Storm-0324 gathers over Microsoft Teams

A threat actor tracked in Microsoft’s taxonomy as Storm-0324 has been observed switching up its tactics to incorporate social engineering phishing attacks conducted via Microsoft Teams, Redmond has revealed.

Storm-0324, a so-called initial access broker (IAB), is linked to several prolific and dangerous ransomware operations, including some known to have deployed the Clop, Gandcrab, Maze and REvil lockers.

“Beginning in July 2023, Storm-0324 was observed distributing payloads using an open source tool to send phishing lures through Microsoft Teams chats,” wrote the Microsoft Threat Intelligence team.

“This activity is not related to the Midnight Blizzard social engineering campaigns over Teams that we observed beginning in May 2023. Because Storm-0324 hands off access to other threat actors, identifying and remediating Storm-0324 activity can prevent more dangerous follow-on attacks like ransomware.

From 2018 up until quite recently, the group’s activity has centred on distributing its malware, JSSLoader, on behalf of the ransomware-as-a-service (RaaS) actor Sangria Tempest – aka Elbrus, Carbon Spider, and FIN7 – using what Microsoft described as “highly evasive infection chains with payment and invoice lures” linking to a SharePoint site from whence the unwary download a malicious ZIP archive containing the payload.

But the threat actor now appears to be exploiting an issue in Teams that was first identified by Jumpsec researchers in June 2023, but left unpatched by Microsoft at the time, supposedly on the basis that it was not serious enough to fix right away.

This activity began in July – after the Jumpsec disclosure had received some attention – and likely involves the use of a publicly available tool called TeamsPhisher, a Python program that lets Teams tenant users attach files to messages sent to external tenants.

It is no stretch to see how this feature can be abused and this seems to be what Storm-0324 is doing, using it to send phishing lures leading to the malicious SharePoint site. Its lures are identified by the Teams platform as external ones, should external access be enabled, meaning they get through to potential victims quite easily.

Defenders have a number of options to harden their networks against these attacks, as set out by Microsoft.

“Microsoft takes these phishing campaigns very seriously and has rolled out several improvements to better defend against these threats,” the Threat Intel team wrote.

“In accordance with Microsoft policies, we have suspended identified accounts and tenants associated with inauthentic or fraudulent behaviour. We have also rolled out enhancements to the Accept/Block experience in one-on-one chats within Teams, to emphasise the externality of a user and their email address so Teams users can better exercise caution by not interacting with unknown or malicious senders.

“We rolled out new restrictions on the creation of domains within tenants and improved notifications to tenant admins when new domains are created within their tenant. In addition to these specific enhancements, our development teams will continue to introduce additional preventative and detective measures to further protect customers from phishing attacks.”