Core Lightning patch fixes revoked-channel penalty bypass

Core Lightning fixed a bug in release v26.06.7 that could allow a peer to broadcast an old, revoked Lightning channel commitment without triggering the normal cheating penalty. The project released a follow-up update, v26.06.8, and published source and test changes in the weeks after the initial fix.

The problem was in how Core Lightning classified closing transactions. When a counterparty broadcasts a revoked commitment, the other side can normally claim a penalty. Before the repair, the software could misidentify certain funding spends as cooperative closes if the transaction outputs matched shutdown scripts already on record. The project added checks of a transaction’s locktime and sequence encoding so the client recognizes a commitment transaction before using outputs to decide whether a close is mutual.

Exploitation required a specific sequence of channel messages. If a channel opened without an upfront shutdown script, a peer could later include the revoked commitment’s output script in a shutdown message, abandon the cooperative-close flow and then broadcast the old commitment. Matching outputs alone previously could make the old transaction appear legitimate and bypass the penalty path. The project’s regression test and release notes describe this possible route but do not report any confirmed cases of funds being taken by the flaw.

The initial fixed release, v26.06.7, was published on Aug. 28. The project posted the release source on Sept. 11 and merged related follow-up changes into the main branch on Sept. 15. A security release, v26.06.8, followed on Sept. 22 and includes the revoked-close repair plus additional fixes and immediately available source. An explanatory write-up published on Sept. 25 clarified the practical risk to operators still running older builds.

Operators who used Docker images during the early rollout should verify the image digest. Images served under the v26.06.7 tag and related tags between Aug. 28 and Sept. 1 reported the new version at startup but did not contain the fixes. The project published corrected digests and instructed users with mismatched digests to re-pull images. Running a vulnerable binary or an image with the wrong digest could leave a node exposed to the described edge-case exploit, though the conditions needed to trigger it are specific to certain channel setups.

The flaw is limited to Core Lightning’s channel-handling logic and does not change Bitcoin’s base-layer rules. The project recommends updating any Core Lightning build older than v26.06.7, moving to v26.06.8 where possible, and following the release notes to verify Docker image digests and binaries. Background documentation on Lightning channel commitments explains that peers replace earlier commitments as balances change and that penalty mechanisms are used to discourage broadcasting revoked states; the recent fixes restore the client’s checks for distinguishing commitment transactions from cooperative closes.