A flaw in older Lightning Terminal software program may mark a Bitcoin Lightning bill paid after the fee was cancelled and returned to its sender, Lightning Labs disclosed on Sept. 21, 2026.
A service provider counting on that bill standing may launch items or credit score with out receiving funds. The corporate describes that threat however offers no tally of precise service provider losses.
The problem was a mismatch between the software program’s bill file and the fee’s consequence. A Lightning fee makes use of a hashed time-locked contract (HTLC) to hold funds.
On this case, the HTLC was canceled on the community and returned to the sender, whereas the receiving node nonetheless recorded the bill as settled. The advisory doesn’t describe a failure of Bitcoin’s base chain.
Lightning Terminal bundles tapd, software program for Taproot Belongings, with the lnd Lightning node. Within the affected setup, tapd enabled its bill interceptor and handled any HTLC carrying customized wire data as an asset fee.
Some sender implementations added an experimental endorsement file even to bizarre BTC funds, inflicting tapd’s strict-forwarding rule to instruct lnd to cancel the HTLC set. The set off didn’t require the service provider to have any open asset channels.
The second defect sat in lnd. When an interceptor canceled the HTLC set, affected variations canceled the fee on the wire however nonetheless marked its bill as settled within the database.
That meant the error stood in one other consumer of lnd’s HtlcModifier interface that canceled an HTLC set may produce the identical mismatch. Lightning Labs charges the vulnerability excessive severity as a result of a false paid standing may lead an operator or fee service to launch worth in opposition to a fee that by no means accomplished. Based on the advisory, the sender’s funds weren’t in danger.
Fixes shipped earlier than disclosure
Lightning Terminal v0.15.0-alpha bundles fixes for each defects. Lightning Labs lists earlier Terminal variations as affected, together with taproot property via v0.5.0 and lnd 0.18.4-beta via 0.18.5-beta.
The tapd set off was fastened in v0.5.1 on Feb. 12, 2025. Terminal v0.14.1-alpha included that repair, however its bundled lnd model nonetheless had the underlying invoice-state defect.
lnd v0.19.0-beta fastened the accounting error on Could 22, 2025. The September 2026 advisory due to this fact disclosed a vulnerability whose related fixes had shipped in 2025. For Terminal operators who can’t replace and don’t have any asset channels, Lightning Labs identifies --taproot-assets-mode=disable as a method to keep away from the noticed tapd set off.
