HollowGraph Malware Abuses Microsoft 365 Calendars to Conceal Stolen Files and C2 Traffic
A newly discovered espionage malware called HollowGraph turns compromised Microsoft 365 calendars into hidden channels for receiving commands and exfiltrating stolen data. The implant schedules events for 13 May 2050, a distant future date chosen to reduce the likelihood of discovery by account owners. Developed as a Windows DLL, HollowGraph leverages the Microsoft Graph API to blend its traffic with legitimate corporate service activity, eliminating the need to contact attacker-controlled servers directly. It supports two primary commands: “get” to retrieve instructions attached to operator-created appointments and “send” to encrypt collected data with RSA and AES-256-GCM before uploading it as calendar attachments. Separate encryption keys are used for inbound commands and outbound data, while stolen files are written to a disguised logAzure.txt file. Researchers have identified at least 12 compromised systems, three of which remained active, with operations observed between 3 June and 9 July 2026 and primarily targeting Israeli entities.
BoletimSec•Malware & Botnets
