How I almost fell for a Microsoft 365 Calendar invite scam

How I almost fell for a Microsoft 365 Calendar invite scam

As a tech journalist, I've seen my fair share of phishing scams, but one recent incident had me questioning the trustworthiness of even the most familiar technology. A reader shared their story with me: Paul from Cape Coral, Florida, received a series of suspicious calendar invites that almost had him hooked.

"I had a very disturbing experience with a phishing attempt that almost had me hooked," Paul wrote. "I'm a Microsoft 365 subscriber and recently got the usual renewal emails. But a few days later, I started getting meeting invites saying my payment failed — they showed up directly on my calendar, even though I never opened or clicked anything."

Paul's experience highlights how easily this type of scam can slip through our defenses. So, let's dive into the details and explore what makes these attacks so effective.

How the Microsoft 365 Calendar invite scam works

This type of phishing combines fake calendar events, Microsoft branding, and social engineering tactics to trick users into handing over personal information or clicking on malicious content. Here's how it starts:

  1. The message appears to be from Microsoft 365, warning that your subscription renewal has either failed or been renewed.
  2. Some versions include an .htm attachment designed to look like a billing portal that captures credit card details.
  3. The calendar invite adds pressure: Many of these scams include a calendar file (.ics) that places the event directly on your calendar. If your Microsoft 365 or Outlook settings automatically accept invites, the event appears without you doing anything.

Even if you never click a link, just seeing the event may prompt panic or confusion. Deleting can confirm your identity: If your only option is "Delete and Decline," that sends a response back to the sender, which makes you a bigger target.

Why Microsoft 365 phishing invites bypass email filters

This tactic is effective because it exploits a loophole in how Microsoft 365 processes calendar invitations. Even if a phishing email is flagged or blocked, the calendar event associated with it can still appear on your calendar. Here's how:

  1. It bypasses traditional email filters: Tools like Microsoft Defender scan incoming messages for bad links and attachments, but in this case, the attacker sends a malicious calendar invite that gets processed by Microsoft's backend calendar services.
  2. You don't have to click or open anything: If your settings allow calendar invites to be added automatically, that fake billing alert can show up instantly, making it feel urgent and legitimate, especially when it looks like it's from Microsoft.

What to do if you get a phishing calendar invite in Microsoft 365

If a suspicious calendar event shows up and you didn’t accept it yourself, do not interact with it. Don't click links, don't download attachments, and don't decline the invite; even that response can confirm your account is active. Here's what to do next:

  1. Don't click or decline the phishing calendar invite
  2. How to delete a phishing calendar event without alerting the attacker: New Outlook (desktop or web) users may find it tricky to handle suspicious invites, but there are ways to get rid of them. Option 2: Use "Ignore" from the inbox - This will move the email to your Trash without sending any response or showing RSVP tracking.
  3. Change Outlook settings to block calendar spam and phishing invites
  4. How to report a phishing calendar invite without alerting the sender
  5. Check your Microsoft account for signs of phishing or hacking and install strong antivirus software
  6. Monitor your identity after a phishing attempt
  7. Remove your personal info from data broker sites to avoid future scams

If you're concerned about the security of your Microsoft 365 account, it's essential to report suspicious calendar events and double-check your account settings. Until Microsoft adds stronger controls, calendar scams will continue to sneak through, but a few careful steps can keep you protected.

What responsibility does Microsoft have to protect users from security flaws in its own ecosystem?

This is an excellent question. As users of Microsoft 365, we rely on the company to maintain our accounts and prevent phishing attacks. However, the lack of stronger controls in place means that these scams can continue to evade detection.

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