By Tej Tulachan
The Cofense Phishing Defense Center (PDC) has detected a phishing campaign that delivers Trickbot embedded in a Google Docs link. Trickbot has been making the rounds for a long time now and is still considered one of the biggest malware threats targeting business today. Threat actors frequently utilize legitimate applications or trusted file sharing sites like Google Docs to bypass the email gateway and lure users to click on the link to deliver malware. In this case, the email made it through Proofpoint’s gateway utilized by our PDC customer.
The email attempts to lure curious users to click on the link: “Have you already received documentation I’ve directed you recently? I am sending them over again.” This is a legitimately generated email by Google Docs when a file is shared by one of its subscribers. Unknowingly, the recipient is directed to a document hosted on Google that contains a malicious URL.
Fig 1. Email body
When the recipient clicks on the link it directs to a genuine Google Docs page as shown below, which contains a fake 404 error message and another embedded link. The threat actor baits the recipient into downloading the document: “Downloading the document manually via the link”. This link hxxps://docs[.]google[.]com/uc?id=112QLCdDtd4y-mAzr8hobCs0TP5mQmKfL downloads the malicious payload.
Fig 2. Google doc page
Once the URL links to a file hosted on Google drive, it downloads a Review_Rep.19.PDF.exe which has been disguised as PDF file. Many recipients will not see the .exe file extension. It’s something that you need to specifically enable in Windows. So, to them it looks like a legitimate PDF file since the attacker uses the icon for a PDF.
Fig 3. Pdf Icon
If we look at the file in a hex editor, we see that in fact it’s an executable file and not a PDF.
Take a look below in the editor, indicated by the magic bytes MZ which denotes a windows executable.
Fig 4. Magic Number
Once the payload is executed it creates a copy of itself (egолаСывЯыФЙ) in C:ProgramData, where it undertakes control over execution of the malware.
Fig 5. egолаСывЯыФЙ.exe
Furthermore, it creates another copy in “C:UsersREMAppDataRoamingspeedLan” that also includes the config file for Trickbot (settings.ini) (The directory depends on the Trickbot version.)
Fig 6. speedlan
If we look inside the settings.ini we see a lot of the “obfuscated” text.
Fig 7. Obfuscated text
Additionally, if we open up the Task Scheduler, we can see it also sets a task that starts the malicious file from the “Speedlan” folder.
Fig 8. Start Task Scheduler
Looking at the Triggers tab, we can see it has been set to repeat itself every 11 minutes for 596843 minutes (414 days) for this particular version of Trickbot. The scheduled task checks to see if the binary is running in memory every 11 minutes over a 1-year period. This means that the binary will stay persistent on the system if the process is terminated. The 414 day counter just insures that the scheduled task stays running for as long as the system is online (generally, people will reboot their computer at least once a year).
Fig 9. Trigger
This then hollows out Svchost, injects its malicious code, and launches it. It keeps launching more and more Svchost’s if you let it run. Each of these are typically responsible for a module of Trickbot.
Fig 10. Hollows Svchost
Indicators of Compromise (IOCs):
Filename: Review_ Rep.19.PDF.exe
File Size: 404,320 Bytes
HOW COFENSE CAN HELP
89% of phishing threats delivering malware payloads analyzed by the Cofense Phishing Defense CenterTM bypassed email gateways. Condition users to be resilient to evolving phishing attacks with Cofense PhishMeTM and remove the blind spot with Cofense ReporterTM. Cofense PhishMe offers a phishing scenario, “Shared Google Doc – TrickBot,” to help users identify the attack described in today’s blog.
Thanks to our unique perspective, no one knows more about REAL phishing threats than Cofense™. To understand them better, read the 2019 Phishing Threat & Malware Review.
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