Two recent malware discoveries suggest that attackers are turning to new modular downloaders that allow them to modify and update their software at will after it has been installed on a victim's system.

Security vendor Proofpoint says its researchers have observed a previously undocumented downloader, called Advisorsbot, being used in a malicious email campaign targeting workers in the restaurant, hotel, and telecommunications industries since at least May 2018.

The malware is designed in such a way that attackers can add new payloads and capabilities to it post-infection. For the moment, at least, all that AdvisorsBot is doing is loading a fingerprinting module on infected systems presumably so it can identify systems of interest.

The data being collected and sent back to the malware's command-and-control server includes system ID, operating system version, domain-related information, Microsoft Outlook account details, information on any antimalware tools on the system, and some unknown hard-coded values. AdvisorsBot currently can receive and execute only two commands — one to load a module and the other to load shellcode in a threat, Proofpoint says.

The malware is identical in function to another modular downloader named Marap, which researchers at Proofpoint also recently discovered. It is being used in a relatively large email campaign targeting users in the financial sector.

Like AdvisorsBot, Marap is designed to let attackers update their malware at will after it has been installed on a victim's system, but it, too, currently only has a system fingerprinting module.

Growing use of such malware could pose new problems for enterprises.

"Modular downloaders tend to be small and 'quiet' in contrast, for example, to ransomware that lets victims know quite clearly that they are infected," says Sherrod DeGrippo, director of threat research and detection at Proofpoint. "For the enterprise, having stealthy malware installed on clients and servers capable of carrying out a variety of malicious actions in the future presents significant risks to organizations and real benefits to threat actors."  

Both AdvisorsBot and Marap employ features aimed at making it harder for security researchers to analyze the malware. For example, AdvisorsBot contains a lot of extra loops, statements, instructions, and other junk code designed to slow down reverse-engineering, according to Proofpoint. It also includes features for detecting anti-malware tools and for when it might be running in a sandbox so it can exit without executing.

The threat actor behind the campaign — an entity that Proofpoint identifies as TA555 — has been distributing AdvisorsBot via phishing emails containing a macro that initially executed a PowerShell command to download the malware. Since early August, the attacker has been using a macro to run a PowerShell command, which then downloads a PowerShell script capable of running AdvisorsBot without writing it to disk first, Proofpoint said.

Interestingly, since first releasing the malware in May, its authors have completely rewritten it in PowerShell and .NET. Proofpoint has dubbed the new variant as PoshAdvisor and describes it as not identical to AdvisorsBot but containing many of the same functions, including the ability to download additional modules.

"At this time, it is unclear why the author might have rewritten the malware in PowerShell," DeGrippo says. It is certainly unusual for malware authors to do so and may be an attempt to further evade defenses.

"For the enterprise, more variety in the threat landscape and newly coded malware increase complexity for defenders and should be driving investments in threat intelligence, robust layered defenses, and end user education," she says.

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