Sites are on the same host ? If yes you can use basic authentication to the same file for all web's
If your auth is on php with mysql and are not on the same host , try to do a simple script what is returning true or false when you try to auth to him.
For example: when you access
https://www.mysite.com/auth?user=xxx&pass=yyy returning true if is auth succesul or false if is not
Regards