<p>The other two nodes would attempt to validate the invalid block, find it to contain invalid transactions, and thus those nodes will discard and ignore it.</p>
<p>Those two nodes will continue to follow the longest chain that is valid according to the rules they enforce. There could be one million other nodes that all follow the invalid block, but that would be without consequence to the two ('honest') network peers.</p>
<blockquote>
<p>Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks</p>
</blockquote>
<p>If they continue to follow the invalid chain, then the malicious nodes would create a <em>new</em> network, due to an incompatible (hard forking) rule change. The original network would continue to operate, otherwise unaffected by this.</p>

CC BY-SA 4.0
2021-12-07T10:13:02.867Z
is this called a Sybil attack or a 51% attack? (my guess is it cannot be called a 51% attack because we don't have the majority hash rate but only influence majority of full nodes)
Manav Kampani
CC BY-SA 4.0
2021-12-07T21:03:01.510Z
@ManavKampani A majority attack (51% attack) involves controlling a majority of the hashrate. What you've described isn't really a sybil attack either, as the dishonest nodes will just be ignored/banned by other nodes if they continuously misbehave. What you're imagining is perhaps most similar to an eclipse attack (which can overcome by making a connection to only one single honest peer).
chytrik
CC BY-SA 4.0
2021-12-08T05:52:53.903Z
so basically even if there's a single working neutral node in the network (assuming it's also a miner node) the blockchain will work just fine and secure.
Manav Kampani
CC BY-SA 4.0
2021-12-08T08:15:00.650Z
@ManavKampani well, if there were zero honest nodes, then there wouldn't really be a network to speak of in the first place. No number of dishonest nodes (or miners) can convince an honest node to accept an invalid block. For eclipse attacks, see: https://bitcoin.stackexchange.com/questions/tagged/eclipse-attack?tab=Votes
chytrik
CC BY-SA 4.0
2021-12-08T08:21:45.547Z
okay but I'm asking about a single honest full miner node, will the network work then?
Manav Kampani
CC BY-SA 4.0
2021-12-08T08:29:03.277Z
@ManavKampani if there were no miners on the network, then the network would not progress (no new blocks being added). If there were only one miner, the network could progress, but users of that network would likely have censorship (majority attack) concerns. Having many competing miners on the network alleviates this concern.
chytrik
CC BY-SA 4.0
2021-12-07T10:06:33.467Z
2021-12-07T10:06:33.467Z
chytrik
<p>What if I deployed 3 of my personal tampered/hacked full nodes(with one of them a miner too) that accepts invalid blocks with proper PoW (containing invalid transactions) and rejects valid blocks. Assume there were previously 2 neutral full nodes in the blockchain with one of them a miner. Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks.</p>
<p>Also is this what's called a sybil attack in a blockchain?</p>

<block><blockchain-fork><full-node><sybil-attack>
Can a hacked/tampered full node be deployed in Bitcoin to accept blocks containing invalid transactions?
Manav Kampani
# Can a hacked/tampered full node be deployed in Bitcoin to accept blocks containing invalid transactions?

(Asked by: username_0 on 2021-12-07)

What if I deployed 3 of my personal tampered/hacked full nodes(with one of them a miner too) that accepts invalid blocks with proper PoW (containing invalid transactions) and rejects valid blocks. Assume there were previously 2 neutral full nodes in the blockchain with one of them a miner. Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks.

Also is this what's called a sybil attack in a blockchain?

**Tags:** block, blockchain-fork, full-node, sybil-attack


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## 1 answers

### Answer by username_1 on 2021-12-07. Score: 2 | Accepted answer

The other two nodes would attempt to validate the invalid block, find it to contain invalid transactions, and thus those nodes will discard and ignore it.

Those two nodes will continue to follow the longest chain that is valid according to the rules they enforce. There could be one million other nodes that all follow the invalid block, but that would be without consequence to the two ('honest') network peers.

> