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I have just sent bitcoins from one account of mine to another, and after I sent it I checked on blockchain.info to get updated about confirmation. There, I noticed something peculiar, it said that I made two transactions at the same time, when I made only one. The first transaction was to the correct address and the correct amount. But the second transaction was not one that I made. The second thing I noticed is the SUM of both transactions, together they form the exact amount I last received to this account a few days ago. This looks to me like a hack. At this moment these transactions have net yet been confirmed, can I do anything about it? Can a transaction be created fraudulently and accepted by the miners? How can a hack like this be accomplished (if it is a hack).

  • Did you create the transaction yourself or did simply send bitcoin wuth the help of a wallet? – Ivano Donadi Aug 23 '17 at 11:11
  • I am running a full node, so I guess I created it myself – Adam Shem-Tov Aug 23 '17 at 16:46
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You are not being hacked. These are not two separate transactions; it is one transaction with two outputs. The second output was automatically created by your wallet client and sends back to a new address which also belongs to you. This is a normal and necessary feature of how Bitcoin works, called "change". See How does change work in a bitcoin transaction?

If this transaction were malicious, it would mean that someone had access to your private key; if so, they would also have stolen your earlier transaction. Moreover, they would have stolen the coins as soon as they obtained the key; they wouldn't have waited for you to make your own transaction.

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  • Thank You Nate Eldredge for your response, I had no idea about this feature, and because the change address is not written in the bitcoin client, that idea couldn't of crossed my mind. After googling I found that when using bitcoin-qt it is possible to enter the command "listaddressgroupings" in the debug console, to see all addresses including change addresses. bitcoin.stackexchange.com/questions/9077/…. – Adam Shem-Tov Aug 23 '17 at 16:56
  • @AdamShem-Tov: Yeah, I think the Bitcoin Core developers took the view that this is a technical detail that end users don't need to be bothered with. But of course when you look "under the hood" with a block explorer like blockchain.info, you see the details, and they can be confusing when they don't appear to match the high-level view presented by the client. – Nate Eldredge Aug 23 '17 at 17:00

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