Subject: [Isis-wg] ISIS Cryptographic Authentication - Replay attacks

From: "Jonnala, Nagi" <[email-masked]>
Date: Mon, 15 Apr 2002 05:47:05 -0400

Tony/Atkinson,

I have a few comments on ISIS Cryptographic Authentication mechanism
in the context of Replay Attacks. Sorry for the inconvenience if it was
already discussed.

Though the Draft says "This mechanism does not prevent replay attacks, 
however such attacks would trigger existing mechanisms in the IS-IS protocol
that would effectively reject old information.", My interpretation is that
we 
do not have any mechanism protecting from replay attacks of LSP-Sequence-0, 
Hellos, CSNPs and PSNPs".

Here is the description.

1) LSP with Sequence Number 0 
=============================

ISO 10589 specifies that we use this special LSP to request the latest LSP
from the intended recipient. It can be replayed as many times as the
intruder wants. The generation/processing of that LSP may not be triggered
by this attack...But I believe the real threat is  "Intruder
can capture all the LSP-Sequence-0 packets and can replay again and again to
consume the resources."  The lack of Key-ID mechanism even worsens the
situation because a cryptographic LSP-Sequence-0 packet may not be changed
very often.

2) CSNPs,  PSNPs  replay
========================

The worst situation in this case is when intruder
replays a particular xSNP packet which could cause a huge
transmittals of LSPs.  One can replay the same packet repeatedly
to consume the maximum resources and perhaps network
will never seem to stabilize.

3) Hello PDUs replay-back
=========================

As Jerome Etienne points out in "OSPFv2 authentication flaws" Internet
Draft,  Hello PDUs
can be replayed back by MAC address spoofing(Please correct me if MAC
address spoofing is not possible) to break the adjacency.

Please correct me if I wrongly interpreted anything.

Thanks
Nagi.

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---

From: Tony Li <[email-masked]>
Date: Tue, 16 Apr 2002 01:04:53 -0700

| Though the Draft says "This mechanism does not prevent replay attacks, 
 | however such attacks would trigger existing mechanisms in the IS-IS protocol
 | that would effectively reject old information.", My interpretation is that
 | we 
 | do not have any mechanism protecting from replay attacks of LSP-Sequence-0, 
 | Hellos, CSNPs and PSNPs".

Mebbe we need to make the wording stronger here.  It was not our intent to
claim that we are protected against such attacks.  Just that the protocol
mechanisms will cause such attacks to be transient.

As such, and given the fact that you can DoS anything to death with enough
bandwidth and a big enough gun, replay prevention seems low on the list of
things to fix.

Tony