<p>The simplest protection is to not consider a payment to have been made to you until it receives some number of confirmations. The number can depend on the value of the payment. For small payments, one is probably sufficient. For large payments, some have decided to wait for as many as six confirmations.</p>

CC BY-SA 3.0
2017-02-10T18:49:57.403Z
2017-02-10T19:09:11.517Z
2017-02-10T19:09:11.517Z
David Schwartz
<p>Yes, it is possible to make such a transaction. It's possible to make all kinds of transactions. Of course, it's much more interesting whether such a transaction would be valid. And it would. A requirement for a valid transaction is that the money that goes into it is at most as much as leaves it. The difference between what goes into a transaction and what leaves it is called a fee. The fee can be 0.</p>

<p>However, that such a transaction would be valid doesn't mean it will be confirmed, as you already suspected. But to defraud someone who accepts zero-confirmation, they need to at least know of the transaction. This is going to be difficult if you don't include a fee: <a href="https://bitcoin.org/en/glossary/minimum-relay-fee" rel="nofollow noreferrer">https://bitcoin.org/en/glossary/minimum-relay-fee</a></p>

<p>The sender then can send a different transaction spending the money. Such a double-spending transaction usually will not be propagated much if some significant amount of time passed before the double-spending transaction was made public because nodes usually don't propagate transactions if they spend funds a different transaction in their mempools already attempts to spend. However, if the first transaction had a very low fee, a lot of nodes did not even add it to their mempools, meaning they don't remember the first transaction. Furthermore, some may store and/or propagate double-spending transactions if the fee of the second one is higher that the fee of the first one. </p>

<p>There are 3 ways to defend against this.</p>

<p>The first one is obvious: Don't accept zero-confirmation transactions.</p>

<p>The second one is to listen to an alarm which goes off as soon as a double-spending transaction has been found. </p>

<p>The third one is to only allowed zero-confirmation transactions from people you can identify. For example, an online shop may accept zero-confirmation transactions without worrying about them much as they know where the people they send their goods to live. </p>

<p>If you should have received money but a double-spending transaction made it into the blockchain instead, you don't own that money. Who owns which money is defined by the longest valid blockchain and if a conflicting transaction is in there, the one sending money to you can't get into it, too. </p>

CC BY-SA 3.0
2017-02-10T20:16:18.507Z
2017-02-10T20:16:18.507Z
UTF-8
<p>I would like to know if it is possible to make a transaction without fee (probably this transaction will never be confirmed) and some time later to take this money back and spend in another transaction?
If yes, can I protect myself against this fake payment?</p>

<p>While the transaction is waiting a confirmation, it's possible for the receiver uses this amount?
If yes, if the confirmation never happens, the third person loses this money?</p>

CC BY-SA 3.0
2017-02-10T17:20:52.007Z
related: [How secure is zero confirmations?](http://bitcoin.stackexchange.com/q/20845/5406), [How do I accept bitcoin payments at a real world store?](http://bitcoin.stackexchange.com/q/327/5406), [How do I safely accept bitcoins without waiting for confirmations?](http://bitcoin.stackexchange.com/q/328/5406)
Murch
<unconfirmed-transactions><doublespend><fraud><zero-confirmation>
It's possible to spend a BTC without fee to forge a payment?
Davi cesar
# It's possible to spend a BTC without fee to forge a payment?

(Asked by: username_0 on 2017-02-10)

I would like to know if it is possible to make a transaction without fee (probably this transaction will never be confirmed) and some time later to take this money back and spend in another transaction?
If yes, can I protect myself against this fake payment?

While the transaction is waiting a confirmation, it's possible for the receiver uses this amount?
If yes, if the confirmation never happens, the third person loses this money?

**Tags:** unconfirmed-transactions, doublespend, fraud, zero-confirmation

> **Comments:**
> **username_1**: related: [How secure is zero confirmations?](http://bitcoin.stackexchange.com/q/20845/5406), [How do I accept bitcoin payments at a real world store?](http://bitcoin.stackexchange.com/q/327/5406), [How do I safely accept bitcoins without waiting for confirmations?](http://bitcoin.stackexchange.com/q/328/5406)  

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## 2 answers

### Answer by username_2 on 2017-02-10. Score: 4

The simplest protection is to not consider a payment to have been made to you until it receives some number of confirmations. The number can depend on the value of the payment. For small payments, one is probably sufficient. For large payments, some have decided to wait for as many as six confirmations.


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### Answer by username_3 on 2017-02-10. Score: 2

Yes, it is possible to make such a transaction. It's possible to make all kinds of transactions. Of course, it's much more interesting whether such a transaction would be valid. And it would. A requirement for a valid transaction is that the money that goes into it is at most as much as leaves it. The difference between what goes into a transaction and what leaves it is called a fee. The fee can be 0.

However, that such a transaction would be valid doesn't mean it will be confirmed, as you already suspected. But to defraud someone who accepts zero-confirmation, they need to at least know of the transaction. This is going to be difficult if you don't include a fee: <https://bitcoin.org/en/glossary/minimum-relay-fee>

The sender then can send a different transaction spending the money. Such a double-spending transaction usually will not be propagated much if some significant amount of time passed before the double-spending transaction was made public because nodes usually don't propagate transactions if they spend funds a different transaction in their mempools already attempts to spend. However, if the first transaction had a very low fee, a lot of nodes did not even add it to their mempools, meaning they don't remember the first transaction. Furthermore, some may store and/or propagate double-spending transactions if the fee of the second one is higher that the fee of the first one.

There are 3 ways to defend against this.

The first one is obvious: Don't accept zero-confirmation transactions.

The second one is to listen to an alarm which goes off as soon as a double-spending transaction has been found.

The third one is to only allowed zero-confirmation transactions from people you can identify. For example, an online shop may accept zero-confirmation transactions without worrying about them much as they know where the people they send their goods to live.

If you should have received money but a double-spending transaction made it into the blockchain instead, you don't own that money. Who owns which money is defined by the longest valid blockchain and if a conflicting transaction is in there, the one sending money to you can't get into it, too.


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