Google Calendar Service Poses Potential Malware Threat

Google has issued a warning regarding potential exploitation of its Google Calendar service for covertly sending commands to malware on a computer. This threat involves the “command and control” infrastructure that hackers use to communicate with their malware after infecting an IT system. Hackers often send commands to their malware via a “C2” server, but some criminals hide their C2 activity by using legitimate services to host commands for the malware. In the past, this has involved using cheap or free cloud services like Dropbox, Amazon Web Services, Google Drive, and Gmail. Such tactics make it challenging for antivirus programs and cybersecurity experts to detect the hacker’s activities, as the C2 commands appear as legitimate traffic.

Google is now warning that its own calendar service could potentially be exploited for this purpose. In a report on future cybersecurity threats, the company mentions a cybersecurity researcher known as “MrSaighnal,” who demonstrated a proof-of-concept technique that leverages Google Calendar as a command-and-control system.

This proof-of-concept, called Google Calendar RAT (GCR), involves placing C2 commands in the event description of a Google Calendar entry. The hacker’s malware can then periodically connect to the Google Calendar account to retrieve and execute these commands on the infected computer. According to the developer, GCR communicates solely through legitimate infrastructure operated by Google, making it difficult for defenders to identify suspicious activity, as mentioned in Google’s report.

Fortunately, Google has not observed any instances of hackers using Google Calendar to host C2 commands yet. However, the report underscores that multiple actors have shared this proof of concept on underground forums, indicating an ongoing interest in abusing cloud services.

In response to this potential threat, Google’s report provides some mitigations but acknowledges that there is no straightforward solution. Instead, the company encourages organizations to monitor their networks for unusual activity and develop baselines for network traffic to help cybersecurity professionals identify anomalous behavior.