Hi Fernando+list
I'm glad to see that someone takes aim at this issue.
However, it seems that your proposal only attempts to address one
consequence of predictable TCP source ports, namely blind TCP attacks
(in all fairness, it appears that the object of your proposal is to
solve the blind TCP attacks, rather than the issue of predictable TCP
source ports; I look at it the other way around...). Naturally this is a
major outcome, but there are still other consequences, perhaps less
severe, such as traffic analysis. For example, the nave (and as
explained in your draft, flawed) algorithm in Fig. 1 of your IETF draft
advances next_ephemeral globally. Therefore, if the attacker can force
the target host to periodically establish a new TCP connection to an
attacker controlled machine (or through an attacker observable routing
path), the attacker can subtract consecutive source port values to
obtain the number of outoing TCP connections established globally by the
target host within that time period (up to wrap-around issues and
5-tuple collisions, of course).
However, note that algorithm #3 in your proposal is also susceptible to
the same technique.
Algorithm #4 is affected as well, to some degree. The "table" array
compartmentalize the space into TABLE_LENGTH sections. An attacker can
perform traffic analysis for any section into which the attacker has
"visibility", namely that the attacker can force the server to establish
connection whose G(offset) points to this section. The attacker has
little control over to which section exactly the host will map the
attacker's traffic, but once there, the attacker can monitor traffic
volumes (new outgoing TCP connections) for this arbitrary section.
Again, I don’t know if this is in scope for your draft, but I do believe
that looking at the generic problem here, this should be a factor.
Thanks, and good luck,
-Amit
> -----Original Message-----
> From: Fernando Gont [mailto:fernando.gont@gmail.com]
> Sent: Friday, December 07, 2007 02:45
> To: bugtraq@securityfocus.com
> Subject: TCP Port randomization paper
>
> Folks,
>
> We have published a revision of our port randomization paper.
> This is the first revision of the document since it was accepted as a
> working group item of the tsvwg working group of the IETF (Internet
> Engineering Task Force). Any feedback on the proposed/described
> algorithms will be welcome.
>
> The document is available at:
> http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-port-rand
> omization-00.txt
>
> Additionally, it is available in other fancy formats (PDF and HTML)
> at: http://www.gont.com.ar/drafts/port-randomization/index.html
>
> Thanks,
>
> --
> Fernando Gont
> e-mail: fernando@gont.com.ar || fgont@acm.org PGP
> Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1
>
>
>
>
>
>
