I'm sure this is a conceptual error on my part. I understood the idea of a replay attack as being the capturing of a txn body with its signature on one chain and 'replaying' it on the other (ETH to ETC or vice versa).

What I don't see is how an attacker can benefit from this assuming the txn body signature has the destination address embedded in it. Wouldn't the attacker have to control that specific address to get the coin value?

How can an attacker use a txn body signature to validly send value to his own address?

1 Answer 1


An attacker cannot take a signed transaction, and change the destination address (to one that the attackers controls) since that would invalidate the signature.

However, here's an example of what could happen:

  • Mallory buys ETC from Alice.
  • Alice has not taken any precautions and sends the ETC to Mallory.
  • Mallory replays the transaction and gets the (corresponding) ETH from Alice.

Mallory did not have to pay anything more to get the ETH: Alice lost her ETH to Mallory due to a replay attack.

  • I see. The attacker has to be someone you have already intentionally transacted with, who then wants to exploit you by replicating that txn on the other chain...at least you would know who did it, right? Aug 28, 2016 at 21:21
  • 1
    It's really less of an "attack" than just a consequence of having two chains where a transaction is valid. You should just assume any replayable transaction will be replayed, even if on accident Aug 28, 2016 at 23:54

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