Can the DAO hardfork be viewed as a (a publicly planned) 51% attack on the network?

Motivation / ethics / justice aside, I'm asking this from a purely technical point of view.

If not, what specifically differentiates it from a "typical" 51% attack?

If so, is this the first ever successful 51% attack on a major blockchain / cryptocurrency? By "successful" I guess I mean having a notable impact, in the short term at least.

2 Answers 2


The classic 51% attack attempts to rewrite the ledger so that transactions you thought had gone through are reversed. IIUC there was a successful attack on CoiledCoin on this definition.

A broader, slightly strained definition is miners colluding to do affect which transactions are processed, and orphaning the blocks of people who don't join the collusion. On this definition any "soft fork" would fit, and there have been lots of those.

However the DAO fork was a "hard fork" where you specifically had to opt in and change your software to apply the new rules. This is completely different from a 51% attack, and allows people to continue getting transactions followed according to the old rules even if your rules don't have 51% of available hashpower, as ETC users are doing.


A 51% attack is where 51% of the miners collude to alter the history of the blockchain. The way to do that is fork from previous blocks, basically erasing all the blocks that happened after. And that is exactly what happened.

Even worse is that if you were a normal miner and just updated your software you were automatically agreeing with the fork. You were supposed to explicitly opt out of the fork by giving a flag to the software.

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    Technically this isn't correct. Even on the new chain, it wasn't a rollback that forked from a previous block and orphaned the ones that came after. The old blocks are still there, unorphaned, in the histories of both chains. Commented Aug 3, 2016 at 9:30

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