Re: [bitcoin-dev] Full Disclosure: CVE-2023-40231 / CVE-2023-40232 / CVE-2023-40233 / CVE-2023-40234 "All your mempool are belong to us"

[Antoine Riard <antoine.riard@gmail.com>]

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Hi,

As I've been shown offline Twitter posts misrepresenting my previous mail,
I think it's good to correct them. The security flaws are not "intentional
backdoor" or whatever misrepresentation that would question the competence
and know-how of the Bitcoin and Lightning development community.

The replacement cycling issue discovered has been known by a small circle
of Bitcoin developers since December 2022. As it appears to some experts
and it has been commented publicly, changes at the bitcoin base-layer might
be the most substantial fixes. Those changes take time and here this is
akin to how the linux kernel, bsds and OS vendors are working [0].

All I can say is that we had recently had internal discussion on how to
improve coordinated security fixes and patching processes for the coming
decades. This is an area of concern where I've always been at the forefront
as early as 2020 / 2021 [1].

In the meanwhile, lightning experts have already deployed mitigations which
are hardening the lightning ecosystem significantly in face of simple or
medium attacks. More advanced attacks can only be mounted if you have
sufficient p2p and mempool knowledge as was pointed out by other bitcoin
experts like Matt or Peter (which take years to acquire for average bitcoin
developers) and the months of preparation to attempt them.

If you're a journalist reporting on the information in mainstream crypto
publications, I'll suggest waiting to do so before expert reporters of
bitcoin circles who have more in-field knowledge can do so and qualify the
technical situation with more distance. As I've already been interviewed by
top financial publication years ago for my work on bitcoin, as a journalist
you're engaging your own reputation on the information you're reporting.
Thanks for being responsible here.

This is the nature of the electronic communication and contemporaneous
media that information is extremely fluid and there is no native anti-DoS
mechanism to slow down the propagation of sensitive information where
mitigations are still in deployment. A reason I'm not on social media of
any kind [2]. In the meanwhile, it's good to go to read senecca and marcus
aurelius take the situation with stoicism and with a zelt of meditation [3].

All my previous statements are mostly technically correct (even if some
could have been written with more clarity once again I'm not an english
native [4]). While I wish to wait the week of the 30th Oct o discuss
further what is best fix and what are the trade-offs as a community as a
wide (give time some laggard lightning implementations ship fixes), though
I'll comment further on the mailing list if the flow of information on
"social media" is DoSing the ability of the bitcoin community to work on
the long-term appropriate fixes in a responsible and constructive fashion.

[0] See meltdown class of vulnerability and how operating systems are
handling hardware-sourced vulnerabilities
https://en.wikipedia.org/wiki/Meltdown_(security_vulnerability). Most of
the time they do their best on the software side and they go to see with
hardware vendors how to do the necessary updates.

[1]
https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-April/003002.html

[2] And for the wider analysis on contemporaneous culture of information
propagation and network effect, I can only recommend to read venkatesh
rao's ribbonfarm essays http://ribbonfarm.com

[3] There are very good reasons why some executives at top modern
technology companies are doing meditation daily, some even hours. "mind
illuminated" is a good read.

[4] While my former employer, Chaincode Labs, paid for my english lessons
back in 2020. Generally it was a good insight from them to train people on
how to communicate in a crisis.


Le ven. 20 oct. 2023 à 07:56, Antoine Riard <antoine.riard@gmail.com> a
écrit :

