Is it possible to create a username/password login with Xojo? I’m sure it is, but how secure is it? Can I host my own application on my own server?
answer is YES for all your question.
Please search the forum for discussions on the proper way to handle passwords. It was discussed in several threads. There are also several threads on hosting your web app, reverse proxies, etc.
I would suggest to search using google in this manner: site: forum.xojo.com term1 term2 term3
It’s only as secure as you make it. Make sure you’re using an SSL certificate to start.