c2d68ff890709b9eb6ef5654d7a5c19dc0ee1b1b82a3c75da2(screenLocker) -  W32/TrickBot.AK!tr

93522b01a8335630a0903303a336bbd7013eeeefaac6efdf7320bb6623c88ed8(mimikatz) -  Riskware/Mimikatz

C2

https[:]//31.134.52.42:449 (main Trickbot)

23.95.97.59 (squlDll)

 

 

 

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