> Hi,
>
> After writing the mail reply on the economics of sequential malicious
> replacement of honest HTLC-timeout, I did write one more test to verify the
> behavior on core mempool, and it works as expected.
>
>
> https://github.com/ariard/bitcoin/commit/30f5d5b270e4ff195e8dcb9ef6b7ddcc5f6a1bf2
>
> Responsible disclosure process has followed the lines of hardware issues
> affecting operating system, as documented for the Linux kernel, while
> adapted to the bitcoin ecosystem:
>
> https://docs.kernel.org/6.1/process/embargoed-hardware-issues.html
>
> Effective now, I'm halting my involvement with the development of the
> lightning network and its implementations, including coordinating the
> handling of security issues at the protocol level (I informed some senior
> lightning devs in that sense before).
>
> Closed the very old issue which was affected to me at this purpose on the
> bolt repository:
>
> https://github.com/lightning/bolts/pull/772
>
> I think this new class of replacement cycling attacks puts lightning in a
> very perilous position, where only a sustainable fix can happen at the
> base-layer, e.g adding a memory-intensive history of all-seen transactions
> or some consensus upgrade. Deployed mitigations are worth something in face
> of simple attacks, though I don't think they're stopping advanced attackers
> as said in the first full disclosure mail.
>
> Those types of changes are the ones necessitating the utmost transparency
> and buy-in of the community as a whole, as we're altering the full-nodes
> processing requirements or the security architecture of the decentralized
> bitcoin ecosystem in its integrality.
>
> On the other hand fully explaining why such changes would be warranted for
> the sake of lightning and for designing them well, we might need to lay out
> in complete state practical and critical attacks on a ~5 355 public BTC
> ecosystem.
>
> Hard dilemma.
>
> There might be a lesson in terms of bitcoin protocol deployment, we might
> have to get them right at first try. Little second chance to fix them in
> flight.
>
> I'll be silent on those issues on public mailing lists until the week of
> the 30 oct. Enough material has been published and other experts are
> available. Then I'll be back focusing more on bitcoin core.
>
> Best,
> Antoine
>
> Le lun. 16 oct. 2023 à 17:57, Antoine Riard <antoine.riard@gmail.com> a
> écrit :
>
>> (cross-posting mempool issues identified are exposing lightning chan to
>> loss of funds risks, other multi-party bitcoin apps might be affected)
>>
>> Hi,
>>
>> End of last year (December 2022), amid technical discussions on eltoo
>> payment channels and incentives compatibility of the mempool anti-DoS
>> rules, a new transaction-relay jamming attack affecting lightning channels
>> was discovered.
>>
>> After careful analysis, it turns out this attack is practical and
>> immediately exposed lightning routing hops carrying HTLC traffic to loss of
>> funds security risks, both legacy and anchor output channels. A potential
>> exploitation plausibly happening even without network mempools congestion.
>>
>> Mitigations have been designed, implemented and deployed by all major
>> lightning implementations during the last months.
>>
>> Please find attached the release numbers, where the mitigations should be
>> present:
>> - LDK: v0.0.118 - CVE-2023 -40231
>> - Eclair: v0.9.0 - CVE-2023-40232
>> - LND: v.0.17.0-beta - CVE-2023-40233
>> - Core-Lightning: v.23.08.01 - CVE-2023-40234
>>
>> While neither replacement cycling attacks have been observed or reported
>> in the wild since the last ~10 months or experimented in real-world
>> conditions on bitcoin mainet, functional test is available exercising the
>> affected lightning channel against bitcoin core mempool (26.0 release
>> cycle).
>>
>> It is understood that a simple replacement cycling attack does not demand
>> privileged capabilities from an attacker (e.g no low-hashrate power) and
>> only access to basic bitcoin and lightning software. Yet I still think
>> executing such an attack successfully requests a fair amount of bitcoin
>> technical know-how and decent preparation.
>>
>> From my understanding of those issues, it is yet to be determined if the
>> mitigations deployed are robust enough in face of advanced replacement
>> cycling attackers, especially ones able to combine different classes of
