Umbraco: Delivery API leaks protected (Public Access) content through Content Picker / Multi-Node Tree Picker expansion

Umbraco is an ASP.NET CMS. Prior to 13.15.1, 17.5.3, and 18.0.2, the Content Delivery API applies member and Public Access checks to the directly requested node but not to referenced nodes serialized through Content Picker or Multi-Node Tree Picker properties, including pickers nested in Block List, Block Grid, or Rich Text Editor blocks. When DeliveryApi:PublicAccess is enabled, an anonymous caller can retrieve a protected node's name, route, and id through an unprotected referencing node and use ?expand to retrieve full property values. When the Delivery API is instead gated by the organization-wide API key, a key holder can still bypass per-node Public Access through the same expansion path. The same RequestContextOutputExpansionStrategyV2 and ElementOnlyOutputExpansionStrategy path also bypasses allowed or disallowed content-type alias restrictions for referenced content. Direct requests for the protected node still return 401, and no integrity or availability impact is established. This issue is fixed in versions 13.15.1, 17.5.3, and 18.0.2.