[Lightning-dev] Full Disclosure: CVE-2020-26896 LND "The (un)covert channel"

Antoine Riard antoine.riard at gmail.com
Tue Oct 20 22:05:18 UTC 2020

# Problem

In case of a relayed HTLC hash-and-amount collision with an expected
payment HTLC on the same channel, LND was releasing the preimage for the
later while claiming onchain the former. A malicious peer could have
deliberately intercepted a HTLC intended for the victim node, probe the
victim to learn the preimage and steal the intercepted HTLC.

Prior to v0.11, LND had a vulnerability in its invoice database while
claiming onchain a received HTLC output, it didn't verify that the
corresponding outgoing off-chain HTLC was already settled before releasing
the preimage. In case of hash-and-amount collision with an invoice, the
preimage for an expected payment was instead released. A malicious peer
could have deliberately intercept a HTLC intended for the victim node,
probe the preimage through a colluding relayed HTLC and steal the
intercepted HTLC.

Note that MPP payments (invoices with `s` field`) aren't subject to this
vulnerability as an attacker wouldn't have the payment secret to
authenticate the probe HTLC. As of today, this class of invoices isn’t
well-deployed as it outlaws non-upgraded payers.

This vulnerability was exposing routing nodes also receiving payments (e.g
merchant nodes). A peer servicing as an intermediary hop of a payment path
could have guessed that the next hop is the final receiver by comparing the
HTLC's `amount_msat` with any goods/services advertised by the victim node
merchant website. This is not affecting non-routing nodes.

Note, this is a case of _indirect_ fund loss of which the exploitation
scope depends on application logic. As the invoice state would have been
marked as settled, an application directly subscribing to it might have
automatically released the offered good/service. Otherwise, the loss may
have been characterized if the preimage had a direct liquid value (e.g
atomic data exchange where the preimage is a decryption key).

# Solution

The current spec requirement is the following :

"A local node if it receives (or already possesses) a payment preimage for
an unresolved HTLC output that it has been offered AND for which it has
committed to an outgoing HTLC MUST resolve the output by spending it, using
the HTLC-Success transaction".

This point could be clearer and precise the risk of HTLC hash-and-amount
collision : "MUST NOT reveal a preimage for an incoming HTLC if it has not
learnt the preimage for the claiming of the outgoing HTLC" [0]

I engage the LN ecosystem to consider the mandatory deployment of
`payment_secret`, reducing the surface for this class of bugs, among other

# Background

Alice, a merchant, is sending an invoice for amount A and hash H to Bob.

Bob is routing the HTLC A/H to Alice through Mallory.

Channel topology [0]

                HTLC (A,H')         HTLC (A,H')          HTLC (A,H)
             _____________       _____________          _____________
            /             \     /              \       /             \
           V               \   V                \     V               \
      Mallet <-----------> Alice <------------> Mallory <-----------> Bob
                             \                                        ^

                                          invoice (A,H)

Mallory intercepts HTLC (A,H) from Bob and doesn't relay it forward.
Mallory, knowing that Alice node pubkey is tied to a merchant node, is
browsing through her goods/services index to decide if Bob's HTLC is
intended for Alice by comparing the relayed amount with the price of an

If there is a match, more-or-less some noise of sats, Mallory draws a new
payment path to Mallet, a colluding node. Mallory sends a HTLC (A,H') to
Alice, which relays it forward to Mallet.

Mallet holds the HTLC and doesn't do it immediately. Instead, Mallet routes
back a new HTLC Y to Mallory through Alice. This HTLC Y won't be failed by
Mallory, thus forcing Alice to force-close the channel when HTLC Y
off-chain settlement deadline is crossed.

Alice goes onchain with her commitment for chan Alice-Mallory. She claims
back offered output for HTLC Y and received output for HTLC H'. She reveals
preimage P, with H'=sha256(P) equivalent to H=sha256(P).

Mallory learns preimage P onchain and sends it out-of-band to Mallet which
claims the incoming HTLC H' from Alice. Mallory claims the previously
intercepted HTLC H from Bob.

Bob learns the preimage P.

The coalition Mallet and Mallory gain the value of HTLC H minus Alice's
routing fee for HTLC H'. Also, they may have to pay the channels
opening/closing fees depending on who initiates.

Alice reveals a preimage P which corresponds to a provided good/service
without receiving the payment for it, thus being at loss.

Alice might not have learnt about her loss until reconciling her merchant
inventory with her HTLC accounting, thus this exploitation might have been
stealth for a while.

# Discovery

While working on Rust-Lightning, I observed that the specification was
silent on the risk of hash collision during the onchain operations. I
surveyed deployed Lightning implementations on this behavior. A quick
manual test against LND (65f5119) confirmed this vulnerability, which has
been immediately disclosed to the LND team.

We agreed for a 6-month embargo period, as at that time a LND release
(0.10) was undergoing and it was too late to implement a covert fix in this

If this vulnerability has been exploited, the original sender would have
discovered the preimage, according to the pre-agreed invoice but without
the issuer effectively being paid. In case of legal disagreement if the
corresponding good/service should be settled, and assuming parties were
subject to the same jurisdiction, it could have been an interesting case to
decide if the invoice/preimage pair is legally binding.

# Timeline

2020-04-19: Vulnerability discovered, LND team notified
2020-04-29: lnd v0.10.0-beta released (without fix)
2020-07-31: lnd v0.11-0-beta released (with fix)
2020-10-08: Partial Disclosure, encouraging lnd users to upgrade to lnd
v0.11.x ASAP
2020-10-09: CVE assigned (
200-10-15: Preventive Disclosure to c-lightning, Electrum and Eclair teams
2020-10-20: Full Disclosure

[0] See gist if it doesn’t survive ML formatting:
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