Okay, today I ran into this problem. Microsoft Outlook Web Access (OWA) prompted twice for username and password when trying to open a word or excel document. Apparently, I am not the only one that is having this problem. I did some researching, it turns out that this is a known problem from Microsoft. It all point to how the “Office Protocol Discovery” works. For other attachments (e.g. texts, jpgs, and pdfs), all of them work fine.

Here are some tests I did with different OS, browser, and office

Working:

• OS: XP; Browser: IE 6; Office: 2003
• OS: ALL; Browser: Firefox; Office: 2003,2007

Not Working:

• OS: XP; Browser: IE 6; Office: 2007
• OS: XP; Browser: IE 7; Office: 2003
• OS: Vista; Browser: IE 7; Office: 2007

Some solutions:

1. Enable Forms Based Authentication — this uses cookies instead of browser session
*** I just did a test today, it turns out that this doesn’t work with Microsoft Vista, IE 7 or 8 and Office 2007

2. Check the ‘remember my password’ option
3. Just cancel the username and password prompt

I think enabling the ‘forms based authentication’ is a better option. This provides better security.