Bitcoin Core developers have disclosed a high-severity bug that could stop affected nodes from building new blocks. The flaw surfaced during chain reorganizations, corrupting the node’s mempool.
Fortunately, developers fixed the problem over a year ago, but older nodes may still face risk.
How a Chain Reorg Could Break Bitcoin Block Building
A block template is the candidate block that a node prepares for miners to work on. Without it, a mining node cannot produce new blocks.
On November 10, Bitcoin Core published the advisory for CVE-2025-62861. These are the key details:
- Affected versions: Bitcoin Core 28.0 to 28.2 and 29.0 to 29.1.
- Root cause: A rule called sibling eviction misbehaved during chain reorganizations.
- Glitch: Nodes could drop a restored transaction but keep its descendants.
- Result: Leftover transactions spent funds that no longer existed.
The problem involved TRUC transactions, a newer type that allows only one unconfirmed child. Under sibling eviction, introduced in version 28.0, a second child paying higher fees can replace the first.
However, that logic failed when nodes restored transactions from disconnected blocks. In simple terms, the node built its next block on a broken foundation.
Who Was at Risk From the Bitcoin Core Bug?
Mainly, miners and pool operators running vulnerable versions. Once the mempool became inconsistent, block template creation failed its validity check.
Consequently, the affected node could no longer produce blocks. The only fix was a restart, which forced the node to rebuild its mempool from scratch.
Security researcher Niklas Goegge reported the flaw responsibly on September 26, 2025. Shortly after, developer Greg Sanders submitted the fix.
Patched releases followed in October 2025 with versions 30.0, 29.2, and 28.3. Bitcoin Core published the details only after the last vulnerable 29.x version reached end of life. That is why developers typically delay details until most nodes have updated.
Notably, the same disclosure batch included two other bugs fixed in version 30.0. One could stall block processing, while another wrongly granted permissions to inbound Tor connections. The disclosure also follows an October batch of three other bugs, which an AI-powered scanner called Project Loupe flagged.
Therefore, operators still running versions 28.0 through 29.1 should upgrade immediately. Restarting an affected node also restores a clean mempool.
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