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Bitcoin Core discloses CVE-2025-62861: TRUC sibling-eviction bug halted block templates

Patched in October 2025 but kept embargoed until Nov 10, 2026. Affected 28.0–28.2 and 29.0–29.1. A reorg could jam mining nodes; two medium bugs disclosed alongside.

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Bitcoin Core published three CVE advisories on November 10, 2026: CVE-2025-62861 (high), CVE-2025-62859 (medium) and CVE-2025-62860 (medium). All three were fixed thirteen months earlier in 30.0 (Oct 10, 2025) and 29.2 (Oct 14, 2025), with CVE-2025-62861 also backported to 28.3 (Oct 17, 2025). The embargo lifted a fortnight after the last vulnerable series (29.x) reached end of life on Oct 27, 2026. The Block's Nov 11 write-up is the first secondary coverage.

The high-severity bug (CVE-2025-62861)

Reported by Niklas Goegge on the Bitcoin Core security list on Sept 26, 2025. Fixed in PR #33504 by Greg Sanders, merged into master on Oct 2, 2025. Affected versions: 28.0–28.2 and 29.0–29.1.

The defect lives in TRUC (v3) transaction policy, added to relay policy in v28.0:

  • TRUC rules cap an unconfirmed parent at one unconfirmed descendant. Sibling eviction allows a new child to replace an existing sibling when it pays more under RBF rules.
  • During a chain reorganization, disconnected block transactions are re-added to the mempool with bypass_limits=true. Policy checks are supposed to be skipped on that path.
  • The bug: the sibling-eviction branch was not gated on bypass_limits. A reorg could therefore evict a TRUC transaction while its descendants stayed in the mempool, leaving entries that spent outputs no longer present.

The inconsistent state fails block-template validity checks. In the advisory's wording, "a node that performed such a reorg cannot produce blocks until it is restarted." The pool itself comes back consistent on restart; the fix gates the eviction on bypass_limits.

The two medium bugs

CVE-2025-62859 — witness-malleation block-processing stall. Reported by Eugene Siegel in April 2025. A peer announcing a compact block with a malleated witness can delay block processing by up to 10 minutes on the victim node. Affects the same 28.x / 29.x window.

CVE-2025-62860 — inbound Tor connections inheriting -whitelist permissions. Reported September 2025. Nodes using the non-default -whitelist flag could grant DoS/privacy exemptions to arbitrary inbound Tor peers. Bitcoin Core notes that the Tor-reachable mainnet surface at the time was in the "thousands" of nodes.

Numbers

- CVEs disclosed             : CVE-2025-62861 (high)
                               CVE-2025-62859 (medium)
                               CVE-2025-62860 (medium)
- Fixed versions             : 30.0 (Oct 10, 2025)
                               29.2 (Oct 14, 2025)
                               28.3 (Oct 17, 2025, 62861 only)
- Public disclosure          : 2026-11-10
- Embargo length (62861)     : ~13 months
- Last vulnerable EOL        : 29.x, Oct 27, 2026
- CVE-2025-62861 PR          : bitcoin/bitcoin#33504 (Greg Sanders)
- CVE-2025-62861 reporter    : Niklas Goegge (Sept 26, 2025)
- CVE-2025-62859 reporter    : Eugene Siegel (April 2025)
- CVE-2025-62859 impact      : block processing stall up to 10 min
- CVE-2025-62860 scope       : nodes running -whitelist with Tor

What to watch

  1. Operators still running 28.x or 29.x. 29.1 and earlier are now EOL and have no further security backports. Any mining pool, custodian or service that pinned a sub-30 release because of operational constraints is exposed to CVE-2025-62861 and should move to 30.0 (preferred) or 28.3 (the final 28.x) before the next reorg of material depth.
  2. Mining-pool restart playbooks. 62861's symptom is that block-template assembly silently fails after a reorg until the node restarts. Pools that monitor template-generation latency rather than restart their vote-nodes on reorg detection should check that their alerting catches the "template build fails validity check" path.
  3. Downstream forks and reimplementations. Knots, libbitcoin-style reimplementations and any client that cherry-picks TRUC sibling eviction without the bypass_limits gate inherit the same bug. Review of PR #33504 is the minimum due diligence.
  4. -whitelist configurations. Services whitelisting trusted peers over Tor should audit whether that intent still holds on pre-30 nodes; the 62860 fix only lands at upgrade.

Context

Bitcoin Core's coordinated-disclosure policy, formalised since the May 2023 publication of the historical CVE backlog, aims to let unpatched versions go end-of-life before an advisory lands. 62861 is the first high-severity advisory published under that policy where the fix was in master and shipped a full year before disclosure. The 13-month gap is on purpose — it buys time for pools, custodians and exchange hot wallets to drift forward through normal release cadence before an attacker learns the specific exploit shape. It only works if those operators actually upgrade; running 29.1 today leaves a public, documented reorg-based DoS on a mining pipeline.

TRUC itself is also the mechanic underneath the ephemeral-anchor channel-closing patterns Lightning Labs and Core Lightning have been building against since v28.0. CVE-2025-62861 does not touch TRUC's consensus-level behaviour — this is a mempool-side policy bug — but it is the first production-impacting defect in the TRUC code path to get a CVE.

Sources:

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