A malicious document builder named EtterSilent is gaining more attention on underground forums, security researchers note. As its popularity increased, the developer kept improving it to avoid detection from security solutions.

Cybercriminals behind operations with notorious malware started to include EtterSilent in their campaigns more often to increase the payload delivery success rate.

Using macros and exploits

Ads promoting EtterSilent maldoc builder have been published on underground forums since at least mid-2020, boasting features like bypassing Windows Defender, Windows AMSI (Antimalware Scan Interface), and popular email services, Gmail included.

In a blog post today, researchers at threat intelligence company Intel 471 note that the seller offered weaponized Microsoft Office (2007 through 2019) documents in two ‘flavors’: with an exploit for a known vulnerability or with a malicious macro.

One of the vulnerabilities leveraged is CVE-2017-8570, a high-severity remote code execution. The author also mentioned two other vulnerabilities (CVE-2017-11882 and CVE-2018-0802), albeit some restrictions applied, and demonstrated them in a video.

According to Intel 471, the variant with the macro is the more popular variant, likely because of the “lower pricing and higher compatibility when compared to the exploit.”

An EtterSilent maldoc with macro code can pose as a DocuSign or DigiCert document that asks users to enable support for macros that downloads a payload in the background.

Because it uses Excel 4.0 XML macros, EtterSilent does not depend on the Visual Basic for Applications (VBA) programming language, which is commonly seen with malicious macros.

“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” – Intel 471

Low detection attracts big names

The researchers note that an EtterSilent maldoc was included in a recent spam campaign that dropped an updated version of Trickbot. The gang used the same method in a campaign on March 19 to infect systems with BazarLoader/BazarBackdoor.

Intel 471 says that other cybercriminal groups leveraged EtterSilent services for their operations. Some examples are banking trojans IcedID/BokBot, Ursnif/Gozi ISFB, and QakBot/QBot. Along with Trickbot, most of them have been used to deliver various ransomware strains (Ryuk, Conti, Maze, Egregor, ProLock).

Gangs as prolific as these are constantly looking for new ways to distribute their payloads while drawing as little attention as possible and the EtterSilent maldoc service appears to provide a good cover.

In early March, some of the weaponized documents built with this tool went completely undetected by all antivirus engines included in a scanning service.

A week ago, less than a handful of antivirus engines detected one weaponized document built with this tool. At the time of writing, the detection increased to 20/40 engines in VirusTotal. For another file, the detection increased over six days from 16/62 to 20/62.

Last year, the price for generating an EtterSilent malicious document was $130. However, the service offered a more expensive tier ($230) for a custom stub to make malicious files unique by encrypting them.

Intel 471’s blog provides a list of indicators of compromise for EtterSilent malicious documents as well as for the payloads they delivered: Trickbot, IcedID, QBot, Ursnif, and BazarLoader.