Re: Hidden Ports

["David J. Bianco" <bianco () jlab org>]

Eduardo Sorensen wrote:
> Can a port scanner not see a port that is opened?
>
> The question is: can a backdoor be on a machine, and with nmap -p 1-,
> for example, you couldn't see it?

Yes, this is quite common these days.  Rootkits like SucKIT can monitor
all IP sessions on a host, and only open up the backdoor port when a
certain trigger arrives via one of the already-open services.  For example,
if an attacker sends a certain string of bytes to the HTTP server on port
80 (even if the string is invalid HTTP).  Some tools also look for
connections to ports in certain order (eg, the same host contacts port
80, port 22 and then port 443 within a few seconds).  Unless the trigger
is received, then the backdoor isn't listening, and thus wouldn't show
up in a portscan.

There may be other more innovative triggers, too.  It's a hard problem.  If
you think you might have a backdoor, you shouldn't depend solely on
portscanners like nmap to detect it.  Anti-virus, tripwire and tools like
chkrootkit are also necessary.

        David

--
David J. Bianco, GSEC GCUX GCIH         <bianco () jlab org>
Thomas Jefferson National Accelerator Facility
GPG Fingerprint:  516A B80D AAB3 1617 A340  227A 723B BFBE B395 33BA

     The views expressed herein are solely those of the author and
            not those of SURA/Jefferson Lab or the US DOE.



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