Pinning Attacks

Pinning = an attacker publishes a tx that blocks your tx from confirming by attaching as a descendant chain that exhausts mempool descendant limits or replacement-fee budgets.

Classic pinning

You publish parent P (commitment, HTLC-timeout, etc.). Attacker publishes child C spending P's output:

• C has very low fee rate but has 24+ unconfirmed ancestors/ descendants of its own (Tx graph).
• Mempool admits C; descendant chain reaches policy limits.
• Now ANY new descendant of P is rejected: descendant limit exceeded.
• You can't CPFP P; you can't RBF (your replacement would conflict with C, but BIP125 rule 5 makes it economically prohibitive).

Result: your honest tx P is "stuck" until mempool eviction (which might never happen if attacker keeps refilling).

Lightning impact

In Lightning context:

• After force-close, you publish HTLC-timeout to claim funds before CLTV expiry.
• Attacker pins HTLC-timeout's anchor / output → can't bump.
• CLTV expires upstream (Alice's HTLC times out before you can claim Bob's).
• You lose the in-flight HTLC.

Mitigation 1: Anchor outputs (option_anchors, BOLT-3, ~2020)

Each commitment tx gets two 330-sat anchor outputs, one keyed to each party's funding key:

• Spendable immediately by that party.
• Spendable by anyone after 16 blocks (UTXO-set hygiene).

You can spend your anchor with a high-fee child to CPFP the commitment. Defeats simple pinning of commitment. Two separate keyed anchors are what stops a peer from monopolising the descendant limit on a shared fee-bump output.

But: HTLC-timeout txs themselves still vulnerable. And because every non-anchor output is CSV-locked under option_anchors, the anchor is the only CPFP handle, so its child needs an external wallet UTXO.

Mitigation 2: TRUC v3 + P2A / ephemeral dust (2024-2025)

• TRUC v3 (BIP431): ≤ 1 unconfirmed ancestor, ≤ 1 unconfirmed descendant, ≤ 10,000 vB per TRUC tx, ≤ 1,000 vB for a TRUC child that has an unconfirmed ancestor, plus sibling eviction. Standard policy since Bitcoin Core 28.0 (October 2024).
• P2A (BIP433): the keyless anchor script OP_1 <0x4e73>, spent by anyone with an empty witness. Standard output type since Core 28.0; standard dust limit 240 sat.
• Ephemeral dust (Core 29.0, April 2025): a tx may carry one dust output provided the tx pays zero fee and that output is spent in the same package.

"Ephemeral anchors" was the 2023 working name for this idea. What actually shipped is the P2A output type plus the ephemeral-dust rule, so prefer those terms. BIP431 and BIP433 are both still Status: Draft as of September 2026 — the policy shipped as standardness in Core, the BIPs themselves were never finalised.

Combined: pinning surface is removed. Attacker can't keep stacking descendants; honest party can sibling-evict bad children cheaply.

Mitigation 3: Package submission and 1p1c relay

submitpackage (RPC, since Bitcoin Core 26.0, December 2023) lets you submit parent + your CPFP child atomically to your own node. Mempool computes effective package fee rate. No race against attacker.

Over the wire, Core 28.0 (October 2024) added opportunistic 1-parent-1-child (1p1c) relay, carried on the existing tx-relay protocol. BIP331 "Ancestor Package Relay" is still Status: Draft and has never been deployed — its sendpackages / ancpkginfo / pkgtxns messages do not exist in the Bitcoin Core source tree as of September 2026. Do not design around a negotiated package-relay protocol on the wire; design around 1p1c plus local submitpackage.

Mitigation 4: Zero-fee commitments (zero_fee_commitments, 2026)

Merged into the BOLT spec on 2026-05-04 (lightning/bolts PR #1228, BOLT-9 feature bits 40/41). This replaces the anchor + update_fee model rather than tuning it:

• Commitment tx version is 3 (TRUC), so commitment + one child form a TRUC package under TRUC's topology limits.
• feerate_per_kw is fixed at 0: commitment, HTLC-timeout and HTLC-success all have base fee 0, and update_fee is never sent.
• The two keyed anchors are replaced by a single shared_anchor: the standard P2A script OP_1 <0x4e73>, spendable by anyone with an empty witness. Its amount is the sum of trimmed outputs plus rounded-down msat; below the 240-sat P2A dust limit that is legal only under the ephemeral-dust rule, which applies precisely because the parent pays no fee. Above 240 sat the anchor caps at 240 and the remainder becomes a real mining fee.
• The 1-block CSV is scoped to option_anchors, so here to_remote is a plain P2WPKH. You can CPFP a remote commitment by spending your own main-balance or HTLC output — no external wallet UTXO needed.

Anti-pinning consequence: a single keyless anchor is safe here only because of TRUC. With at most one unconfirmed descendant and sibling eviction, an attacker's child can always be replaced by a higher-feerate sibling, so there is nothing to monopolise — which is why the per-party keyed anchors of Mitigation 1 are no longer needed. Requires a relay path accepting v3/TRUC (Core 28.0+) and ephemeral dust (Core 29.0+).

State as of September 2026

• option_anchors + TRUC v3 + P2A/ephemeral dust + 1p1c package submission → robust against known pinning variants.
• zero_fee_commitments is the current end-state design, merged into the BOLT spec but shipped in only two implementations so far (see below).
• Legacy non-anchor channels still exist; gradual migration. Eclair v0.14.0 (2026-05-21) dropped support for channels that do not use anchor outputs.
• TRUC/v3 is mempool policy, not a consensus rule, so there is no "activation" to key channel defaults off — what matters is whether your relay path runs Core 28.0+ (and 29.0+ for ephemeral dust).

Implementation status (as of September 2026)

LDK's 0.2 release notes flag that 0FC channels currently only interoperate with LDK peers and that feature signalling may still change. Do not cite a 0.0.1xx LDK version as the floor: the 0.0.x series ended at 0.0.125 (2024-10-14) and was succeeded by 0.1.0 (2025-01-16).

Common confusions

• Pinning vs replacement cycling — both attack mempool policy but differently:
  • Pinning: block your tx from confirming.
  • Cycling: drain your bumping budget while attacker's tx sits.
• Pinning vs channel jamming — different layers (on-chain vs off-chain).
• Anchor outputs mitigate commitment pinning but not necessarily HTLC pinning. Full fix: TRUC v3 commitments with the single shared P2A anchor (zero_fee_commitments).
• BIP331 vs 1p1c relay — BIP331 is an undeployed Draft; 1p1c over ordinary tx relay is what Core actually ships.
• Two keyed anchors vs one shared_anchor — option_anchors needs one anchor per party because descendant limits are shared; zero_fee_commitments needs only one keyless P2A because TRUC caps the parent at one evictable descendant.

See also

• replacement-cycling/SKILL.md
• channel-jamming/SKILL.md
• channels/SKILL.md
• ../../protocol/package-relay/SKILL.md
• ../../protocol/transactions/quick-ref/v3-truc.md