I'm building a web application that connects to other web services (using strictly anonymous binding, so no user passwords are being used). However the web application maintains its own users itself, and is required to ask certain details such as e-mail addresses and public linking information to these other web services (for example, a username but not a password).

I want to deter or prevent users from reusing passwords in my application that they have also used in the applications I'm linking to. For example, if I ask for their e-mail and provide me with their gmail address, I don't want them using their gmail password for my system. Another example would be reusing a password to a linked system in which they also gave me their username.

One idea I had was to simply try using the information they gave me, along with the password they are trying to store and log in to these external web applications to test the password - then immediately unbind if I was successful and ask the user to use a different password. However I suspect there is a host of morale and legal issues there.

The reason this is a big deal to me is accountability. My application is simply not funded enough to invest properly in security around user passwords. A salted, hashed password in a public SQL-like database is as secure as it gets. So if passwords and linked usernames or e-mails get out, I don't want my userbase compromised.