VAST error 403, aka VAST response declared unsupported MIME types for all Media Files, is an error that can be caused by a VAST response that declares unsupported MIME types for all available Media Files (for example, Flash on mobile, or WebM on iOS). It may indicate that the wrong creative type attempted to play. This error type is more common on mobile. To fix the error, the publisher should ensure that the creative format or technology requested is supported by the video player’s device and platform. For example, VPAID creatives should not be requested from mobile apps. The buyer should ensure that all returned creatives respect the video format in the bid request.