ResetPassword fails - UPDATE permission denied on AspNetUsers

My application (using ASP.NET Identity) works when I run it within VS Express 2013 for Web. Now I am trying to get it to work on the production server.
In my code I reset a user's password like this:
var manager = Context.GetOwinContext().GetUserManager<ApplicationUserManager>(); string resetToken = manager.GeneratePasswordResetToken(user.Id); manager.ResetPassword(user.Id, resetToken, password); The call to ResetPassword fails with "The UPDATE permission was denied on the object 'AspNetUsers'".
I connect to the database with Integrated Security=SSPI. The site has its own application pool, but its identity is ApplicationPoolIdentity. I suppose that all I need to do is grant the necessary SQL permissions to the ApplicationPoolIdentity user. Is it documented anywhere how I should do that? I don't mean "GRANT UPDATE ...". I mean which user needs which permissions so that operati ...