[Lightning-dev] Disclosure of a fee blackmail attack that can make a victim loose almost all funds of a non Wumbo channel and potential fixes

René Pickhardt r.pickhardt at googlemail.com
Wed Jun 17 10:29:42 UTC 2020

Hey everyone and of course good morning ZmnSCPxj (:

about 11 months ago I discovered a potential blackmail attack with HTLCs
after answering this question on stack exchange (c.f
This attack is similar to the one that was possible with tx malleability on
the funding transaction without the segwit upgrade (c.f.
Meaning an attacker can force a victim to lose money and use this fact to
blackmail the victim, to potentially gain / steal some of the lost funds.

* Depending on the circumstances this attack allows an attacker to make
channel partners lose a substantial amount of BTC without substantial costs
for the attacker.
* Depending on the exact circumstances this could be for example ~0.15 BTC.
In particular it demonstrates why opening a channel is not an entirely
trustless activity.
* The attacker will reliably only be able to force the victim to lose this
amount of Bitcoin.
* It is not clear how in practice the attacker could gain this amount or
parts of it as this would involve not only game theory but also rather
quick communication between attacker and victim and customized Lightning
nodes which at least for the victim would be unlikely to exist.
* None of the suggested fixes seems to be satisfying though the current
solution of lowering the maximum amount of HTLCs that can concurrently be
in flight seems to be a reasonable start for now.

Timeline on Disclosure
I have disclosed this attack on Sunday July 21st 2019 to Fabrice Drouin
(and shortly after to Christian Decker) in a phone call who in turn has
discussed it with people from the other implementations. From his feedback
I understood that people working on implementations have been more or less
aware of the possibility of this attack. Fabrice also mentioned that he
believed implementations currently try to mitigate this by setting low
limits of allowed / accepted HTLCs in flight. However at that time this was
only true for e-clair. It is now also true for c-lightning and as far as I
know still not true for lnd. Fabrice said that the people he talked to have
suggested that I should eventually describe the attack in public to raise
awareness (also from the group of node operators) for the problems related
to this attack. He also suggested that - if I wanted to - I should update
the rfc with recommendations  and warnings. While I already have in mind
how to change the rfc I wanted to start the discussion first. Maybe some
people find better fixes than just a warning that I have in mind. So far I
didn't do anything because I wanted to also give lnd the chance to handle
the problem.

There are two reasons I disclose this attack today:
1.) I think almost 1 year is enough time to do something about it. The only
implementation that afaik didn't yet is lnd (see below) but I got roasbeefs
ok last week to go ahead and publish the attack anyway so that we can have
a broader discussion on mitigation strategies.
2.) The attack seems actually very similar to the one described in the
"Flood & Loot: A Systemic Attack On The Lightning Network" - paper which
came out 2 days ago (c.f.: https://arxiv.org/abs/2006.08513 ). I believe
any person reading that paper will understand the possibility of the attack
that I describe anyway so I believe it is now more or less public anyway
and thus time for an open / public discussion.

The main difference between the two attacks (if I understand this novel
paper correctly) is: In the "flood and loot"-attack one tries to steal the
HTLC output of the victims. Where in the "flood and blackmail"-attack that
I describe I try to to force the victim to lose almost all its funds due to
high on chain fees (Which I could use to blackmail the victim)

Description of the attack
Let us assume the victim has funded a channel with an attacker meaning it
will have to pay the fees for the commitment transaction in case of a force

During a fee spike (let us assume fee estimators suggest 150 sat / byte)
the attacker spams this channel with the maximum possible amount of HTLCs
that the protocol allows. The HTLCs can be of a small value but need to be
bigger than the dust limit so that additional outputs are actually added to
the commitment transaction which makes it quite large in Bytes. According
to the BOLTs these are 483 additional outputs to the commitment
The direction of HTLCs are chosen so that the amount is taken from the
`to_remote` output of the attacker (obviously on the victims side it will
be the `to_local` output) For the actual attack it does not matter in which
direction the HTLCs are spammed but economically the direction I propose
makes even more sense for the attacker and can be achieved with circular

The attacked channel partner will happily - according to the protocol - use
a higher fee than the current fee rate. I quote from BOLT 02 which suggests
a buffer of a factor of 5

> The node responsible for paying the Bitcoin fee [...] SHOULD send
update_fee to ensure the current fee rate is sufficient (by a significant
margin) for timely processing of the commitment transaction. [...] Given
the variance in fees, and the fact that the transaction may be spent in the
future, it's a good idea for the fee payer to keep a good margin (say 5x
the expected fee requirement); but, due to differing methods of fee
estimation, an exact value is not specified.

This overpayment of fees will result in 750 sat / byte for this fee spike
scenario. This is by the way not completely unrealistic [I recently opened
a channel with 2.56 sat / byte (c.f.:
and the channel was force closed a couple minutes later due to an
implementation bug resulting in fees of 101.17 sat / byte (c.f.:

As far as I understand the appendix of BOLT 03 offered HTLCs are 43 Byte in
size (c.f.:
resulting in the following additional fees for the spammed commitment

fee (for 483 htlcs)  = 483 * 43 byte * 750 sat / byte = 15576750 sat =
0.1557675 BTC

Additionally the victim will also have to swipe all offered HTLCs (which
will be additional costs but could be done once the fees came down) so we
neglect them.

