Udemy: Able to view others' gifts on /gift/share URL, giftId is predictable, and easy to manipulate

I recently purchased a gift for a friend, and noticed the share URL gift ID was simply numeric. I managed to access other people's gifts simply by incrementing and decrementing the ID by 2, I was able to verify that the price was dropped to "Free", regardless of if I was logged in or not, and I w...