Sometime ago I thought of the following idea for a covert channel. Although the idea of covert channels is *not* new
at all, I couldn't find anything in Google related to the following method of implementing a covert channel.
The scenario is the following. The victim is a host with a host-level firewall which is blocking *all* incoming
traffic. Somehow the attacker still needs to communicate with a backdoor planted in this host. Use a reverse shell
and job done, you might say.
Actually, there is another way which I thought would be more creative (IMHO).
It works like this: the backdoor enables logging in the host-level firewall for all dropped packets, say Windows XP
SP2 Firewall. Then the backdoor receives commands from the attacker by interpreting the properties of the dropped
packets which were logged by the firewall. In other words, the backdoor is constantly reading the logs and parsing
commands which were sent by the attacker embedded in packets which are being dropped (but logged) by the firewall.
attacker sends packets -> packets are dropped by firewall -> packets properties are captured in logs -> backdoor
reads logs and finds encoded commands -> commands are executed
Now, for the way the backdoor would reply back to the victim is really up to you. One method that comes to my mind is
by posting the responses to a PHP script which is located in some free-hosting webpage. The attacker would then
access this webpage.
Please, if you know anything related to backdoors intercepting commands from log files send me some links. Ideas,
comments and flames are more than welcome :-) .
pagvac (Adrian Pastor)
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