This question might sound kind of naive, but why isn't available the source code for the WebMatrix.Security assembly?

The WebMatrix templates use it to provide OAuth features, together with the class WebMatrix.WebData.WebSecurity.

That assembly has been renamed to Microsoft.Web.WebPages.OAuth. The source is available here: http://aspnetwebstack.codeplex.com/SourceControl/changeset/view/c91461f723f4#src%2fMicrosoft.Web.WebPages.OAuth%2fOAuthWebSecurity.cs

Wow! You totally made my day.
Now it all makes sense.

Thank you so very much.

@marcind What about the SimpleMembershipProvider and all related classes, e.g. ExtendedMembershipProvider, DatabaseConnectionInfo, etc? 

Found them! They're now in Webmatrix.Webdata

That's actually where they've been since v1 of ASP.NET Web Pages. :) Glad you found them. Let us know if you have any other questions.

Is Microsoft.AspNet.WebPages.OAuth the updated Nuget package that replaces the WebMatrix.Security?

Yup!