>> transaction-relay jamming such as pinnings or vetted with more privileged
>> capabilities.
>>
>> Please find a list of potential affected bitcoin applications in this
>> full disclosure report using bitcoin script timelocks or multi-party
>> transactions, albeit no immediate security risk exposure as severe as the
>> ones affecting lightning has been identified. Only cursory review of
>> non-lightning applications has been conducted so far.
>>
>> There is a paper published summarizing replacement cycling attacks on the
>> lightning network:
>>
>> https://github.com/ariard/mempool-research/blob/2023-10-replacement-paper/replacement-cycling.pdf
>>
>>  ## Problem
>>
>> A lightning node allows HTLCs forwarding (in bolt3's parlance accepted
>> HTLC on incoming link and offered HTLC on outgoing link) should settle the
>> outgoing state with either a success or timeout before the incoming state
>> timelock becomes final and an asymmetric defavorable settlement might
>> happen (cf "Flood & Loot: A Systematic Attack on The Lightning Network"
>> section 2.3 for a classical exposition of this lightning security property).
>>
>> Failure to satisfy this settlement requirement exposes a forwarding hop
>> to a loss of fund risk where the offered HTLC is spent by the outgoing link
>> counterparty's HTLC-preimage and the accepted HTLC is spent by the incoming
>> link counterparty's HTLC-timeout.
>>
>> The specification mandates the incoming HTLC expiration timelock to be
>> spaced out by an interval of `cltv_expiry_delta` from the outgoing HTLC
>> expiration timelock, this exact interval value being an implementation and
>> node policy setting. As a minimal value, the specification recommends 34
>> blocks of interval. If the timelock expiration I of the inbound HTLC is
>> equal to 100 from chain tip, the timelock expiration O of the outbound HTLC
>> must be equal to 66 blocks from chain tip, giving a reasonable buffer of
>> reaction to the lightning forwarding node.
>>
>> In the lack of cooperative off-chain settlement of the HTLC on the
>> outgoing link negotiated with the counterparty (either
>> `update_fulfill_htlc` or `update_fail_htlc`) when O is reached, the
>> lightning node should broadcast its commitment transaction. Once the
>> commitment is confirmed (if anchor and the 1 CSV encumbrance is present),
>> the lightning node broadcasts and confirms its HTLC-timeout before I height
>> is reached.
>>
>> Here enter a replacement cycling attack. A malicious channel counterparty
>> can broadcast its HTLC-preimage transaction with a higher absolute fee and
>> higher feerate than the honest HTLC-timeout of the victim lightning node
>> and triggers a replacement. Both for legacy and anchor output channels, a
>> HTLC-preimage on a counterparty commitment transaction is malleable, i.e
>> additional inputs or outputs can be added. The HTLC-preimage spends an
>> unconfirmed and unrelated to the channel parent transaction M and conflicts
>> its child.
>>
>> As the HTLC-preimage spends an unconfirmed input that was already
>> included in the unconfirmed and unrelated child transaction (rule 2), pays
>> an absolute higher fee of at least the sum paid by the HTLC-timeout and
>> child transaction (rule 3) and the HTLC-preimage feerate is greater than
>> all directly conflicting transactions (rule 6), the replacement is
>> accepted. The honest HTLC-timeout is evicted out of the mempool.
>>
>> In an ulterior move, the malicious counterparty can replace the parent
>> transaction itself with another candidate N satisfying the replacement
>> rules, triggering the eviction of the malicious HTLC-preimage from the
>> mempool as it was a child of the parent T.
>>
>> There is no spending candidate of the offered HTLC output for the current
>> block laying in network mempools.
>>
>> This replacement cycling tricks can be repeated for each rebroadcast
>> attempt of the HTLC-timeout by the honest lightning node until expiration
>> of the inbound HTLC timelock I. Once this height is reached a HTLC-timeout
>> is broadcast by the counterparty's on the incoming link in collusion with
>> the one on the outgoing link broadcasting its own HTLC-preimage.
>>
>> The honest Lightning node has been "double-spent" in its HTLC forwarding.
>>
>> As a notable factor impacting the success of the attack, a lightning
