When downloading the Microsoft Update offline catalog (wsusscn2.cab) before each attempt to scan, downloaded files are checked for a valid Microsoft digital signature before being used. When the file is missing, or the digital signature cannot be verified, these errors may occur. To avoid this error message, it is recommended that the update (see Microsoft Knowledge Base article 835732) be installed on all Windows 2000 computers running MBSA to perform scanning. This error may also appear if you use a proxy server such as Microsoft ISA 2000 Server if it does not include a policy that permits anonymous access to the Microsoft Update site. A resolution for this is described in Microsoft Knowledge Base article 885819. Another cause for this issue may be that Internet Explorer is set to offline mode (Work Offline). MBSA requires either IE to be in online mode and be able to successfully pass through any proxy servers to obtain the wsusscn2.