ntop is a tool that shows the network usage, similar to what the popular top Unix command does. Starting ntop in web mode (with the -w parameter) starts ntop with it's own built in HTTP server, to allow remote access to the functions it provides. ntop does not properly authenticate requests and is vulnerable to a ../../ request whereby unauthorized files can be retrieved, including files which are only readable by root.
The default directory ntop serves HTML from is /etc/ntop/html so to retrieve /etc/shadow one can request the following URL: http://URL:port/../../shadow
ntop running in web mode (-w) contains a flaw that allows a remote attacker to access arbitrary files outside of the web path. The issue is due to the internal web server not properly sanitizing user input, specifically traversal style attacks (../../) supplied via the URI.
Upgrade to version 1.3.1 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.