Cybercriminals have discovered a troubling new method to control malware on infected devices – by abusing Google Calendar’s event description functionality. This technique allows hackers to hide in plain sight, making detection extremely difficult.
Proof-of-Concept Code Circulating Online
According to a recent warning published by Google, proof-of-concept code called “Google Calendar RAT” (GCR) is currently circulating on the dark web and hacker forums. It was first published to GitHub in June 2023.
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GCR exploits the calendar service to create a covert command & control channel (C2) right under the nose of security tools.
How the Exploit Works
Once a device is infected with malware that uses GCR, it will periodically check calendar event descriptions for commands from the attacker. The malware is able to parse these text fields for instructions to execute on the victim’s machine.
After running these commands, the malware will then update the same event description with the output or results.
“The target will connect directly to Google Calendar, so there will be no evidence of DNS requests to a malicious domain or IP address,” explained the creator with the alias MrSaighnal. This means that typical threat detection based on tracking DNS and network connections is ineffective.
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The infected calendar also belongs to the target user rather than the attacker. Combined with the use of HTTPS encryption, this makes GCR nearly impossible to catch through network monitoring alone.
Real-World Attacks Not Seen Yet
So far, Google has not observed any real-world attacks actively exploiting this technique. However, history shows that inventive tactics like this often proliferate rapidly across the criminal underground once they are publicly disclosed.
Increased abuse of legitimate apps and services for delivering malware is an escalating trend in recent years. For example, some threat actors have weaponized Google Docs by sending notifications of shared documents containing malicious links or attachments.
How to Detect Google Calendar Malware C2
While calendar-based command and control remains confined to proof-of-concepts for now, businesses should be vigilant about monitoring Google Calendar activity for telltale signs of abuse. Unexpected changes to events or seemingly random text may indicate compromise.
Advanced endpoint detection solutions that include behavior analysis and malware scanning can also potentially detect known malware strains abusing GCR. However, keeping systems patched and training staff to avoid suspicious links and files provides the best defense against infection in the first place.
