NAI VirusScan NT 4.0.2 does not properly modify the scan.dat virus definition file during an update via FTP, but it reports that the update was successful, which could cause a system administrator to believe that the definitions have been updated correctly.
https://euvd.enisa.europa.eu/vulnerability/EUVD-1999-1176
http://www.securityfocus.com/bid/169