A threat actor is using compromised Skype and Microsoft Teams accounts to distribute DarkGate, a troublesome loader associated with multiple malicious activities, including information theft, keylogging, cryptocurrency miners, and ransomware such as Black Basta.

Forty-one percent of the targets of the campaign — which appears to have begun in August — are organizations in the Americas, according to researchers at Trend Micro who are tracking the activity.

In a report this week, Trend Micro also said its researchers had observed the developer of DarkGate begin to advertise the malware on underground forums and renting it out on a malware-as-a-service basis to affiliate threat actors. The pivot, after years of going it alone, has resulted in a recent surge in DarkGate activity after a relative lull.

Microsoft Phishing Via Skype and Teams

The operator of the DarkGate campaign that Trend Micro is currently tracking is using both Skype and Teams to distribute the malware. In one of the attacks, the threat actor took control of a Skype account belonging to an individual at an organization with whom the target recipient's organization had a trusted relationship. The adversary basically used the compromised Skype account to hijack an existing message thread and send a message that appeared to contain a PDF file but was actually a malicious VBS script. When the recipient executed the file, it initiated a sequence of steps to download and install DarkGate on the target computer.

In another attack that Trend Micro analyzed, the threat actor attempted to achieve the same outcome using a Teams account to send a message with a malicious .LNK file, to a target recipient. Unlike the Skype caper, where the threat actor purported to be someone belonging to a trusted third party, in the Teams variation, the recipient received the malicious message from an unknown, external entity. 

"In this case, the organization’s system allowed the victim to receive messages from external users, which resulted in them becoming a potential target of spam," Trend Micro said.

"We also observed a tertiary delivery method of using a VBA script wherein a .LNK file arrives in a compressed file from the originators' SharePoint site," the security vendor said. In this attack variation, the threat actor attempts to lure the victim to a specific SharePoint site, to download a file named "Significant company changes September.zip."

DarkGate: A Potent Threat

DarkGate is malware that has targeted users in various regions around the world since at least 2017. It integrates multiple relatively potent functions; for instance, the malware can execute commands for gathering system information, mapping networks, and doing directory traversal. It also implements remote desktop protocol (RDP), hidden virtual network computing, AnyDesk, and other remote access software. Other "features" include ones related to cryptocurrency mining, keylogging, privilege escalation, and stealing information from browsers.

For payload delivery and execution, DarkGate uses AutoIT, a legitimate Windows automation and scripting tool that authors of other malware families have used for obfuscation and defense evasion.

DarkGate Offers Multiple Potential Payloads

Trend Micro's analysis showed that once DarkGate is installed on a system, it drops additional payloads. Sometimes those are variants of DarkGate itself or of Remcos, a remote access Trojan (RAT) that attackers previously haveused for cyber-espionage surveillance and for stealing tax-related information.

Trend Micro said it was able to contain the DarkGate attacks it observed before any actual harm came to pass. But given the developer's apparent pivot to a new malware leasing model, enterprise security teams can expect more attacks from varied threat actors. The objectives of these adversaries could vary, meaning organizations need to keep an eye out for threat actors using DarkGate to infect systems with different kinds of malware.

While the attacks that Trend Micro observed targeted individual Skype and Teams recipients, the attacker's goal clearly was to use their systems as an initial foothold on the target organization's networks. "The goal is still to penetrate the whole environment, and depending on the threat group that bought or leased the DarkGate variant used, the threats can vary from ransomware to cryptomining," according to Trend Micro.

The security firm recommends that organizations enforce rules around the use of instant messaging applications such as Skype and Teams. These rules should include blocking external domains, controlling the use of attachments and implementing scanning measures if possible. Multifactor authentication is also crucial to prevent threat actors from misusing illegally obtained credentials to hijack IM accounts, Trend Micro said.