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Failure to reject option_scid_alias on an announced channel

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
52/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Tranquilla
Stack tecnologico
scala
Ambito
payments

Direzione di ricerca

Start with the channel-open validation in sanityCheckNonInitiator, validateParamsSingleFundedFundee and validateParamsDualFundedNonInitiator, and check how channelFlags.announceChannel and the option_scid_alias bit in channel_type are read there. Confirm the report against the BOLT 2 open_channel rule before changing anything, since the issue notes the spec is silent on the acceptor's behaviour. Done means the fundee's handling of this combination is decided and tested, which probably needs a maintainer decision on reject versus accept first.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

BOLT 2 forbids the sender of open_channel from setting the option_scid_alias bit in channel_type when announce_channel is true:

The sending node:

  • if it includes open_channel_tlvs:
    • MUST set channel_type:
      • if announce_channel is true (not 0):
        • MUST NOT send channel_type with the option_scid_alias bit set.

Eclair enforces this on itself as opener, but it does not reject the combination on receipt. Neither sanityCheckNonInitiator nor validateParamsSingleFundedFundee/validateParamsDualFundedNonInitiator checks the option_scid_alias bit against channelFlags.announceChannel, so the fundee happily replies with accept_channel/accept_channel2 echoing back option_scid_alias on a channel it will then announce. The spec does not specify rules for the acceptor in this case, so accepting it is not itself a violation, but the resulting channel is internally inconsistent.

Impact

Both single and dual funded channels are affected. The effect is that both can exchange announcement_signatures and publicly announce the channel using its real short_channel_id, since announcement is gated purely on announceChannel. However, per BOLT, when the agreed channel type has option_scid_alias set, a node MUST NOT allow incoming HTLCs to that channel using the real short_channel_id -- and eclair implements exactly that: it drops the real scid from the routing scids. So if the peer returns its signatures, eclair ends up announcing a channel to the whole network and then refusing every HTLC (addressed to that short_channel_id) it is asked to forward to the peer, which only affects the peer.

Only forwards towards the peer are affected, inbound HTLCs still work. For eclair, this means gaining some bad reputation for unnecessarily failing HTLCs. Other than that, this is purely a spec-compliance issue.

Discovery

This bug was found while fuzzing the funding protocol with smite.

Lingua principale
Scala
Stelle
1.3k
Fork
285
Merge medio
20h 22m
PR unite (30g)
28

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