GCR – Google Calendar RAT

Google Calendar RAT is a PoC of Command&Control (C2) over Google Calendar Events, This tool has been developed for those circumstances where it is difficult to create an entire red teaming infrastructure.

To use GRC, only a Gmail account is required. The script creates a ‘Covert Channel’ by exploiting the event descriptions in Google Calendar. The target will connect directly to Google.”

It could be considered as a layer 7 application Covert Channel (but some friends would say it cannot be 🙂 very thanks to my mates “Tortellini” 

POC

How It Works

GCR attempt to connect to a valid shared Google Calendar link and after generating a unique ID check for any yet-to-be-executed commands.

If it is not able to find any command, it creates a new one (fixed to “whoami”) as a proof of connection. Every event is composed by two part:

1. The Title, which contains the unique ID, it means you can schedule multiple commands creating events having the same unique ID as name

3. The Description, which contains the command to execute and the base64 encoded output using the pipe symbol as separator “|”

Workflow Attack

What A SOC Analyst/Blue Teamer Will See?

Focusing specifically on the network aspect, the only connections established will be to Google’s servers, making the connection appear completely legitimate. Let’s check with process hacker:

which results in this

How To Use It

• Setup a Google service account and obtain the credentials.json file, place the file in the same directory of the script
• Create a new Google calendar and share it with the new created service account
• Edit the script to point your calendar address
• Once executed on the target machine an event with a unique target ID is automatically created autoexecuting the “whoami” command
• Use the following syntax in the event description for the communication => CLEAR_COMMAND|BASE64_OUTPUTExamples:
  • “whoami|”
  • “net users|”
• The date is fixed on May 30th, 2023. You can create unlimited events using the unique ID as the event name.