IIS 7.5 Missing Authorization Header
Solution 1:
Although IIS 7 supports ISAPI filters, Microsoft recommends that you extend Web server functionality by using modules instead of by using ISAPI filters.
Is there any reason why you have chosen to develop something on IIS 6 and now moving it to IIS 7.5?
Also have you followed the correct procedure of installing the ISAPI filter on IIS 7.5, as that differs from IIS 6. Perhaos you can share some more details on your ISAPI filter and it's configuration?