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RE: Port-Knocking vulnerabilities?
From: "Craig Wright" <Craig.Wright () bdo com au>
Date: Tue, 1 Jan 2008 08:46:24 +1100

Lets look at the issues.

You rely on obscurity in a manner that changes flags in IP and makes the packets stand out. Most IDS's will alert to 
this, many routers will. A TCPdump filter for unusual flags and IP ID's is common in many ISP's. So we have a security 
mechanism that is advertising itself but relies on secrecy. Paradox and inconsistency No. 1.

The IP ID field is 16 bits. With a 4 packet knock we have a functional equivalent of a 3 character all symbol or 4 
character alpha numeric password. I do not believe that this was ever considered secure.

IP ID fingerprinting will make the flag stand out. Without the SPA encryption mechanisms it is a simple capture (or 
sniffing). Using SPA (not Port knocking) you can sniff packets and capture for analysis. The “encryption” can be 
silently cracked in seconds (it is functionally equivalent to 13 bit DES) in microseconds given any modern PC.

 Please explain how this is more than a script kiddie toy and a security boon?

 As Brent stated, why not deploy a REAL crypto solution. It is 1. Easier. 2. Supported and 3 More secure (i.e. 128 or 
256 bit keys take a LONG time to break).

Dr Craig Wright (GSE-Compliance)
PS Happy New Year

Craig Wright
Manager of Information Systems

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From: listbounce () securityfocus com [listbounce () securityfocus com] On Behalf Of Ansgar -59cobalt- Wiechers 
[bugtraq () planetcobalt net]
Sent: Tuesday, 1 January 2008 7:50 AM
To: security-basics () securityfocus com
Subject: Re: Port-Knocking vulnerabilities?

On 2007-12-31 Robert Inder wrote:
On 29/12/2007, Ansgar -59cobalt- Wiechers <bugtraq () planetcobalt net> wrote:
On 2007-12-28 Jay wrote:
Portknocking is a security mechanism as it is a type of
authentication. "Something you know" in this case the sequence of
ports to knock before a unstarted service or daemon begins listening
for connections.

Since everything is transmitted in the clear port-knocking is as much
of a security mechanism as cleartext passwords. Technically: maybe
(depending on your definition). Realistically: no.

I think your dismissal of port knocking (and, indeed, plain text
passwords) is unrealistic.

If you can intercept my interaction with some remote server, you can
steal the relevant secrets (the password or the sequence of ports).

But isn't that quite a substantial "if"?

The substantial "if" is the question if intercepting the transmission
will allow an attacker to learn the secret without having to compromise
either the sender or the receiver of the communication. If an attacker
can do that, then the authentication mechanism is insecure and thus mere
obscurity. Period.

How are you going to do it?  Aren't you going to have to compromise
some other machine, either where I am, or where the server is (or, I
guess, where the relevant DNS records are), and then plant software to
deliberately wait and watch until a relevant interaction takes place?


There are other attack vectors as well.

I'm not saying that's impossible.  But it would take considerable
knowledge, planning and effort.

Why doesn't that make it a substantial defence against most kinds of
casual attack?

Because "substantial" is the opposite of "casual". A measure that won't
also stop a determined attacker is just obscurity, not security.

Ansgar Wiechers
"All vulnerabilities deserve a public fear period prior to patches
becoming available."
--Jason Coombs on Bugtraq

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