We have two sites, app.company.com and www.company.com. Right now, users are trained only to enter their login credentials on app.company.com. Both sites use SSL. The app.company.com uses CSRF and we actively scan for security vulnerabilities. www.company.com, hosted at a separate hosting company, is not treated with the same level of care because there's no customer data on it.
Now, we'd like to be able to present a login form on www.company.com which submits to app.company.com, so the login form can be more easily skinned; colours changed, graphics inserted, that sort of thing. But we'd like to do this securely. I can't immediately see a secure way of doing so.
The closest I can think of is to have an IFRAME, but even that appears to be vulnerable to clickjacking. Additionally, if www.company.com was compromised, there'd be nothing stopping them from just replacing the IFRAME with a regular form and capturing the form contents.
So, my question is this. Is there a way to allow users to submit their username and password from an unsecure site without compromising security?