I have an Intranet application with Windows Authentication. Everything works fine. However, for some sensitive operations (it could be approving AP check, or grading a student) I need to get user's id and password. The goal is to prevent somebody walking to unattended terminal, and generally for non-repudiation. Trivial with forms authentication - but I can't figure out how to do it with Windows authentication.
I don't think it matters, but it's MVC application and authentication is done against Active Directory
Typically, server application needs to issue HTTP Status Code 401 to tell client for authentication information. However, in case of windows authentication, once user is authenticated, the token is cached by client system or browser and used subsequently whenever needed. So in your case, even if you issue 401, client will send the same token again - so your main problem of abusing unattended terminal will not get solved (as user already logged in there into the system).
One of the way could be simulate forms authentication from the application - i.e. on sensitive operations, prompt user for his/her windows password again (note that you will be already having user's name(identity) if you are using windows authentication) and then re-validate that password using active directory API (or logon user windows API).
Personally, I feel that you are trying to solve the problem at server which is wrong end - I would rather have a IT policy that prohibits unlocked unattended terminals - either user should lock the workstation or have a password protected screen saver. Typically, these things can be somewhat enforced via group policy.