Validating Eternal Form with Current WP Login Info

I am creating a login that logs members into a WordPress site as low level users, giving them access to a members dashboard.
The dashboard will have a variety of content, links and functions.

One function I am having trouble with however is the contact upload form. This form is created for members to upload their information to a separate virtual dedicated server contact database. (This virtual dedicated server requires the same username and password for each member as the WordPress site does.) For this reason, I need the submit button on this form to validate the username and password of each member by referencing the username and password that they are already logged into WordPress with. (AKA: the form action needs to look at the WordPress username & password to get the username & password it needs to validate access.)

I could very easily just have the people reenter their username & password to access the import form.. but that doesn’t seem ideal.
No one wants to keep reentering their info.
Do you have any suggestions on how I can fix this?
If you did I would be so so grateful. Thank you so much.