What the developer wrote
Authored by f321x
LNWallet: fix fee budget check regression
The fee budget during local pathfinding route creation incorrectly
compared the full payment amount against the full budget on every
route instead of the partial amount against its fraction of the budget.
This is a regression introduced by df5c8c4c9 and makes it possible
for a payment to exceed the budget or a route to be incorrectly considered
too expensive.
What changed, and why it matters
This commit fixes a bug in Electrum's Lightning Network payment routing. When splitting a payment into multiple parts, the app was checking each partial route's fees against the entire payment's fee budget instead of that part's fair share. This could cause a payment to silently overspend on fees, or wrongly reject a valid route as too expensive. The fix makes each partial route responsible only for its proportional share of the fee budget.
Users relying on Lightning payments should upgrade to a version containing this commit. Review related multi-part payment tests and consider adding regression tests for fee-budget checks with number_parts > 1.
Security signals we found
Fee-budget logic regression in payment routing
Multi-part Lightning payment fee overpayment possible
Incorrect comparison of full amount/budget against partial route
Potential payment failure or excessive fee acceptance
Evidence from the diff
In electrum/lnworker.py, LNWallet’s local pathfinding compared the full payment amount (amount_msat) and full budget against each candidate route during multi-part payments. The regression, introduced by df5c8c4c9, is corrected by passing part_amount_msat and a budget with fee_msat divided by sc.config.number_parts(). This aligns the per-route fee-budget check with the actual partial amount being routed.
Changed components
electrum/lnworker.pyLNWallet local pathfindingLightning Network payment routingFeeBudgetExceeded handlingInspect captured patch +2 / −2
diff --git a/electrum/lnworker.py b/electrum/lnworker.py
index db5d29f..9681951 100644
--- a/electrum/lnworker.py
+++ b/electrum/lnworker.py
@@ -2524,8 +2524,8 @@ class LNWallet(Logger):
full_path=full_path,
))
if not is_route_within_budget(
- route, budget=budget,
- amount_msat_for_dest=amount_msat,
+ route, budget=budget._replace(fee_msat=budget.fee_msat // sc.config.number_parts()),
+ amount_msat_for_dest=part_amount_msat,
cltv_delta_for_dest=paysession.min_final_cltv_delta):
self.logger.info(f"rejecting route (exceeds budget): {route=}. {budget=}")
raise FeeBudgetExceeded()
Why this scored 63/100
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