source: http://www.securityfocus.com/bid/7397/info

AN HTTPd contains a sample script named count.pl that may be used as a web counter. This script does not perform adequate access validation on paths containing directory traversal (../) character seqences. The vulnerable script may be used to overwrite any file to which it has write permissions by supplying that file name as input. 

http://www.victim.com/isapi/count.pl??../../../../../../../../../../../../../../../../../../../../../../winnt/explorer.exe