Which Membership Provider implements users stored in web.config?

Having a code-blind moment.

ASP.NET 4.0.

Web.config:

<?xml version="1.0"?>
<configuration>
  <system.web>
    <authentication mode="Forms">
      <forms name="DataViewer" loginUrl="login.aspx">
        <credentials passwordFormat="Clear">
          <user name="devuser" password="test" />
        </credentials>
      </forms>
    </authentication>
    <authorization>
      <deny users="?" />
    </authorization>
  </system.web>

and a login control:

  <asp:Login ID="login" runat="server" />

If I enter a username and password, and click Login, it hangs.

If I break, I can see in the call stack that login.AuthenticateUsingMembershipProvider() is in the middle of calling SqlMembershipProvider.ValidateUser(). There is no database defined or involved in this project at all, and I haven't specified that SqlMembershipProvider should be used.

So my question is, what membership provider should I use to get ASP.NET to use the usernames and passwords in the <credentials> element of web.config?