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This question already has an answer here:

Yesterday the Ethereum project made a fork. Was it a soft or hard fork? How did the attacker perform his attack?

marked as duplicate by Richard Horrocks, eth Oct 19 '16 at 22:46

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To answer your first question it was a hardFork. for the second i suggest you to read the related post on the ethereum blog :

https://blog.ethereum.org/2016/09/22/ethereum-network-currently-undergoing-dos-attack/ https://blog.ethereum.org/2016/09/22/transaction-spam-attack-next-steps/

where they explain how the attacker exploit an EVM opcode to perform his attack.

the network was attacked by a transaction spam attack that repeatedly called the EXTCODESIZE opcode (see trace sample here), thereby creating blocks that take up to ~20-60 seconds to validate due to the ~50,000 disk fetches needed to process the transaction. The result of this was a ~2-3x reduction in the rate of block creation while the attack was taking place;

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