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Malware Hides Commands in Future Calendar Events

Guavy AI Editorial Team Published August 17, 2026

Instruments

MSFT

A sophisticated malware called HOLLOWGRAPH has been discovered using compromised Microsoft 365 mailboxes as command and control channels against Israeli organizations. The malware, identified by Group-IB's Threat Intelligence team, uses a unique tactic of hiding its instructions in calendar events dated May 13, 2050, which are unlikely to be noticed by users or retention rules.

HOLLOWGRAPH communicates with Microsoft Graph using OAuth2 credentials of an app registered inside the victim tenant on Entra ID. It combines 2048-bit RSA-OAEP with AES-256-GCM cryptography and uses separate RSA key pairs for inbound and outbound streams, making it difficult to intercept or decrypt.

The malware's choice of date has operational value, as no user typically scrolls their calendar view all the way to 2050. This makes it a stable dead-drop spot that even attentive users may miss. Additionally, the DNS channel used by HOLLOWGRAPH renews credentials when they are rotated, making manual rotation ineffective.

Malware Hides Commands in Future Calendar Events

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