How Attackers Are Using Google Calendar to Hide Malicious Activity?
This week, we're exploring an advanced stealth technique used by attackers to blend in seamlessly with trusted cloud services. We’ll unpack the attack chain with key cybersecurity insights!
Post-compromise, every attacker faces two critical challenges:
To remotely send commands to the compromised device.
To stealthily receive data back without tipping off defenders.
But the classic predator’s dilemma comes up:
How to reach your prey without getting noticed?
In the digital world, that’s what C2 communication (command-and-control) is all about.
But how do attackers pull this off in a world of firewalls, network monitoring, and vigilant defenders?
There are many ways to do this but here's one method I recently came across that uses Google calendar to achieve it. Here's how (credits: Google TIG):
🔍 The Attack Flow: A Step-by-Step Look
Attacker sends a phishing email containing a ZIP file hosted on a previously hacked government website > Victim extracts it.
The archive contains: A Windows LNK file disguised as a PDF.
Victim opens the PDF file > Victim is shown a legitimate decoy PDF to avoid suspicion> But silently launches a DLL file that kicks off the malware chain.
Once the malware is executed, it creates a Google Calendar event on a hardcoded date > Attacker polls this date and if event is created, they know that system is compromised.
Post that, attacker places encrypted commands in Calendar events on predetermined dates hard-coded into the malware.
Malware then begins polling Calendar for these events > Retrieves the event > Decrypts the event description > Executes the command it contains on the compromised host.
Results from the command execution are encrypted and written back to another Calendar event > Attacker can poll and retrieve the results.
💡Key Insights & Lessons
In nature, some predators are deadlier because of their ability to camouflage and to blend seamlessly with their environment. This stealth lets them get dangerously close to their prey. The same principle applies to modern cyber attackers. They understand that blending in, by hiding in plain sight, is the key to evading detection. Legitimate traffic is the perfect cover.
At the heart of every attack is the need for C2 communication. In the past, attackers relied on:
Dedicated servers with hardcoded IP addresses.
Obscure protocols like IRC or FTP.
Unencrypted traffic that was easy to sniff.
Direct TCP/UDP channels on unusual ports.
But these traditional C2 techniques left footprints: static IPs, weird traffic patterns, and suspicious-looking servers that defenders learned to spot and block.In today’s landscape, attackers are adapting with clever new strategies: Hijacking legitimate services (like Google Drive, Slack, Dropbox, or in this case, Google Calendar) to hide in normal traffic. Encryption everywhere—HTTPS, TLS, and custom crypto makes inspection harder. This Google Calendar technique is part of that broader shift: using trusted cloud services as shields.
This attack is more than just a clever use of Google Calendar. It highlights a fundamental shift in the attacker’s playbook:
Legitimacy as Cover: Attackers know defenders often trust traffic to major cloud providers—Google, Microsoft, Amazon. They’re betting on this trust to slip under the radar.
The Versatility of Polling: With polling, the attacker doesn’t need an inbound connection (which defenders often block), just outbound requests to a legitimate service. This can be done with any tool that supports polling.
The Challenge of Detection: Detecting this kind of stealthy C2 is incredibly hard. Traffic to Google services looks normal. Encrypted connections hide the content.
Event descriptions in a Calendar? Most tools don’t even parse them.
LNK files are Windows shortcut files that can launch applications or commands. They’re commonly used for legitimate shortcuts—like a desktop icon that opens Word. But attackers weaponize them by embedding commands or file paths that execute malicious code.
In this specific case, Google as the platform owner was able to detect the campaign by crafting custom fingerprints (for APT41 actor) to identify and neutralize malicious Calendars and associated Workspace projects. But identifying these scenarios normally by traffic inspection is extremely tough. You mostly identify these in an indirect way. For ex, in this case, the initial phishing link that was hosted on government infra unraveled the campaign.
💭 Final Thoughts: Adapt or Be Left Behind
This incident is a reminder of how attackers adapt to our defenses. Attackers are always probing for ways to blend in, to use our trusted tools against us.
For defenders, it’s a call to action:
Expand your visibility beyond the traditional perimeter.
Challenge assumptions about what’s “safe” or “normal.”
Stay curious—because the threats are constantly evolving.