as stated in mandubian, the best option is to have a central service that provides authentication tokens that all applications recognize and use to authenticate the user. An example of such a service is CAS , if, as you say, you do not have experience in SSO, I would read about it to understand how this should work.
Just a warning: authentication and authorization are an important part of the application, even more of a web application. I would advise you not to implement your solution, you probably would end up hacking with problems (if applications are not on the intranet, risks exist, but are usually simpler). Try using an existing solution, such as CAS or JOSSO
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