I had something similar happen with one of my services. I have an "Update your Payment Details" page for paid subscribers that lets them enter new card details

I had something similar happen with one of my services. I have an "Update your Payment Details" page for paid subscribers that lets them enter new card details when their old card expires or they just want to switch the card they're using with us. It normally gets used a few times a month, and anywhere from zero to one time over the lifetime of a customer. But then the bad guys found it. And individual users started updating their card details dozens of times each day. This went on for several days before I noticed it in the logs. It was easy enough to fix: users now get one update per year, unless they email me and ask what's wrong with that card update page. The bad guys moved on to greener pastures and life went back to normal.