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Still think Outlook is harmless?
Today I released my custom C2 framework called BL4CK0UT that weaponizes Outlook calendar invites (.ics) to deliver covert command and control on low‑privileged, air‑gapped systems. Instead of relying on typical network beacons or HTTP callbacks, bl4ck0ut embeds commands inside calendar invitations and uses Outlook itself as the execution trigger and exfiltration channel.
How It Works
A PowerShell implant is deployed to a target system. Note: Microsoft Outlook must be present on the machine. The operator then sends a calendar invite with a subject formatted like invite "<PAYLOAD>".
When the invite is received, the implant parses the subject, executes the command, and embeds the output back into the invite body as base64 encoded content.
Results may be retrieved by the C2 operator simply by viewing the calendar item, no obvious outbound traffic required.
Why It’s Interesting
BL4CK0UT demonstrates how trusted enterprise tools can be abused as covert communication channels, making detection more difficult in environments where Outlook traffic is assumed to be benign. An external sender can send an arbitrary calendar invite to the target and it gets synced and shown as tentatively accepted in the target calendar, which allows for immediate command registration on the target. Also defenders are constantly looking to prevent exfiltration mechanisms, but using this technique, no exfil traffic is generated. Outlook IS the exfiltration tool.