Cybercrims deploying DragonForce ransomware appear to have gained access to a major US services company's network, then spent two months up to no good while disguising their command-and-control activities as legitimate Microsoft Teams traffic.

Researchers at security firm Symantec said the intrusion began with attackers gaining access to the victim's environment before deploying a custom Go-based backdoor, tracked as "Backdoor.Turn," to maintain communication with the compromised systems. Rather than reaching out to attacker-controlled infrastructure that might raise alarms, the backdoor hid its activity inside traffic associated with Microsoft's widely used collaboration platform.

To anyone monitoring network traffic, the compromised systems appeared to communicate only with legitimate Microsoft servers.

"The attackers in this campaign use exceptionally sophisticated cyber tradecraft," Symantec said. "The configuration of Backdoor.Turn means that security products only see C&C traffic going to legitimate Teams servers, leaving defenders unaware that data is being siphoned away by malicious actors."

Symantec said the attackers installed Backdoor.Turn on systems after deploying DragonForce ransomware, potentially giving them a way back into compromised networks or access they could later sell to other criminals.

To connect to Microsoft's infrastructure, the backdoor first requested an anonymous visitor token from Microsoft Teams and Skype back-end services. It then used a Microsoft-operated TURN relay server – infrastructure typically used to help establish communication between users – before establishing a direct QUIC connection to a malicious command-and-control server.

Symantec said this is the first known case of malware using this particular technique.

The security firm did not identify the victim beyond describing it as a major US services company, nor did it say whether the Teams-based communications channel had been observed in other DragonForce incidents.

The ransomware operation has become increasingly prominent over the past year, operating a ransomware-as-a-service model that allows affiliates to conduct attacks under the DragonForce banner. It has been linked to the prolific Scattered Spider group, which has conducted a string of high-profile attacks, including intrusions targeting major retailers in the UK.

While attackers have long abused legitimate cloud services to conceal malicious traffic, Symantec's findings suggest that DragonForce operators continue to look for ways to blend into the software and infrastructure that organizations trust most. ®