Hackers Are Using Google Calendar for Command and Control: A New Threat Emerges
Cybercriminals are constantly innovating, finding new ways to infiltrate networks and evade detection. A significant evolution in their tactics involves “Living off the Land” (LotL), a technique where attackers use legitimate, trusted software and services to carry out their malicious activities. A powerful new example of this has emerged: a framework that uses Google Calendar as a covert Command and Control (C2) channel.
This method moves away from traditional C2 servers, which can be identified and blocked, to a serverless model that hides in plain sight. By leveraging the universal trust and ubiquity of Google’s infrastructure, attackers can maintain persistent access to a compromised system with a significantly lower chance of being discovered.
Understanding Command and Control (C2)
Before diving into the specifics, it’s essential to understand what a C2 server does. In a typical cyberattack, after a device is infected with malware (the “implant”), it needs to communicate with the attacker. This communication line is the Command and Control channel.
The implant “calls home” to the C2 server to receive instructions—such as “steal files,” “download more malware,” or “encrypt data for ransom”—and to send stolen information back to the attacker. Security teams often focus on detecting and blocking this C2 traffic to neutralize a threat.
How Attackers Are Abusing Google Calendar
This new technique ingeniously repurposes Google Calendar events to facilitate two-way communication between the attacker and the compromised machine. The process is both simple and highly effective.
Here’s a breakdown of how it works:
Initial Compromise: The attack begins with a standard breach, where a malicious implant is installed on a target machine. This implant is configured with credentials for a Google Service Account, giving it API access.
Issuing Commands: The attacker, operating from anywhere in the world, creates a new event on a Google Calendar that is shared with the service account. The malicious command (e.g.,
whoami,dir C:\Users\) is not placed in the event title but is hidden within the event’s description field. To avoid detection and special character issues, the command is typically encoded in Base64.Command Execution: The implant on the compromised machine is programmed to periodically poll the Google Calendar via the official Google Calendar API. It scans for new events. When it finds one, it reads the encoded command from the description.
Exfiltrating Data: After decoding and executing the command, the implant takes the output, encodes it in Base64, and writes it back into the same event’s description field, overwriting the original command.
Retrieving Results: The attacker simply checks the event description to see the output of their command, confirming successful execution and retrieving any stolen data. The event is then deleted to clean up the trail.
Why This C2 Method is So Dangerous
The use of a trusted service like Google Calendar as a C2 channel presents a formidable challenge for cybersecurity professionals.
- Extreme Stealth: Network traffic generated by this method consists of standard, encrypted API calls to
calendar.google.com. This activity is indistinguishable from legitimate Google Calendar usage by employees. Traditional firewalls and intrusion detection systems are unlikely to flag this traffic as malicious. - High Resilience: There is no malicious server IP address or domain for security teams to block. Blocking access to Google Calendar is not a viable option for nearly any organization, as it’s a critical business tool.
- Abuse of Trust: The entire framework operates on Google’s robust and trusted infrastructure. This allows the attacker to bypass reputation-based security filters that would normally block connections to newly created or known-malicious domains.
How to Defend Against Abused Service Attacks
Protecting your organization from such sophisticated threats requires a shift from traditional perimeter defense to a more behavior-focused and proactive security posture.
- Monitor API Usage: Closely monitor logs for anomalous Google Workspace API traffic. Look for unusual patterns, such as a service account frequently creating and deleting a high volume of single-attendee calendar events or making oddly timed API calls.
- Audit Service Accounts: Regularly review the permissions granted to all service accounts. Apply the principle of least privilege, ensuring that accounts only have the absolute minimum access required for their legitimate functions. A service account with overly broad permissions is a significant security risk.
- Employ Endpoint Detection and Response (EDR): Since network detection is difficult, endpoint security is critical. An EDR solution can detect the initial implant and monitor for suspicious process behavior, such as a non-browser application making frequent API calls to Google.
- Implement Egress Traffic Filtering: While blocking Google entirely is impractical, consider restricting which applications and services are allowed to make outbound connections. If a server has no legitimate reason to access Google Calendar, its access should be blocked by default.
Ultimately, this technique is a stark reminder that attackers will always seek to weaponize the tools we trust most. As they continue to innovate, our defensive strategies must evolve to focus on identifying anomalous behavior, enforcing strict access controls, and maintaining deep visibility into both network and endpoint activity.
Source: https://securityaffairs.com/181940/security/meetc2-a-serverless-c2-framework-that-leverages-google-calendar-apis-as-a-communication-channel.html
