Until Microsoft releases a patch (this week hopefully), you can change in web.config to provide a single custom error page like:
<customErrors mode="RemoteOnly" defaultRedirect="/Error.aspx"/>
Do not use seperate error page for 404 and 500 which is the default configuration i lemoon 4.
Please see http://stevesmithblog.com/blog/asp-net-custom-errors-security-flaw/ for more information.