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Date: Fri, 21 Aug 2015 18:48:06 -0700
From: Peter Todd <pete@petertodd.org>
To: Matt Corallo <lf-lists@mattcorallo.com>
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References: <55D6AD19.10305@mattcorallo.com> <20150821053819.GA18176@muck>
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Cc: bitcoin-dev@lists.linuxfoundation.org, Mike Hearn <mike@plan99.net>
Subject: Re: [bitcoin-dev] Revisiting NODE_BLOOM: Proposed BIP
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On Sat, Aug 22, 2015 at 01:08:13AM +0000, Matt Corallo wrote:
> > Well actually, we can reference the DoS attacks that Bitcoin XT nodes
> > are undergoing right now - part of the attack is repeated Bloom filter
> > requests to soak up disk IO bandwidth. I've CC'd Gavin and Mike - as far
> > as I know they haven't published details of those attacks - a write-up
> > would be very helpful.
> >=20
> > While so far those are being directed only at XT nodes, obviously this
> > is a potential issue for Core nodes as well. Like I mentioned last time
> > around, it's critical that miners aren't affected by these attacks -
> > nodes simply serving SPV wallet clients are much less latency sensitive,
> > so a good DoS attack mitigation strategy would be to have the two
> > classes of nodes out there "in the wild"
>=20
> Ehh, I was going more for the oldest mention.

One of the oldest mentions is the to-be-published-later portion of my
Litecoin Audit report; attached.

(see http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2013-July/00304=
4.html
for the original report/timestamping/verification)

--=20
'peter'[:-1]@petertodd.org
00000000000000000939524874a2896a46ea96bf59776ed869ccff95679cb087

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Security vulnerabilities related to the Litecoin v0.8.3.6 release
=================================================================

Nonce: c0854ae01b1ed8526af3bb6fb82550ff
Date:  Jul 29 2013

To be released in full on January 29, 2014 to the public.


LevelDB
=======

Something not well appreciated outside of the Bitcoin developers is that the
temporary limits on block size and complexity introduced in response to the
fork, automatically removed on May 15th, were simply voluntary measures to
protect against accidental triggering of the fork. The measures did not protect
against malicious attempts to trigger the the fork.

Litecoin is no different. Because of that I strongly recomend that miners be
encouraged to transition to v0.8.3.6 as soon as possible to ensure as much
hashing power as possible is consolidated on one version. Transitioning for
users/merchants is also important, however with sufficient mining power on the
new version it would be difficult to pull off a double-spend attack against an
older version simply because it would take so long to get the required number
of confirmations.

The development team may want to clarify this point to the Litecoin community
and encourage users to be suspicious if it takes an especially long time to get
confirmations. Merchants with automatic systems should use fail-safes triggered
by unusually long confirmation times, and as always ensure that total possible
losses are limited. It would be good if popular "blockchain information" sites
upgraded to v0.8.3.6 along with miners to give users accurate information on
the state of the blockchain. In any case users and merchants should take this
advice in general regardless of specific threats.

I would not be surprised to see someone deliberately attempt to fork Litecoin
by exploiting the issue; there is a lot of hostility torwards and within the
alt-coins.


SPV and network-wide DoS attacks
================================

Bitcoin is quite vulnerable currently to network-wide DoS attacks due to the
maximum connections limits; we do not have any form of filtering on incoming
connections so an attack can be made simply by making sufficient connections to
all nodes that they hit that incoming connections limit. This is public
knowledge, as is the suggestion to stop the attack by having concepts of peer
"usefullness" so that "useless" peers can be dropped. Of course SPV nodes are
badly impacted by such measures.

What isn't as well-known publicly is that Litecoin will make this attack
significantly less costly to the attacker in v0.8.3.6 by adding support for
bloom filters. That support allows the attacker to reduce their bandwidth
consumption to a minimum, making the attack easier to pull off on a wide scale.

The Bitcoin team is aware of the issue. Our plans in the event of an attack are
to ensure that large pools and merchants connect to each other via a private
"darknet" while fixes are implemented. These plans are also useful in the event
of an attack exploiting the vulnerability discussed below.


Bloom filters and disk IO
=========================

However it gets worse: there is nothing limiting peers from requesting blocks.
Without bloom filters bandwidth is a natural limit, however with bloom filters
a malicious peer can simply set the filter to match almost nothing and simply
submit getdata messages to the target requesting all blocks. The target will do
an extremely large amount of disk IO at almost no cost to the attacker.

To quote one core developer in a private conversation regarding bloom filtering
"I think we didn't think hard enough before implementing this"

A good first measure would be to assign a service bit to advertise bloom filter
support:

 /** nServices flags */
 enum
 {
     NODE_NETWORK = (1 << 0),
+    NODE_BLOOM_FILTER = (1 << 1),
 };

Secondly add a command line switch that allows bloom filtering to be turned on
or off entirely. I would suggest that the next version of Litecoin be released
soon and have bloom filters *disabled* by default unless the user specifically
turns them on.

There is a *very* good chance of an attack being launched targetting this
vulnerability by people using it as a way to show their opinion of Mike Hearn;
lots of people strongly dislike him for what they regard as very poor judgement
on security and scalability issues and would be happy to show that a design he
promoted is flawed.

Disabling bloom filering does of course cause problems for SPV clients, however
in the Litecoin realm such clients are non-existent. In the long run DNS seeds
should allow for the specification of desired service bits and rate limiting of
getdata operations implemented. However given that attacks are likely imminent
I strongly suggest a quick solution.

Bitcoin should have done bloom filtering as a service bit on day one; not doing
so was a mistake.

The Bitcoin team is aware of this issue. Please contact me to discuss the
release process for a fix; I will also be happy to review it. Unfortunately due
to the impact on SPV clients this issue is political as well as technical on
the Bitcoin side of things.


CHECKMULTISIG dummy value
=========================

The AreInputsStandard() test does not check the contents of the dummy value
required to spend a BIP11 CHECKMULTISIG scriptPubKey; anything that be put in
that space as a way to get data into the blockchain. Less well appreciated is
that because scriptSigs are unsigned any node, even a non-miner, can change the
txid of a CHECKMULTISIG transaction by modifying the dummy value.

This issue will probably be fixed in Bitcoin soon, not to mention revealed
publically; I'm only including it for the sake of completeness.

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