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Good Griefing: A Lingering Vulnerability On Lightning Community That Nonetheless Wants Fixing

01/19/2021

What occurs when your Lightning Community routing node is fed with rubbish transactions that by no means resolve? Briefly, it causes a variety of grief for routing nodes. What was as soon as a easy, international cost system could be locked up with trivial effort from a savvy script author.

Working in a small group of routing nodes, we efficiently ran a check of the assault with actual funds and demonstrated the “griefing” assault described by Joost Jager. The assault is known as a grief assault since it’s not a theft of funds, however it causes a sufferer’s Lightning funds to be frozen: a significant upset. What we discovered is that griefing is a critical menace to giant “wumbo” channels anticipating to earn a yield on their bitcoin, solely to have their funds frozen for a time period. 

That is largely a grief assault: no lack of funds, however the sufferer could also be pressured to pay for an costly channel drive shut. It is a identified vulnerability on mainnet Lightning and it must be understood and prioritized, particularly at this early market stage of Bitcoin’s Lightning Community.

Because of Clark Burkhardt and Phillip Sheppard for his or her willingness to take part on this check and to Jager for his tireless work to deliver consideration and precedence to this vulnerability. Jager performed the position of the attacker for our demonstration, whereas Burkhardt and Sheppard joined me as related sufferer routing nodes.

How The Assault Works

The attacker saturates one (or a number of) channel(s) with Hashed Time Locked Contracts (HTLCs) that don’t resolve as a finalized cost. These are a particular breed of HTLCs often known as HODL invoices. Solely 483 of those unresolved HTLCs are required to overwhelm a channel per path. As soon as these HTLCs are within the channel, any transactions utilizing that very same channel path are unattainable, together with a transaction to cooperatively shut that channel.

In principle, an attacker may contact the sufferer (maybe through a keysend message or in an “onion blob”) and demand a ransom be paid to halt the assault. As soon as the ransom is paid, the attacker may take away the unresolved funds, ending the assault. The assault could be sustained indefinitely, halting all routing and cost exercise in that channel. This freezes the funds within the Lightning channel.

Each instructions of funds could be stalled in a channel by utilizing 483 HTLCs in every path, each inbound and outbound.

Thunderhub view of my balanced channel to Burkhardt underneath assault. The channel exhibits as “Not Lively,” as if Burkhardt had been offline, however he wasn’t. The quantity in blue is the native stability in sats, the quantity in inexperienced is the distant stability in sats owned by Burkhardt. Supply: Thunderhub.

Why Would An Attacker Do One thing Like This?

The primary motive that involves thoughts is to demand a ransom. This assault causes ache for the sufferer and paying a ransom could also be engaging to a sufferer, even with out assurance that the assault would cease. Contacting the sufferer may be dangerous for an attacker, however a ransom cost may not be the one purpose somebody would do that.

A secondary incentive for launching a griefing assault could be to disrupt routing competitors. Jamming a competitor’s route may create extra demand for a route owned by an attacker.

As a benchmark, contemplate that Lightning Labs’ Loop node has an ongoing demand for liquidity for which it can generally pay a 2,500 components per million of the cost (ppm) (0.25 %) price fee. In my expertise, they’d usually exhaust 16 million sats’ price of liquidity in about two weeks (5.2 % annual proportion fee), however that’s with competitors current. 

If an attacker may disable any competing route with decrease price charges, Loop could also be prepared to pay a better price fee (because the provide of liquidity is now lowered). Let’s say Loop would pay 3,000 ppm (0.3 %), in addition to use that liquidity extra rapidly since no different channels are functioning. Loop would possibly use that liquidity in half the time, say one week. The attacker would greater than double their typical yield to fifteen.6 % APR on this instance. The one price to the attacker is the price of operating a script on an present channel and the psychological price of doing one thing immoral/damaging to the Lightning Community. With a single attacker channel, a malicious actor may jam about 9 channels (see Jager’s tweets about this).

What Would The Sufferer Of This Assault Expertise?

The sufferer of this assault wouldn’t actually know that this assault was occurring except they’d some particular alerts set for pending HTLCs. For Thunderhub customers (a extremely beneficial instrument), the house display will present a chart of pending HTLCs in addition to a warning stating that channels can solely maintain 483 pending HTLCs.

