HollowGraph Malware Hides C2 and Stolen Files in Microsoft 365 Events Dated 2050
Jul 20, 2026
Cyber Espionage / SaaS Security
A newly discovered espionage implant has been using a hijacked Microsoft 365 calendar as its command channel, planting operator instructions and smuggling out stolen files as attachments on calendar events dated to the year 2050. Group-IB , which named the malware HollowGraph , says the approach moves tasking and stolen data through legitimate Microsoft Graph API traffic, so the activity looks like ordinary Microsoft 365 chatter and network controls keyed to attacker-owned destinations have nothing to flag. The implant is a .NET DLL that supports just two commands, get and send , and it never reaches out to an attacker-owned server for payloads. Instead, it treats the compromised mailbox's calendar as a two-way dead drop. To pull tasking, it queries the calendar for an event the operator has planted, buried at 2050-05-13 , so the mailbox owner is unlikely to ever scroll to it, and reads its instructions from an attached file. To exfiltrate, it does the reverse: encrypts th...