From: /dev /fd0 <alicexbtong@gmail•com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: [bitcoindev] Re: A "Free" Relay Attack Taking Advantage of The Lack of Full-RBF In Core
Date: Fri, 19 Jul 2024 05:41:06 -0700 (PDT) [thread overview]
Message-ID: <18a5e5a2-92b3-4345-853d-5a63b71d848bn@googlegroups.com> (raw)
In-Reply-To: <Zpk7EYgmlgPP3Y9D@petertodd.org>
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Hi Peter,
> I didn't get a substantive
> response from Bitcoin Core, other than Core closing the my pull-req
enabling
> full-RBF by default that would fix this specific vulnerability.
The last comment in the pull request suggests opening a new pull request to
enable full RBF by default, referencing the one closed due to off-topic
comments.
> But read on, this is quite an odd case of Core politics, and the story is
not
> as simple as Core refusing to fix a vulnerability.
It seems that you are the one trying to politicize this issue.
/dev/fd0
floppy disk guy
On Thursday, July 18, 2024 at 4:04:26 PM UTC Peter Todd wrote:
> # Summary
>
> This is a public disclosure of a vulnerability that I previously disclosed
> to
> the bitcoin-security mailing list. It's an easy vulnerability to fix.
> Although
> as with other "free" relay attacks I've disclosed, I didn't get a
> substantive
> response from Bitcoin Core, other than Core closing the my pull-req
> enabling
> full-RBF by default that would fix this specific vulnerability.
>
> But read on, this is quite an odd case of Core politics, and the story is
> not
> as simple as Core refusing to fix a vulnerability. Also, I've including a
> fun
> homework problem at the end: figure out how TRUC/V3 transactions itself
> creates
> a "free" relay attack.
>
>
> # Background
>
> This is just one of a few "free" relay attacks that I have recently
> disclosed,
> including, but not limited to:
>
> "A Free-Relay Attack Exploiting RBF Rule #6" - Mar 18th 2024
> https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg
>
> "A Free-Relay Attack Exploiting Min-Relay-Fee Differences" - Mar 31st 2024
> https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo
>
> The term "free relay attack" simply refers to any mechanism where
> transaction
> data can be broadcast at unusually low cost; the "free" in "free relay" is
> a
> misnomer as all these attacks do in fact have some cost.
>
> This particular attack isn't significantly different than the other attacks
> I've disclosed. With one important exception: unlike those other attacks,
> fixing this particular attack would be quite easy, by enabling full-rbf by
> default. So I disclosed it to the bitcoin-security mailing list as a test:
> does
> Bitcoin Core actually care about free relay attacks? My hypothesis is that
> Core
> does not, as they know full well that "free" relay is an unavoidable
> problem;
> I've received absolutely no feedback from any Bitcoin Core members for the
> other disclosed attacks, beyond achow using my disclosure of the RBF Rule
> #6
> attack as an excuse to remove me from the bitcoin-security mailing list.
>
> The fact that Core doesn't actually care about "free" relay attacks is
> relevant
> to TRUC/V3 Transactions. As per BIP-431:
>
> "The primary problem with [RBFR proposals] is the potential for free relay
> and DDoS attacks.
>
> Removing Rule 3 and 4 in general would allow free relay [27]."
>
> https://github.com/bitcoin/bips/blob/812907c2b00b92ee31e2b638622a4fe14a428aee/bip-0431.mediawiki#user-content-Alternatives_replace_by_feerate
>
> I believe the authors of that BIP are fully aware of the fact that "free"
> relay
> is an unavoidable problem, making their rational for TRUC/V3 bogus, and
> don't
> want to admit that they've wasted a large amount of engineering time on a
> bad
> proposal. I will be submitting a pull-req to get BIP-431 corrected, as the
> many
> "free" relay attacks I've disclosed clearly show that claiming RBFR would
> "allow" free relay is simply not true.
>
> Notably, full-RBF is _itself_ a transaction pinning fix for many use-cases;
> part of the TRUC/V3 standard is to force full-RBF behavior for V3
> transactions.
> So Core closing my full-RF pull-req is doubling down on TRUC/V3 in a second
> way, and TRUC/V3 proponents were the ones who tried to get the full-RBF
> option
> removed from Core in the first place. If not for this dumb bit of Core
> politics, I'm sure my year-old pull-req to enable full-RBF by default would
> have been merged many months ago, as almost all hashpower has adopted
> full-RBF
> making objections based on "zeroconf" absurd.
>
>
> # The Attack
>
> If you're a competent Bitcoin engineer, familiar with how mempools work,
> you've
> probably figured it out already based on the title: obviously, if a high
> percentage of miners are adopting a policy that Bitcoin Core nodes are
> not, you
> can cheaply consume transaction relay bandwidth by simply relaying
> transations
> that miners are rejecting.
>
> Specifically, do the following:
>
> 1. Broadcast a small, low-fee-rate, tx A with BIP-125 opt-in disabled.
> 2. Broadcast a full-RBF double-spend of A, A2, with a higher fee-rate.
> 3. Spend the outputs of A in a large, low fee-rate, transaction B with
> BIP-125
> opt-in enabled. ~100% of miners will reject B, as it spends an input not in
> their mempools. However Bitcoin Core nodes will waste bandwidth propagating
> B.
> 4. (Optional) Double-spend B repeatedly. Again, Bitcoin Core nodes will
> waste
> bandwidth propagating Bn's that ~100% of miners are ignoring.
> 5. Double-spend A2 to recover your funds and do it all over again (or if
> A2 had
> a high enough fee-rate, just wait for it to be mined).