Supply: Thunderhub

In follow, my node rapidly turned unreliable and skilled a number of app crashes, together with Thunderhub, which was the one app to inform me of the issue. Then, because of my “Stability of Satoshis” Telegram bot, I acquired a channel closing notification. The channel underneath assault force-closed itself! That was not speculated to be a part of the experiment. (For extra technical info on the involuntary drive shut, see beneath for extra force-close information.)

A check cost utilizing the channel with Burkhardt (salmiak) failed because of the assault. This warning studies that Burkhardt’s node is offline, although it was on-line. Supply: Thunderhub.

What Can The Sufferer Do To Cease A Griefing Assault?

As soon as an assault begins, a sufferer basically can’t do something to cease it. The one options out there to halt an ongoing assault could be to force-close the channel being attacked, which signifies that the terrorists win. 

So as to add insult to damage, force-closing the channel will push the unresolved funds to the on-chain transaction information, triggering secondary on-chain transactions for the initiator of the drive shut. At 50 sats/vbyte and 483 on-chain transactions, that’s simply a 1 million sat price ticket to drive shut a single channel underneath assault (a $368 channel shut price at at the moment’s costs). The a number of on-chain transactions solely happen if the output is above the minimal cost “mud” restrict. (See this instance on testnet.)

How To Stop A Griefing Assault

Jager has been engaged on a proof-of-concept program to assist isolate and battle attackers. He’s calling his program “Circuitbreaker.” The Circuitbreaker works at a community degree, which sadly signifies that everybody has to take part for it to be efficient.

Past that, this problem wants prioritization and a focus from devoted engineers/builders to seek out higher options. There have additionally been some good discussions on modifying the protocol within the Bitcoin Optech e-newsletter (problem #122 or #126).

This assault could be executed at the moment. It’s a miracle that it hasn’t already been used maliciously. It’s a mirrored image of the incentives for these utilizing Lightning at the moment in order that it might develop into an open, common cost community. Please share this submit as you see match to encourage and encourage extra work to repair this downside earlier than it causes actual hurt.

Further Technical Data About The Involuntary Pressure-Shut

Listed below are the logs from my node operating LND 0.11 in the intervening time that the above talked about involuntary force-close occured:

2020-11-26 21:24:47.374 [ERR] HSWC: ChannelLink(657759:561:0): failing hyperlink: ChannelPoint (c37bec006b18df172698a84739ca47128935e0a8666fecd1a843e49b01db207c:0): obtained error from peer: chan_id=7c20db019be443a8d1ec6f66a8e035891247ca3947a8982617df186b00ec7bc3, err=rejected dedication: commit_height=455, invalid_commit_sig=3044022076fd65191eb6305b723fa6012be378413b6326e2786c38db58b4c02e1f3999d202207605ca31de8b4c5b1d9cd20dc1581dfa2383e0b4e06c8ad4f718ab5c434d8cf5, commit_tx=02000000017c20db019be443a8d1ec6f66a8e035891247ca3947a8982617df186b00ec7bc300000000008a792e8002210d0000000000002200201031cf10a1efef261edd3d0a1a6a953b27bc25bd7150bb2b07afdc69805e02157213000000000000160014de650929042bef58b71783ae1a44834a902a8f2d542ca720, sig_hash=4e0fb804c74376020e4c44a60969b9206eb0aaa9a89b76017d60f23ad5cf63e5 with error: distant error

The logs present an “invalid_commit_sig” which is a identified problem in LND. Supposedly, this may occur upon reconnecting and isn’t a direct results of the channel jamming. The quantity of pending HTLCs sadly makes it extra more likely to occur. Jager helped clarify the method as channel jamming –> limitless cost loop (bug) –> node down –> reconnect –> invalid commit sig (bug) –> channel force-close.

The “limitless” loop bug is a identified bug that happens when the HTLC restrict is reached and an extra HTLC is shipped. As a substitute of ending in a cost failure, LND will proceed to aim the cost in a loop. To assist with this bug, see LND problem #4656.

It is a visitor submit by Jestopher. Opinions expressed are totally their very own and don’t essentially replicate these of BTC Inc or Bitcoin Journal.

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