How Expiring HTLCs and Force Closes Put Lightning Network Funds at Risk

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Hash Time-Locked Contracts (HTLCs) are conditional payment outputs used in Bitcoin's Lightning Network, governed by real blockchain block heights that enforce strict deadlines. When routing payments, a node simultaneously holds an incoming and outgoing HTLC at different expiry heights, creating a narrow safety window to act before funds are lost. If a downstream peer fails to reveal a payment preimage in time, the routing node must broadcast its commitment transaction on-chain — a process known as a force close — before its own upstream expiry deadline passes. BOLT 5 protocol rules explicitly require nodes to initiate this on-chain action when an outgoing HTLC's expiry deadline is not met, leaving no room for negotiation. Because Bitcoin block times are probabilistic and fee delays can slow confirmation, the practical safe window for action is significantly tighter than the raw CLTV delta values suggest.

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