Once all HTLCs are set up the attacker will stop signing commitment
transactions. In the beginning this is not suspicious as the HTLCs will
take some time to settle anyway. But also when fees go down and the victim
who funded the channel wants to update the fees the attacker will just not
be responsive. Eventually the victim will dare to force close the channel
with all those expensive HTLCs.

Knowing that this will happen and that the victim has to spend those funds
(publishing old state obviously does not work!) the attacker has a time
window to blackmail the victim outside of the lightning network protocol:
"Either you will pay those 0.1557675 BTC of fees or we will collaboratively
close the channel but that will cost you part of the amount you lost." Game
theory suggests that the attacker will be able to claim the major fraction
of the BTC (c.f. https://en.wikipedia.org/wiki/Ultimatum_game that are
frozen in tx fees as the victim effectively already has lost that money and
can only gain something back.

You might say that the blackmail part of this attack is unrealistic as the
blackmailing person will not have enough time to successfully do the
blackmail as the channel that is not operational will fail.

1.) The only thing that lightning nodes might do is a fee update as the
htlcs stuffed the channel so I believe there is actually some time to
contact the victim.
2.) What if the attacker is a mining pool who is just interested in high
fees who does not even need to do the blackmailing stuff but will just
force close the channel once the htlcs are set up?
3.) The attacker might not even be interested in blackmailing the victim.
The attacker could just be interested in harming the victim. Though it
might certainly be a challenge to target a specific victim and trick it
into opening a channel with an attacker.

Also you might say that an attacker needs many incoming channels to execute
this attack. This can be achieved by gaming the autopilot. an attacker can
start by creating many channels making him a highly likely channel partner
for autopilot users (who will also fund the channel). Such a highly
connected node might also be interesting for non autopilot users.


I looked at the code myself. I hope I do oversee things but to me it looks
like only eclaire was somehow mitigating this attack from being exploited.
(by a default config of 30 accepted htlcs which will protect the average
user and is much lower than the 483) and c-lightning has merged a patch
from me which I provided after I disclosed the attack:

## clightning:

c-lightning did not by default set a hard cap on htlcs before version 0.7.2
but then merged my patch
https://github.com/ElementsProject/lightning/pull/2858 which tried to
resemble the eclair defaults

## eclaire:

the max accepted htlc value per channel is set as a constant to 483 which
follows the recommendation of the BOLTs:

The value is tested against at:



However the default config file specifies a maximum amount of 30 accepted
htlcs at:

## lnd
It seems like lnd did not and last time I checked (maybe I oversaw
something) does not set a hard cap on htlcs by default. The way how I
understand the code they allow up to 483 htlcs by default:

The test when adding an htlc if it is beyond the maximum accepted values is

the default configuration seems to be here (and in the most recent commit
the line still exists):

the software will accept up to 483 htlcs according to this line:

I could not find any spec lower than that in the suggested sample config

Ideas for Fixes

I am completely unhappy with each of the following ideas. I hope you will
come up with smarter solutions. I believe the solution is not obvious. Thus
I thought it makes sense in the brainstorm phase to even post some ideas
with obvious drawbacks.

1. The current solution is to just not use up the max value of
htlc's. Eclaire and c-lightning by default only use up to 30 htlcs.
2. Probably the best fix (not sure if I understand the consequences
correctly) is coming from this PR to bitcoin core (c.f.
https://github.com/bitcoin/bitcoin/pull/15681 by @TheBlueMatt . If I get it
correctly with that we could always have low fees and ask the person who
want to claim their outputs to pay fees. This excludes overpayment and
could happen at a later stage when fees are not spiked. Still the victim
who offered the htlcs would have to spend those outputs at some time.
3. Don't overpay fees in commitment transactions. We can't foresee the
future anyway
4. Don't add htlcs for which the on chain fee is higher than the HTLCs
value (like we do with sub dust amounts and sub satoshi amounts. This would
at least make the attack expensive as the attacker would have to bind a lot
of liquidity.
5. Somehow be able to aggregate htlc's. In a world where we use payment
points instead of preimages we might be able to do so. It would be really
cool if separate HTLC's could be combined to 1 single output. I played
around a little bit but I have not come up with a scheme that is more
compact in all cases. Thus I just threw in the idea.
6. Split onchain fees differently (now the attacker would also lose fees by
conducting this attack) - No I don't want to start yet another fee
bikeshadding debate. (In particular I believe that a different split of
fees might make the Flood & Loot attack economically more viable which
relies on the same principle)

Independently I think we should have a hint in our readme file about where
and how people can disclose attacks and vulnerabilities. Implementations
have this but the BOLTs do not.

with kind regards Rene



Skype: rene.pickhardt
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