If there’s one thing we at OSIbeyond consistently warn our clients about, it’s that cybercriminals never stop innovating. One loophole that’s been quietly affecting businesses across the Washington DC area and beyond involves something so routine, so trusted, that many organizations don’t even consider it a security risk: calendar invites.
How Calendar Invite Attacks Work
Calendar invite attacks are a sophisticated form of cyberattacks where criminals weaponize legitimate calendar and meeting systems to deliver malware, steal credentials, or gain unauthorized access to your network. They exploit the inherent trust and automated processing of calendar invitations, which is why they often appear directly in victims’ calendars without any interaction required.
Cybercriminals have developed several distinct methods to execute these attacks, and each exploits different weaknesses in how modern businesses handle digital scheduling and collaboration:
- Direct calendar invites: Attackers send malicious .ics files that bypass email filters because security tools classify them as “safe” file types. Gmail and many other calendar applications automatically add external invites as “tentative” appointments without user approval. In some cases, the calendar event still appears in users’ calendars even when the originating email gets quarantined by security filters.
- Microsoft Teams meeting invites: Threat actors operate their own Microsoft Office 365 service tenants and take advantage of a default Microsoft Teams configuration that permits users on external domains to initiate chats or meetings with internal users. Once victims join the fake meetings, attackers share malicious files through chat, display phishing sites via screen sharing, or guide users to install remote access tools.
- Email-calendar hybrid attacks: Some of the most sophisticated campaigns combine multiple vectors. For example, the Storm-1811 group flooded targets with thousands of spam emails, then followed up with a Teams call claiming to help with the “spam problem.” During the call, they guided victims to install remote access tools. Such multi-stage attacks exploit the trust users have in IT support while abusing calendar invites as just one piece of a larger social engineering puzzle.
- Platform-specific exploits: Each calendar platform has unique vulnerabilities. For example, Apple’s iCloud calendar can be abused using Apple’s own [email protected] domain, making phishing invites appear completely legitimate. Google Calendar invites can embed links to Google Forms or Drawings that lead to credential harvesting pages. Microsoft’s auto-accept features for external calendar invites create additional attack surfaces that don’t exist in other platforms.
The sudden shift to remote work in 2020 created a perfect storm of conditions for calendar invite attacks to flourish. Organizations that once relied on in-person meetings were forced to adopt digital scheduling tools overnight, often without adequate security training or protective measures in place. Employees who previously verified meeting requests by walking down the hall to a colleague’s office suddenly found themselves accepting virtual meeting invites on a daily basis.
Defending Against Calendar Attacks
Protecting your organization from calendar-based attacks requires a multi-layered approach that combines technical controls, user education, and process improvements.
Configure Platform-Specific Security Settings
Each major calendar platform offers security configurations that many organizations never touch.
For example, in Microsoft 365, administrators should navigate to the Teams admin center and modify the external access settings to restrict which domains can initiate meetings with your users. Consider implementing a whitelist approach where only pre-approved partner organizations can send meeting invites.
Implement Calendar-Aware Security Tools
When evaluating security tools, specifically ask vendors about their ability to scan calendar attachments, inspect embedded URLs within meeting descriptions, and detect anomalous meeting patterns like invites from external domains mimicking internal naming conventions. For example, Microsoft Defender for Office 365 can analyze Teams meeting invites for suspicious patterns thanks to its deep integration with the Microsoft ecosystem.
Educate Users on Calendar Hygiene
Technical controls alone won’t stop determined attackers who craft convincing social engineering campaigns. Regular security awareness training should specifically address calendar threats, teaching employees to recognize red flags like generic meeting titles (“Urgent: Action Required”), requests to join meetings on unfamiliar platforms, or calendar invites that bypass normal scheduling protocols.
Show real examples of malicious invites your industry has encountered, and conduct simulated calendar phishing exercises to identify users who need additional training. Emphasize that legitimate IT support will never ask users to install remote access tools through a calendar invite or Teams meeting.
Establish Meeting Verification Protocols
Create clear procedures for verifying unexpected or suspicious meeting requests, especially if they’re claiming urgency or come from executives. Implement a “trust but verify” policy where employees are encouraged to confirm meeting requests through a secondary channel (a quick Teams message or phone call) before joining meetings from unknown organizers.
For high-stakes meetings involving financial transactions, system access, or sensitive data sharing, require verbal confirmation codes that meeting organizers must share through a separate communication channel. This might seem like overkill, but it’s far less disruptive than a ransomware incident.
Conclusion
Calendar invite attacks represent a clear and present danger to businesses of all sizes, but they’re far from unstoppable. By understanding how these attacks work and implementing the defensive strategies we’ve outlined, you can close this security gap before criminals exploit it.
Whether you need help protecting against calendar-based threats or addressing any other cybersecurity challenges your organization faces, OSIbeyond has the expertise and tools to keep your business secure. Schedule a consultation with us today to learn how we can fortify your defenses and make sure technology works for you, not against you.