This Is Why Some Bitcoin Core Nodes Can Stop Building Blocks

Bitcoin Core has published a high-severity advisory disclosing a bug that could prevent affected nodes from creating block templates after a blockchain reorganization.

While the flaw didn’t change Bitcoin’s consensus rules, it could leave a miner unable to produce blocks until the node was restarted.

How a Reorg Can Leave the Mempool Broken

The advisory, CVE-2025-62861, is about an inconsistency involving version 3 transactions, known as TRUC, and Bitcoin Core’s mempool, with the affected releases being versions 28.0 to 28.2 and 29.0 to 29.1.

Under BIP-431, TRUC transactions face extra mempool restrictions where each can only have one unconfirmed ancestor (earlier transactions) and one unconfirmed descendant (later transactions). It should be noted that these are relay policy rules and not consensus requirements, meaning valid blocks can have arrangements that the mempool rejects.

“When a transaction submitted to the mempool would give a TRUC parent a second unconfirmed child, Bitcoin Core’s ‘sibling eviction’ logic (introduced in v28.0) treats the existing child as a replacement candidate and, if the new transaction pays enough, evicts it under replace-by-fee rules,” read the advisory.

According to the Bitcoin Core team, the above flaw usually appears during a chain reorganization. This is when recently confirmed blocks get disconnected from the chain, and the transactions within them have to be put back into the mempool so that they can be reconfirmed again later using “bypass_limits=true.” This flag is meant to faithfully restore the transactions without applying mempool anti-spam limits or topology restrictions.

The bug was that the sibling-eviction branch was not gated on “bypass_limits,” meaning that while transactions were being re-added, TRUC enforcement was still running. As such, a reorganization could restore a transaction arrangement that, while valid in a block, violated TRUC’s mempool limits.

It means that one of the restored transactions could be kicked out, with the ones remaining now spending outputs that were no longer available, thus leaving the mempool inconsistent. And when that happened, the affected node’s block template failed its validity check, and a miner using that node could not produce blocks until they restarted it.

Bitcoin at the Market

At the time of writing, BTC was trading below $74,000, having lost slightly more than 1% of its value in 24 hours, although the damage was much worse across longer timeframes, including a 6% dip in the last 7 days and a nearly 10% plunge over two weeks.

The weakness came in the wake of the release of October’s US CPI data, which ended up being hotter than expected, as headline inflation rose by 0.4% and core CPI went up 0.3%. It has contributed to the OG cryptocurrency dropping nearly 12% from where it was a month ago and 30% lower than its price around the same time last year.

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