This command tells netsed to listen on port 10101 (where
Netfilter will direct all port 80 traffic for the Target-IP) and
replace every occurrence of the legitimate link with the fake
link and every occurrence of the real MD5SUM with the
fake MD5SUM (the %2f is ASCII hex for the / character
and will be properly interpreted by the web server). The
net effect of doing these replacements is to replace the valid
HTML link with a link to a trojaned version of the software
desired by the client. It also manages to replace the
MD5SUMs so the client is assured that the download has
completed safely.
4.2 Experiment Conclusions
This particular implementation is only one particular
way of accomplishing this attack. In fact it is even a rather
naive attack, in that it reveals the real download IP to the
client. In addition to that netsed will not match strings that
cross packet boundaries. These, and other problems, could
easily be addressed by someone with malicious intent. In
fact, there are many variations on this attack. This approach
could be used to do all sorts of nasty things to the client but
we expect that this particular attack sufficiently illustrates
the risks.
5 Solution
The previous example should convince the reader that
even casual web browsing over a wireless link is suscepti-