The Group-IB Threat Intelligence team has identified HOLLOWGRAPH, a new malware sample that we attribute, with high confidence, to the Cavern backdoor framework. This malware is one component of a larger toolkit, and it uses the Microsoft Graph API through a compromised Microsoft 365 account observed in Israel to communicate with its operators — a technique that helps conceal command-and-control traffic within legitimate Microsoft 365 communications.
The malware supports just two commands, get and send, and executes both exclusively through trusted Microsoft cloud infrastructure. Using the Microsoft Graph API, it treats the compromised mailbox’s calendar as a two-way dead-drop: operators plant tasking as calendar events, and the implant exfiltrates stolen files by creating its own events with encrypted data attached. To avoid catching the mailbox owner’s attention, every event is dated far into the future — 13 May 2050 — with payloads attached as files to the event.
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Source: Group-IB
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