If you have multiple products/sites under a common domain, what are the advantages and disadvantages, from user convenience to security, of having a common login page? For example, this site uses https://security.stackexchange.com/users/login for login. Google uses accounts.google.com and Microsoft uses login.live.com. However Apple doesn't have a common page, Facebook has the login form on the home page and most banks do too.
From a security point of view, both have their own issues and both have their advantages. Both can be made more or less equally secure.
If you use a login-page per service, you might want to synchronize passwords, using, as an example, LDAP or something like that. Otherwise, users may need different passwords per site/service. On the other hand, if you do not synchronize passwords, you allow users to have different passwords per service.
If you use a single login page for all your services, you need a security token to allow access to different sites. There are a number of frameworks for this, so try not to re-invent the wheel.
In general, when you make a choice, you will consider at least:
- What is the image that you want to give to the users? A single entity or just a bunch of services?
- Are the development groups of the different services sufficiently aligned with your security-token solution?
- Do you want to allow your users to use different passwords per service? Did you think about password-resets?