LDK’s fix blocks a funding-location probe that required connections to both endpoints of an unannounced channel.

Lightning Development Kit (LDK), software used to build Bitcoin Lightning applications, has a privacy fix that blocks a documented probe linking an unannounced channel to the Bitcoin output backing it. The probe exploited how LDK forwarded onion messages, turning a guessed channel identifier into confirmation of a connection between two suspected nodes.

Bitcoin Optech highlighted the change on October 30. An unannounced channel stays out of the public network map, but that alone did not prevent LDK’s message-forwarding lookup from confirming its funding link.

A real short-channel ID, or SCID, works like blockchain coordinates. The Lightning specification defines it using the block height, the transaction’s position within that block and the output’s position within that transaction. Together, those identify the particular Bitcoin output funding a channel.

An attacker can derive a candidate ID from an output’s location without yet knowing whether it belongs to a channel between the suspected nodes. The documented probe supplies that missing confirmation. It requires the attacker to connect to both endpoints and construct an onion-message path through them that returns to the attacker.

Before the fix, LDK’s channel lookup could resolve a real SCID even for an unannounced channel once its funding confirmed. If the message returned successfully, the attacker could confirm that the guessed output and the channel between those nodes matched.

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