>> node's honest HTLC-timeout might be included in the block template of the
>> miner winning the block race and therefore realizes a spent of the offered
>> output. In practice, a replacement cycling attack might over-connect to
>> miners' mempools and public reachable nodes to succeed in a fast eviction
>> of the HTLC-timeout by its HTLC-preimage. As this latter transaction can
>> come with a better ancestor-score, it should be picked up on the flight by
>> economically competitive miners.
>>
>> A functional test exercising a simple replacement cycling of a HTLC
>> transaction on bitcoin core mempool is available:
>> https://github.com/ariard/bitcoin/commits/2023-test-mempool
>>
>> ## Deployed LN mitigations
>>
>> Aggressive rebroadcasting: As the replacement cycling attacker benefits
>> from the HTLC-timeout being usually broadcast by lightning nodes only once
>> every block, or less the replacement cycling malicious transactions paid
>> only equal the sum of the absolute fees paid by the HTLC, adjusted with the
>> replacement penalty. Rebroadcasting randomly and multiple times before the
>> next block increases the absolute fee cost for the attacker.
>>
>> Implemented and deployed by Eclair, Core-Lightning, LND and LDK .
>>
>> Local-mempool preimage monitoring: As the replacement cycling attacker in
>> a simple setup broadcast the HTLC-preimage to all the network mempools, the
>> honest lightning node is able to catch on the flight the unconfirmed
>> HTLC-preimage, before its subsequent mempool replacement. The preimage can
>> be extracted from the second-stage HTLC-preimage and used to fetch the
>> off-chain inbound HTLC with a cooperative message or go on-chain with it to
>> claim the accepted HTLC output.
>>
>> Implemented and deployed by Eclair and LND.
>>
>> CLTV Expiry Delta: With every jammed block comes an absolute fee cost
>> paid by the attacker, a risk of the HTLC-preimage being detected or
>> discovered by the honest lightning node, or the HTLC-timeout to slip in a
>> winning block template. Bumping the default CLTV delta hardens the odds of
>> success of a simple replacement cycling attack.
>>
>> Default setting: Eclair 144, Core-Lightning 34, LND 80 and LDK 72.
>>
>> ## Affected Bitcoin Protocols and Applications
>>
>> From my understanding the following list of Bitcoin protocols and
>> applications could be affected by new denial-of-service vectors under some
>> level of network mempools congestion. Neither tests or advanced review of
>> specifications (when available) has been conducted for each of them:
>> - on-chain DLCs
>> - coinjoins
>> - payjoins
>> - wallets with time-sensitive paths
>> - peerswap and submarine swaps
>> - batch payouts
>> - transaction "accelerators"
>>
>> Inviting their developers, maintainers and operators to investigate how
>> replacement cycling attacks might disrupt their in-mempool chain of
>> transactions, or fee-bumping flows at the shortest delay. Simple flows and
>> non-multi-party transactions should not be affected to the best of my
>> understanding.
>>
>> ## Open Problems: Package Malleability
>>
>> Pinning attacks have been known for years as a practical vector to
>> compromise lightning channels funds safety, under different scenarios (cf.
>> current bip331's motivation section). Mitigations at the mempool level have
>> been designed, discussed and are under implementation by the community
>> (ancestor package relay + nverrsion=3 policy). Ideally, they should
>> constraint a pinning attacker to always attach a high feerate package
>> (commitment + CPFP) to replace the honest package, or allow a honest
>> lightning node to overbid a malicious pinning package and get its
>> time-sensitive transaction optimistically included in the chain.
>>
>> Replacement cycling attack seem to offer a new way to neutralize the
>> design goals of package relay and its companion nversion=3 policy, where an
>> attacker package RBF a honest package out of the mempool to subsequently
>> double-spend its own high-fee child with a transaction unrelated to the
>> channel. As the remaining commitment transaction is pre-signed with a
>> minimal relay fee, it can be evicted out of the mempool.
>>
>> A functional test exercising a simple replacement cycling of a lightning
>> channel commitment transaction on top of the nversion=3 code branch is
>> available:
