If you have a public web site you might have the ViewFormPagesLockDown feature turned on. This restricts access for anonymous users so they can’t access the pages in your libraries “Forms” folders. There’s a good description of the feature here http://technet.microsoft.com/en-us/library/cc263468.aspx.
If you’re accessing a libraries “Forms” pages, for example by using InfoPath Forms Services, you might need anonymous users to have access to the library.
With the following configuration the user could be prompted for credentials on an anonymous site when trying to access an InfoPath Forms Services form.
- Browser-based InfoPath Form published to a Forms Library
- Site has anonymous access set to “Entire Web Site”
- ViewFormPagesLockDown feature is activated
You can fix this by deactivating the ViewFormPagesLockDown feature
stsadm -o deactivatefeature -url “<site URL>” -filename ViewFormPagesLockDown\feature.xml
Now toggle the anonymous access setting on the site from “Entire Web Site” to “None” and back to “Entire Web Site”.
Then activate the ViewFormPagesLockDown feature again
stsadm -o activatefeature -url “<site URL>” -filename ViewFormPagesLockDown\feature.xml