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What is immutability in blockchain?
Immutability is the property that once a transaction is recorded deep in a blockchain, it is practically impossible to change or delete. Altering an old entry would mean redoing the work behind every later block and outpacing the honest network, so the history stays as written.
Where people disagree
Supporters say a record no one can quietly rewrite is what makes a blockchain useful: balances, ownership and history can be relied on without an administrator, and payments cannot be clawed back after delivery. The Bitcoin whitepaper presents transactions that are computationally impractical to reverse as protection for sellers against fraud.
Critics say a ledger that cannot be corrected leaves fraud victims and mistakes without remedy and clashes with legal rights. A European Parliament study by Michèle Finck finds blockchains' tamper-evident design hard to square with EU data-protection rights to rectification and erasure, and notes the history can be changed anyway if enough participants collude.
What it means for you
A confirmed transfer to a wrong address or a scammer stays on the record; there is no undo button and no one to petition. Each additional block makes reversal harder, which is why recipients wait for several confirmations on large payments. Anything you put on-chain, including messages or links tied to your address, can stay public indefinitely.
A common mistake: “Blockchains are immutable, so nothing on them can ever change.”
In fact: The record is very hard to change, not impossible. Enough mining power could rewrite recent blocks, and a community can change the rules by fork, as Ethereum's did in 2016 after the DAO hack.
How it works
The whitepaper describes a chain of proof-of-work 'forming a record that cannot be changed without redoing the proof-of-work'. To modify a past block, an attacker must redo that block and all after it, then overtake the honest chain; the probability of a slower attacker catching up falls exponentially as blocks are added. Immutability is therefore probabilistic and social, not absolute: a 2019 European Parliament study calls the word 'misleading' because participants can collude to change the ledger in extraordinary circumstances, and Ethereum's 2016 DAO fork moved funds by changing the rules.
Sources: Bitcoin whitepaper (Satoshi Nakamoto), European Parliament (EPRS/STOA): Blockchain and the General Data Protection Regulation (Finck, 2019), ethereum.org: History of Ethereum (DAO fork) · checked 4 October 2026
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