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Can a blockchain be hacked or changed?

Published Sep 18, 2026Updated Sep 19, 2026Last verified Sep 18, 2026Requests recorded in this site’s own server logs, with the crawler identity checked by reverse DNS or IP range.Google8Yandex1First collected Sep 18, 2026Recent requestsGoogleSep 21, 2026GoogleSep 20, 2026GoogleSep 20, 2026GoogleSep 20, 2026GoogleSep 20, 2026GoogleSep 20, 2026GoogleSep 19, 2026GoogleSep 18, 2026YandexSep 18, 2026
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Quick answer

Changing past records is hard but not impossible. Each block contains the hash of the previous block, so editing an old transaction means rebuilding every block after it. On a proof-of-work chain, that would take control of most of the network's computing power, known as a 51% attack. On Ethereum's proof-of-stake chain, it would take at least half of the staked ETH, and attackers can lose that stake. Wallets, keys and smart contract code can also be attacked separately.

79 words12 sources

Answer

The Bitcoin developer guide explains how chaining and proof of work make revisions expensive [5]. Ethereum documents slashing and finality [9]. The OECD lists custody of access credentials and security weaknesses as risks .

Sources

  1. governmentIR 8202, Blockchain Technology Overview — National Institute of Standards and Technology (NIST) · accessed Sep 19, 2026 · Evidence page
  2. governmentGlossary: permissioned — NIST Computer Security Resource Center · accessed Sep 19, 2026 · Evidence page
  3. governmentGlossary: permissionless — NIST Computer Security Resource Center · accessed Sep 19, 2026 · Evidence page
  4. governmentGlossary: smart contract — NIST Computer Security Resource Center · accessed Sep 19, 2026 · Evidence page
  5. officialBlock Chain — Bitcoin Developer Guide — Bitcoin Project (developer.bitcoin.org) · accessed Sep 19, 2026 · Evidence page
  6. governmentThe story of Satoshi Nakamoto and the 1 million bitcoins — European Commission, CEF Newsroom · accessed Sep 19, 2026 · Evidence page
  7. otherStuart Haber — Wikipedia · accessed Sep 19, 2026 · Evidence page
  8. officialThe Merge — ethereum.org · accessed Sep 19, 2026 · Evidence page
  9. officialProof-of-stake (PoS) — ethereum.org · accessed Sep 19, 2026 · Evidence page
  10. officialBlocks — ethereum.org · accessed Sep 19, 2026 · Evidence page
  11. officialIntroduction — What is Hyperledger Fabric? — Hyperledger Foundation (Linux Foundation) · accessed Sep 19, 2026 · Evidence page
  12. academicBitcoin electricity consumption: an improved assessment — Cambridge Judge Business School, University of Cambridge · accessed Sep 19, 2026 · Evidence page