When a Calendar Invite Becomes Chaos: Crashing Apps Like It’s 1999

I've been a daily user of this platform for years. Privacy-first, encrypted, trusted by millions of users and business teams. You know the type. The kind of service you recommend to friends when they ask "what should I use instead of Google?"

And here's the thing about being a regular user who also happens to break things for a living: you start to see the app differently. You know the flows. You know what gets validated where. You know which features feel bolted on versus battle-tested.

So when I started poking at the calendar functionality, I wasn't looking for anything fancy. No zero-days, no memory corruption, no fancy exploits. Just… what happens if I send a weird invite?

The Old School Technique That Refuses to Die 📟

Remember SMS bombing? Zip bombs? Fork bombs? That golden era when hackers crashed systems not with sophisticated exploits but with "what if I just… send a lot of this?"

Turns out that energy is alive and well in 2026.

The attack is embarrassingly simple in concept: send a calendar invite with a recurrence rule so dense that the app chokes trying to render it.

Calendar apps use something called RRULE to define recurring events. Daily standup? FREQ=DAILY. Weekly sync? FREQ=WEEKLY. Standard stuff.

But RRULE also supports selectors. You can say "repeat daily, but specifically at these hours, these minutes, these seconds." And if you specify every hour (0 through 23), every minute (0 through 59), and every second (0 through 59)…

That's 24 × 60 × 60 = 86,400 instances. Per day. Forever.

What Happens on the Device 💀

The app receives the invite. Looks normal enough. Meetings happen, right?

But when you open your calendar to that month, the rendering engine tries to expand the recurrence rule into actual drawable events. It starts allocating memory for each instance. 86,400 per day. Across your visible range. Maybe a few weeks.

The heap fills up. Garbage collection kicks in. Doesn't help. More allocations. GC again. Still not enough.

Then:

OutOfMemoryError: Failed to allocate ... <1% of heap free after GC
Process has died: fg TOP

OutOfMemoryError: Failed to allocate ... <1% of heap free after GC
Process has died: fg TOP

App crashes. You reopen it. It syncs the same event. Tries to render. Crashes again.

And here's the fun part: the event lives server-side. Reinstalling doesn't help. Logging out and back in doesn't help. The poison is in your account now.

Zero-Click? Yeah, About That 🎯

Most calendar apps auto-import invites by default. The event shows up before you even open the email. Before you accept. Before you do anything.

So the attack becomes:

Attacker sends crafted invite from any email (Gmail works fine)
Victim's calendar auto-imports it
Victim opens their calendar (you know, like a normal person)
Boom

No phishing. No "click here". No social engineering. Just… existing as a calendar user.

And because the recurrence has no end date, it affects every future view too. The victim literally cannot access their calendar anymore. Not on mobile. Not on desktop. Not on web.

Cross-platform denial of service from a single email.

The Sanitizer Gap 🕳️

The app actually has validation for dense recurrence rules. It checks for stuff like BYHOUR, BYMINUTE, BYSECOND and rejects them.

But only on certain code paths.

When you import a .ics file directly? Validated. When an invite arrives via email and you open it from within the mail app? Skipped.

Classic "we secured the front door but left the garage open" situation.

The rendering engine also has no upper bound on how many occurrences it will generate. It just keeps going until it hits the end of your visible date range. Or runs out of memory. Whichever comes first.

The Nostalgia Hit 📼

There's something poetic about this. We have end-to-end encryption. Zero-knowledge architecture. Hardened clients. Security audits.

And it all falls over because of a calendar invite that says "repeat every second of every day forever."

The same energy as filling someone's pager with 9999999999. Or faxing a black page on loop. Or SYN flooding before anyone knew what a firewall was.

Some techniques just refuse to die. And sometimes the biggest platforms are still vulnerable to the oldest tricks.

Responsible Disclosure Note 📝

This was reported through the platform's bug bounty program with full technical details, proof of concept, and suggested fixes and was paid $XXXX. My test accounts were left in an irrecoverable state (calendar completely inaccessible) which honestly just proves the point and were later recovered by the team on the server side.

If you're using a privacy-focused calendar and you receive weird invites from strangers… maybe don't open your calendar just yet.

Stay paranoid. Stay curious. And never underestimate the power of a well-crafted RRULE. ✌️