I came across a serious issue with Yahoo! Mail that I hope someone would help to fix or raise awareness for.
I wanted to create an account on Yahoo! Mail. I entered my phone number to the Yahoo! login, and it asked me if I wanted to receive an access key. So I did that, and typed in the access key... Surprise, I logged in ACCIDENTALLY to the Yahoo! Mail of the previous owner of my current phone number! Someone had used the same phone number years ago and connected it to his Yahoo! mail, and once he stopped using the phone number it eventually was bought by me, and I used this phone number to gain access to his email! Yahoo! didn't even ask me for a name or email, just access to the phone number, and I was able to access his email. I immediately logged out.
Is this a normal thing? Looks like a very serious privacy issue... and Yahoo! could fix it just by requiring the person to enter the full name or email.