<p>A miner could create a block with an incorrect transaction, but this wouldn't be accepted by the honest nodes in the network. The honest nodes would attempt to validate the transaction, fail and then not propagate the block to other nodes.</p>

CC BY-SA 3.0
2018-01-07T07:58:30.400Z
What is an example of someone trying to modify a transaction (and then embarking on the impossible task of redoing PoW for every subsequent node). In this case, the change is something validated and thus the need for PoW?
flyinghigh
CC BY-SA 3.0
2018-01-07T09:05:54.050Z
PoW does not really matter here. Blocks are **verified** before PoW even comes into play. If an invalid block is produced, nodes simply ignore it. Then, PoW is used to find the most-work chain out of the set of **valid** blocks.
Pieter Wuille
CC BY-SA 3.0
2018-01-07T21:19:11.143Z
Then what is an example of a valid block that is trying to overtake a legitimate blockchain, but won't be added because it would have to redo immense amounts of PoW.
flyinghigh
CC BY-SA 3.0
2018-01-07T07:47:42.797Z
2018-01-07T07:47:42.797Z
Matthew Stannard
<p>Every full node independently validates that each block is following all consensus rules of Bitcoin. Therefore, a block with invalid transactions would simply not be accepted by any full node, and thus would quickly be forgotten by the whole network.</p>

CC BY-SA 4.0
2018-05-04T17:23:40.453Z
2018-05-04T17:23:40.453Z
Murch
<p>Let's say someone creates a fraudulent block and finds a nonce that solves the hash puzzle. This block is broadcasted, its hash easily verified, and it's added to the blockchain. What prevents a miner from hearing this block and adding it to its blockchain such that it uses the hash of this fraudulent block as the previous hash field in the new block it is trying to solve? Couldn't this then create the longest blockchain at a fork and thus become accepted? </p>

<p>When do mining nodes give up on a certain block? Once a block with the correct nonce is found for a given previous block, don't all other nodes mining a block with the same previous node have to start over?  </p>

CC BY-SA 3.0
2018-01-07T07:41:00.040Z
What do you mean by a fraudulent block?
Matthew Stannard
CC BY-SA 3.0
2018-01-07T07:43:24.033Z
A block that contains an incorrect or malicious transaction.
flyinghigh
<blockchain><block-validity>
How does the protocol prevent miners from building off of a fraudulent blockchain?
flyinghigh
# How does the protocol prevent miners from building off of a fraudulent blockchain?

(Asked by: username_0 on 2018-01-07)

Let's say someone creates a fraudulent block and finds a nonce that solves the hash puzzle. This block is broadcasted, its hash easily verified, and it's added to the blockchain. What prevents a miner from hearing this block and adding it to its blockchain such that it uses the hash of this fraudulent block as the previous hash field in the new block it is trying to solve? Couldn't this then create the longest blockchain at a fork and thus become accepted?

When do mining nodes give up on a certain block? Once a block with the correct nonce is found for a given previous block, don't all other nodes mining a block with the same previous node have to start over?

**Tags:** blockchain, block-validity

> **Comments:**
> **username_1**: What do you mean by a fraudulent block?  
> **username_0**: A block that contains an incorrect or malicious transaction.  

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## 2 answers

### Answer by username_1 on 2018-01-07. Score: 1

A miner could create a block with an incorrect transaction, but this wouldn't be accepted by the honest nodes in the network. The honest nodes would attempt to validate the transaction, fail and then not propagate the block to other nodes.

> **Comments:**
> **username_0**: What is an example of someone trying to modify a transaction (and then embarking on the impossible task of redoing PoW for every subsequent node). In this case, the change is something validated and thus the need for PoW?  
> **username_2**: PoW does not really matter here. Blocks are **verified** before PoW even comes into play. If an invalid block is produced, nodes simply ignore it. Then, PoW is used to find the most-work chain out of the set of **valid** blocks.  
> **username_0**: Then what is an example of a valid block that is trying to overtake a legitimate blockchain, but won't be added because it would have to redo immense amounts of PoW.  

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### Answer by username_3 on 2018-05-04. Score: 0

Every full node independently validates that each block is following all consensus rules of Bitcoin. Therefore, a block with invalid transactions would simply not be accepted by any full node, and thus would quickly be forgotten by the whole network.


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