Microsoft Internet Information Services (IIS), when used in conjunction with unspecified third-party upload applications, allows remote attackers to create empty files with arbitrary extensions via a filename containing an initial extension followed by a : (colon) and a safe extension, as demonstrated by an upload of a .asp:.jpg file that results in creation of an empty .asp file, related to support for the NTFS Alternate Data Streams (ADS) filename syntax.  NOTE: it could be argued that this is a vulnerability in the third-party product, not IIS, because the third-party product should be applying its extension restrictions to the portion of the filename before the colon.