>
> The cost to relay each B transaction depends on the fee-rate of B. Since
> Bitcoin Core defaults to a fairly large mempool, the minimum relay
> fee-rate is
> typically well below the economic fee-rate required for miners to actually
> mine
> a transaction; Core accepts transactions that are uneconomical for miners
> to
> mine for the forseeable future.
>
> For example, at the moment typical mempools require transactions to pay at
> least 1sat/vB, while there are hundreds of MvB worth of transactions paying
> 4sat/vB, the minimum economical fee-rate. Thus, transactions paying less
> than
> 4sat/VB are extremely unlikely to get mined in the nearish future.
>
> Concretely, broadcasting B transactions at 1sat/vB, 2sat/vB, and 3sat/vB
> would
> have almost zero cost as the probability of those transactions getting
> mined is
> nearly zero. This is true _regardless_ of what % of miners are mining
> full-RBF!
> As long as you can get at least one miner to mine the A double-spend, the
> attack only costs what it cost to get A mined.
>
> If B's are broadcast at a higher fee-rate than the minimum economical
> fee-rate,
> then the % of full-RBF miners matters. For example, if only 99% of miners
> mine
> full-RBF, the chance of a B transaction getting mined per block is about
> 1%, so
> the amortized cost of broadcasting B is about 1% of whatever total fee the
> highest fee-rate variant of B pays.
>
> For an attacker who does not need any B to be broadcast, the cost savings
> to
> use of relay bandwidth is approximately the ratio of the difference in size
> between B and and A. With a maximum standard transaction size of 100KvB, or
> 400KB serialized size, this ratio is on the order of 5000:1, times the
> total
> number of B variants broadcast, and the % chance of each B being mined;
> it's a
> few orders of magnitude.
>
> Of course, as mentioned above, this is just one of *many* "free" relay
> attacks,
> so fixing this particular issue doesn't change much.
>
>
> # Attackers Who Benefit From B Getting Mined
>
> Some attackers actually need B to get mined. For example, imagine an
> exchange
> who needs to do large consolidation transactions. They could use this
> attack
> (and some attacks like it) as a way to goad users and miners into mining
> consolidation transactions for them at low cost. In this variant of the
> attack,
> the attacker would pad the size of B with consolidation spends that they
> needed
> to do anyway. Someone who tried to stop the attack by getting B mined (eg
> via
> mempool.space's transaction accellerator) would simply be paying the
> attacker's
> fees for them.
>
> Obviously, this strategy is only relevant for B's below the economic
> fee-rate.
> However, the weaker version of this strategy is to parallize the attack,
> and do
> your consolidation with the _A_ double-spends to reduce the # of bytes
> used per
> full-rbf double-spend.
>
>
> # TRUC/V3 Creates a Free Relay Attack
>
> I'll leave the details of this as a homework problem. But obviously, the
> introduction of TRUC/V3 transactions *itself* creates a free relay attack
> very
> similar to the above! Just like full-RBF, not all miners will mine V3
> transactions. So you can do the exact same type of attack by taking
> advantage
> of this difference in mining policy.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>
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next prev parent reply other threads:[~2024-07-19 18:27 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-07-18 15:56 [bitcoindev] " Peter Todd
2024-07-18 23:04 ` [bitcoindev] " Antoine Riard
2024-07-19 1:05 ` Peter Todd
2024-07-19 13:52 ` Antoine Riard
2024-07-19 14:38 ` Peter Todd
2024-07-19 23:58 ` Antoine Riard
2024-07-20 0:46 ` 'Ava Chow' via Bitcoin Development Mailing List
2024-07-21 2:06 ` Antoine Riard
2024-07-21 20:17 ` 'Ava Chow' via Bitcoin Development Mailing List
2024-07-22 1:59 ` 'Anonymous User' via Bitcoin Development Mailing List
2024-07-24 0:44 ` Antoine Riard
2024-07-24 0:35 ` Antoine Riard
2024-07-19 12:41 ` /dev /fd0 [this message]
2024-07-19 23:56 ` Antoine Riard
2024-07-20 5:57 ` /dev /fd0
2024-07-20 15:08 ` Peter Todd
2024-07-21 2:13 ` Antoine Riard
2024-07-21 6:16 ` /dev /fd0
2024-07-21 2:12 ` Antoine Riard
2024-07-19 18:26 ` [bitcoindev] " Murch
2024-07-20 14:10 ` Peter Todd
2024-07-20 6:41 ` David A. Harding
2024-07-20 15:03 ` Peter Todd
2024-07-20 15:30 ` Peter Todd
2024-07-21 15:35 ` David A. Harding
2024-07-21 20:25 ` Peter Todd
2024-07-24 0:38 ` Antoine Riard
2024-07-21 2:10 ` Antoine Riard
2024-07-22 15:10 ` Peter Todd
2024-07-24 0:41 ` Antoine Riard
2024-07-22 11:45 ` [bitcoindev] RBFR makes the CPFP carve-out obsolete with cluster mempool, without upgrading LN nodes; TRUC/V3 does not Peter Todd
2024-07-22 16:43 ` David A. Harding
2024-07-22 20:06 ` Peter Todd
2024-07-22 22:08 ` David A. Harding
2024-07-23 11:29 ` Peter Todd
2024-07-24 0:42 ` Antoine Riard
2024-07-22 17:13 ` [bitcoindev] A "Free" Relay Attack Taking Advantage of The Lack of Full-RBF In Core Peter Todd
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