---
title: "Bitcoin Core discloses low-severity undo file overflow bug and releases fixes"
description: "Bitcoin Core disclosed a low-severity bug in which a miner could push an undo file past 4 GiB and corrupt undo data on other nodes, causing them to shut down…"
language: en
canonical: "https://whycoinmoves.com/en/events/bitcoin-undo-file-overflow-under-extreme-f1ab709d188592"
published: 2026-10-28T18:18:22.000Z
updated: 2026-10-28T19:19:59.000Z
indexable: true
---

# Bitcoin Core discloses low-severity undo file overflow bug and releases fixes

Bitcoin Core disclosed a low-severity bug in which a miner could push an undo file past 4 GiB and corrupt undo data on other nodes, causing them to shut down…

Attributed sources: [Bitcoin Core](<https://bitcoincore.org/en/2026/10/28/disclose-undo-overflow/>)

## Key facts

Bitcoin Core disclosed a low-severity bug in which a miner could push an undo file past 4 GiB and corrupt undo data on other nodes, causing them to shut down if a reorg of the right blocks followed. Fixes for 64-bit systems shipped in Bitcoin Core versions 29.5, 30.4, 31.2 and 32.1; 32-bit systems and nodes storing blocks on FAT32 remain affected.
Evidence strength: Official source, 1 publisher group(s) (Bitcoin Core).
After site detection, observed returns were 5 minutes: +0.07% (Oct 28, 2026, 18:18 → Oct 28, 2026, 18:23, UTC; Gate · USDT); 1 hour: +0.41% (Oct 28, 2026, 18:18 → Oct 28, 2026, 19:18, UTC; Gate · USDT)
In the preceding 30 days, the 99th percentile of absolute hourly returns was 1.01% (OKX · USDT; cutoff Oct 28, 2026, 18:18, UTC).

## Returns after site detection

| Window | Return | BTC | Difference from BTC | Actual sampled interval (UTC) | Source |
| --- | --- | --- | --- | --- | --- |
| 5 minutes | +0.07% | +0.07% | 0.00 pp | Oct 28, 2026, 18:18 → Oct 28, 2026, 18:23 | Gate · USDT |
| 1 hour | +0.41% | +0.41% | 0.00 pp | Oct 28, 2026, 18:18 → Oct 28, 2026, 19:18 | Gate · USDT |

## Historical hourly baseline

| Metric | Value |
| --- | --- |
| Coverage | 100.0% (720/720) |
| Median absolute hourly return | 0.14% |
| 99th percentile of absolute hourly returns | 1.01% |
| BTC beta | 1.00 |
| Source / cutoff (UTC) | OKX · USDT / Oct 28, 2026, 18:18 |

## Claims and open questions

- The project said a miner can produce valid blocks whose undo data is much larger than the blocks themselves, enough to push a single undo file past 4 GiB, and that exploiting this on 64-bit systems is impractical, requiring several thousand blocks of nonstandard transactions, roughly a month of the entire network's block production.
- Bitcoin Core said a fix for 64-bit systems was released in versions 29.5, 30.4, 31.2 and 32.1, storing block and undo file positions as 64-bit integers while leaving the on-disk format unchanged.
- The project said 32-bit systems remain affected because their positioning calls are limited to 2 GiB, and nodes whose blocks directory is on FAT32 remain affected because that file system caps files at 4 GiB.
- Whether any node has actually shut down from this bug, and whether an official statement or on-chain data would confirm exploitation, is not stated.
- When patched releases for 32-bit systems or FAT32 storage will be issued is not specified.

## Source evidence

- [Undo file overflow under extreme conditions](<https://bitcoincore.org/en/2026/10/28/disclose-undo-overflow/>) — Bitcoin Core; published Oct 28, 2026, 04:00; received Oct 28, 2026, 18:18 (UTC).


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- [Full coin archive](<https://whycoinmoves.com/en/coins/bitcoin>)

## Research for this archive

- [Altcoin beta to Bitcoin: BTC, ETH, SOL and DOGE hourly returns](<https://whycoinmoves.com/en/research/hourly-returns-and-bitcoin-beta.md>)
- [Will Bitcoin go back up? Every crash since 2011 and its recovery time](<https://whycoinmoves.com/en/research/bitcoin-crash-recovery-time.md>)
- [Perpetual funding rates and annualized yield: a 2025 comparison](<https://whycoinmoves.com/en/research/funding-rates-versus-investor-returns.md>)
- [Bitcoin halving and bull markets: evidence from four halvings](<https://whycoinmoves.com/en/research/bitcoin-halving-and-market-cycles.md>)

Evidence levels describe source coverage, not the probability that a report is true. Price moves and their timing cannot establish a cause; minute samples show an observed interval and alert thresholds are heuristics. Explanations are model-assisted, with citations and figures checked. Source titles stay in their original language; full articles are not republished. [Methods and limitations](<https://whycoinmoves.com/en/methodology>)