EtterSilent: the underground’s new favorite maldoc builder
By Intel 471 •
The cybercrime underground often mimics behaviors that we see in everyday facets of life. Intel 471’s latest discovery is an example of one of these patterns: when a product takes off in the marketplace, users will rush to obtain it and find unique ways to use it in order to fit their needs.
The latest “product” is a malicious document builder, known in the underground as “EtterSilent,” that Intel 471 has seen leveraged by various cybercrime groups. As it has grown in popularity, it has constantly been updated in order to avoid detection. Used in conjunction with other forms of malware, it’s a prime example of how ease of use and a concentration of skill sets leads to a commoditization of the cybercrime economy.
How it works
First advertised on a well-known Russian cybercrime forum, the seller offered two types of weaponized Microsoft Office documents (maldocs) to users: one that exploits a known vulnerability in Microsoft Office (CVE-2017-8570) and another that uses a malicious macro. To our knowledge, the maldoc with the macro is the more popular choice, possibly due to lower pricing and higher compatibility when compared to the exploit.
The malicious document, when opened, shows a template that poses as DocuSign, the popular software that allows individuals and organizations to electronically sign documents. The maldoc then leverages Excel 4.0 macros stored in a hidden sheet, which allow an externally-hosted payload to be downloaded, written to disk and executed using regsvr32 or rundll32. From there, attackers can follow up and drop other assorted malware.
EtterSilent DocuSign template from December 2020
EtterSilent DocuSign template from March 2021
A cybercrime extravaganza
Ettersilent is being used in many malware campaigns, many of which will be familiar to most cybersecurity experts.
It was used in a recent spam campaign to drop an updated version of Trickbot. The maldoc was attached in an email that pretended to be from a well-known multinational appliance manufacturer, claiming to be a payment invoice.
On March 19, 2021, EtterSilent was used as part of a Bazar loader campaign. The analyzed maldoc did not use a DocuSign template, but the main Excel sheet was named “DocuSign®.” The maldoc downloads the Bazar payload, which in turn connects to another URL that downloads the related Bazar backdoor.
Everyone needs hosting
Three banking trojans — BokBot, Gozi ISFB and QBot —have also used EtterSilent in conjunction with their schemes. However, what makes these campaigns stand out is their reliance on bulletproof hosting, which is needed to stand up their schemes. All three campaigns use services run by Yalishanda, one of the world’s most notorious BPH providers.
Intel 471 tracked a particular campaign tied to BokBot that had numerous distribution URLs embedded in the EtterSilent maldocs. As of the time this blog was published, all of those domains resolved to one particular IP address. That address is tied to bulletproof infrastructure provided by Yalishanda. The usage of Yalishanda’s BPH service specifically for the delivery URL is reminiscent of years worth of Hancitor campaigns observed by Intel 471.
We have written extensively how bulletproof hosting works hand-in-glove with cybercrime for decades, supplying criminals with the infrastructure they need to carry out their crimes. EtterSilent’s attachment to Yalishanda is another example of that notion.
A piece in a bigger puzzle
The widespread use of EtterSilent shows how commoditization is a big part of the cybercrime economy. Different players specialize in their respective area, whether that be robust hosting, spam infrastructure, maldoc builders, or malware as a service, and find ways to leverage each other’s products in services by working together. A better understanding of not only the malware being used, but how the cybercrime economy works and who the major players are, helps defenders focus on the threats most relevant to our organizations.
Appendix 1 – Indicators of compromise
|Trickbot distribution URL||http://costacars[.]es/ico/ortodox.php|
|IcedID distribution URL||http://188[.]127.254.114/44270.7082388889.dat|
|Qbot distribution URL||http://kfzhm28pwzrlk02bmjy[.]com/mrch.gif|
|Ursnif distribution URL||http://musclemodz[.]com/asrt3.png|
|Bazar distribution URL||http://itelsys[.]ma/prod/education.php|
|Qbot distribution URL||http://pokojewewladyslawowie[.]pl/orlpzhiy/44270.5684626157.dat|
|Trickbot distribution URL||http://mineiro[.]ch/casrtnoar/count.php|
|Ursnif distribution URL||http://holmesservices[.]mobiledevsite.co/ds/2803.gif|