In order to grant authenticated FBA users access to a custom application page (/_layouts page), please follow these steps:
- The Page should inherit from LayoutsPageBase (Making it inherit from UnsecuredLayoutsPageBase will grant anonymous access to this page).
- Override the Properties RightsRequired and RequireDefaultLayoutsRights
protected override SPBasePermissions RightsRequired
{
get
{
SPBasePermissions permissionRequired = SPBasePermissions.ViewPages;
return permissionRequired;
}
}
protected override bool RequireDefaultLayoutsRights
{
get
{
return false;
}
}
This will grant authenticated FBA users access to this _Layouts page.
Emad's Blog 