>> https://github.com/ariard/bitcoin/commits/2023-10-test-mempool-2
>>
>> ## Discovery
>>
>> In 2018, the issue of static fees for pre-signed lightning transactions
>> is made more widely known, the carve-out exemption in mempool rules to
>> mitigate in-mempool package limits pinning and the anchor output pattern
>> are proposed.
>>
>> In 2019, bitcoin core 0.19 is released with carve-out support. Continued
>> discussion of the anchor output pattern as a dynamic fee-bumping method.
>>
>> In 2020, draft of anchor output submitted to the bolts. Initial finding
>> of economic pinning against lightning commitment and second-stage HTLC
>> transactions. Subsequent discussions of a preimage-overlay network or
>> package-relay as mitigations. Public call made to inquiry more on potential
>> other transaction-relay jamming attacks affecting lightning.
>>
>> In 2021, initial work in bitcoin core 22.0 of package acceptance.
>> Continued discussion of the pinning attacks and shortcomings of current
>> mempool rules during community-wide online workshops. Later the year, in
>> light of all issues for bitcoin second-layers, a proposal is made about
>> killing the mempool.
>>
>> In 2022, bip proposed for package relay and new proposed v3 policy design
>> proposed for a review and implementation. Mempoolfullrbf is supported in
>> bitcoin core 24.0 and conceptual questions about alignment of mempool rules
>> w.r.t miners incentives are investigated.
>>
>> Along this year 2022, eltoo lightning channels design are discussed,
>> implemented and reviewed. In this context and after discussions on mempool
>> anti-DoS rules, I discovered this new replacement cycling attack was
>> affecting deployed lightning channels and immediately reported the finding
>> to some bitcoin core developers and lightning maintainers.
>>
>> ## Timeline
>>
>> - 2022-12-16: Report of the finding to Suhas Daftuar, Anthony Towns, Greg
>> Sanders and Gloria Zhao
>> - 2022-12-16: Report to LN maintainers: Rusty Russell, Bastien
>> Teinturier, Matt Corallo and Olaoluwa Osuntunkun
>> - 2022-12-23: Sharing to Eugene Siegel (LND)
>> - 2022-12-24: Sharing to James O'Beirne and Antoine Poinsot
>> (non-lightning potential affected projects)
>> - 2022-01-14: Sharing to Gleb Naumenko (miners incentives and
>> cross-layers issuers) and initial proposal of an early public disclosure
>> - 2022-01-19: Collection of analysis if other second-layers and
>> multi-party applications affected. LN mitigations development starts.
>> - 2023-05-04: Sharing to Wilmer Paulino (LDK)
>> - 2023-06-20: LN mitigations implemented and progressively released. Week
>> of the 16 october proposed for full disclosure.
>> - 2023-08-10: CVEs assigned by MITRE
>> - 2023-10-05: Pre-disclosure of LN CVEs numbers and replacement cycling
>> attack existence to security@bitcoincore.org.
>> - 2023-10-16: Full disclosure of CVE-2023-40231 / CVE-2023-40232 /
>> CVE-2023-40233 / CVE-2023-40234 and replacement cycling attacks
>>
>> ## Conclusion
>>
>> Despite the line of mitigations adopted and deployed by current major
>> lightning implementations, I believe replacement cycling attacks are still
>> practical for advanced attackers. Beyond this new attack might come as a
>> way to partially or completely defeat some of the pinning mitigations which
>> have been working for years as a community.
>>
>> As of today, it is uncertain to me if lightning is not affected by a more
>> severe long-term package malleability critical security issue under current
>> consensus rules, and if any other time-sensitive multi-party protocol,
>> designed or deployed isn't de facto affected too (loss of funds or denial
>> of service).
>>
>> Assuming analysis on package malleability is correct, it is unclear to me
>> if it can be corrected by changes in replacement / eviction rules or
>> mempool chain of transactions processing strategy. Inviting my technical
>> peers and the bitcoin community to look more on this issue, including to
>> dissent. I'll be the first one pleased if I'm fundamentally wrong on those
>> issues, or if any element has not been weighted with the adequate technical
>> accuracy it deserves.
>>
>> Do not trust, verify. All mistakes and opinions are my own.
>>
>> Antoine
>>
>> "meet with Triumph and Disaster. And treat those two impostors just the
>> same" - K.
>>
>

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