Two ransomware groups abuse Microsoft’s Office 365 platform to gain access to target organizations
January 22, 2025
Two ransomware groups exploiting Microsoft 365 services and default settings to target internal enterprise users.
Sophos researchers started investigating two distinct clusters of activity, tracked as STAC5143 and STAC5777, in response to customer ransomware attacks in November and December 2024.
Threat actors used their own Microsoft 365 tenants and exploited a default Teams setting allowing external users to contact internal users for attacks.
Sophos states that the STAC5777’s TTPs overlap with the group Storm-1811 first spotted by Microsoft. STAC5143 is a new cluster mimicking the Storm-1811 playbook, potentially linked to the FIN7/Sangria Tempest/Carbon Spider threat actor.
“Sophos MDR has observed more than 15 incidents involving these tactics in the past three months, with half of them in the past two weeks.” reads the advisory published by Sophos.
STAC5143 relies on Teams’ built-in remote control capabilities and deploys Java-based tools to exploit victim systems. It extracts Python backdoors from ZIP files downloaded via remote SharePoint links and employs techniques associated with the FIN7 threat actor.
STAC5777 was spotted using Microsoft Quick Assist and manual configuration changes to deploy malware. It incorporates a legitimate Microsoft updater paired with a malicious DLL to establish persistence, steal credentials, and discover network resources. The group uses RDP and Windows Remote Management for lateral movement and, in some cases, deploys Black Basta ransomware. The group shares TTPs with the threat actor Storm-1811.
Common tactics include email bombing with spam (in some cases the volume reached as 3,000 in less than an hour), posing as tech support via Teams messages and calls, and using Microsoft remote tools to install malware.
Sophos first observed a STAC5143 attack in November 2024, it began with spam emails followed by a Teams call from an account named “Help Desk Manager.” The attackers requested remote screen control via Teams, enabling them to drop files, execute malware, and run PowerShell commands to fetch a ProtonVPN executable and a malicious DLL. This led to the deployment of a Python payload, installation of backdoors, and execution of commands for user and network discovery.
STAC5777 attacks also began with targeted spam emails followed by a Teams message from an adversary posing as internal IT support. The attacker guided the victim through installing Microsoft Quick Assist, enabling remote control of the victim’s device. Once access was established, the attacker used a web browser to download a malicious payload, which was split into parts, reassembled, and unpacked to deploy malware.
The payload included a legitimate Microsoft-signed executable, unsigned OpenSSL DLLs, and a malicious DLL named winhttp.dll, designed for side-loading. This DLL collected system details, user credentials, and keystrokes. The attacker made registry changes to establish command-and-control connections and used PowerShell to set up persistence, creating a service and startup entry for the malicious executable.
The malware enabled ongoing access and backdoor capabilities, leveraging configuration changes and IP-based connections established by the attacker.
“organizations should take further steps to prevent attacks based on these tactics. First, unless absolutely necessary, organizations should ensure that their O365 service provisions restrict Teams calls from outside organizations or restrict that capability to trusted business partners. Additionally, remote access applications such as Quick Assist should be restricted by policy unless they are specifically used by the organization’s technical support team. Sophos can block unwanted execution of Quick Assist through application control settings in endpoint protection.” concludes the report.
“Sophos strongly recommends use of Microsoft Office 365 integration with the security environment for monitoring of sources of potentially malicious inbound Teams or Outlook traffic.
Organizations should also raise employee awareness of these types of tactics—these aren’t the types of things that are usually covered in anti-phishing training. Employees should be aware of who their actual technical support team is and be mindful of tactics intended to create a sense of urgency that these sorts of social-engineering driven attacks depend upon.”
Sophos published a list of indicators of compromise for these campaigns.
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Pierluigi Paganini
(SecurityAffairs – hacking, ransomware)