If you come across this screen, I have a few steps that will help you out of it.
The screen you are seeing is a virus. It gets you to confirm your email address, and sends a false link to your email account and hacks into your computer, email, Tumblr page, everything. Not to worry though, there is a way to get rid of it!
STEP ONE: LOGOUT/SWIPE ALL HISTORY.
Seeing as the virus will not let you log out, you can only see the ” Security Warning ” screen. What you need to do is delete all of your internet browser’s history so you are forced to log back in again…
When you refresh your page after you get rid of all of your internet history, you will come back to the normal Tumblr page but this doesn’t mean your mission is over. What you need to do is go up to the regular log in page, and instead of logging in you must click the “Forgot your password?” option under log in input boxes and follow the steps from there.
STEP THREE: CHANGE YOUR TUMBLR AND EMAIL PASSWORDS.
It’s very important that you change both your Tumblr and Email passwords, because the virus has successfully hacked into both and the only way to get rid of it is to change the password.
After all of that, you’re home free. No worries! I hope this helps some of you for future reference, and I hope that I can help any one who had the same problem as I did! :)