I have developed several web applications with the own login forms/logic etc. I would like to simplify these by creating a single dedicated web app to handle all the login logic using FirebaseUI. The way I imagine this working is when the user needs to login with one of my apps they are redirected to my login web app. The user supplies their credentials and if successful the login app redirects back to the original app with data to confirm whether the login was successful.
If this is possible I shouldn't have to repeat the login logic in my other web applications and any changes to login code are only required in my login app. I'm not sure if such an approach is possible, however, or how I could securely and safely let each web app determine that the user has successfully logged in. Can anyone offer advice on how to implement this or point to examples where this has been done already?
You might be looking for single sign-on.
Google does something similar. Every time you login you're redirected to accounts.google.com, and after that you're redirected to your app.
You're right in saying that it's trickier than it appears on first sight though. Most web login systems are based around cookies and the whole client-server process around them. Cookies are set per domain. If all your webapps are on the same domain or subdomains of a single domain, you should be OK to go with this approach and get the results you want. If not, you're going to need some extra work to get code from different domains to speak with each other and make everything possible.
In general, if designing a login system from scratch is considered a big endevour, designing a single sign-on system is an order of magnitude harder. OWASP had this to say about them in 2011.