How could someone break into my calendar?

Question

iOS

I recently created a separate iPhone calendar, made it public, and sent the link to friends. Everyone has read-only access. Actually, the link is read-only and maybe.
And now strange entries began to appear in the calendar, and for 2016.

Example

Text

www.rboutletshoppings.com $19.99 Ray-ban&Oakley Black Friday In-Store & Online

Moreover, this garbage sits in the calendar and is sent to all participants in the calendar. It is displayed with them (thanks that at least for 2016, and not for current dates, it climbs).
How did the attackers get into my calendar?

Answer 1

I can’t say for sure about the iPhone, but regarding the google calendar, the vulnerability is widely known in narrow circles. Most likely this is one of its adaptations for the IOS platform. In any case, read it, you can also ask the author a question, perhaps together you will come to a conclusion.
https://codeby.net/threads/ujazvimost-rikardo-milo...

Answer 2

So far I have done the following (as recommended on the Internet):
1. Created a new calendar
2. Moved each spam to a new calendar.
That's all.
Although further recommend:
3. Delete the entire calendar WITHOUT notification.
If you delete it just like that, then this is a signal to spammers that the calendar is active, and the infection will continue to climb.
But the question is still open. After all, this is not a solution.