APT41's technique revolves around leveraging Google Calendar’s event invitation and synchronization features to act as covert C2 channels.

Key Technical Components:

Initial Access: Achieved through spear-phishing emails, watering hole attacks, or exploitation of known application vulnerabilities.
Google Calendar Abuse:
- Attackers create calendar events and embed malicious instructions (e.g., encoded shell commands or URLs) within the event description or notes field.
- These calendar events are sent to the victim, whose system, infected with loader malware, is programmed to periodically check the calendar through the Google Calendar API.
Persistence: Regular calendar sync ensures periodic update of commands, maintaining control without traditional C2 infrastructure.

Execution Flow:

The victim’s device makes authorized API requests to calendar.googleapis.com.
Malware decodes the instructions embedded in event fields.
The decoded command is often executed using PowerShell or a script interpreter.
Outputs (data exfil, beaconing) may be encoded and sent via HTTPS or stored in return calendar events.

Evasion Techniques:

Case Studies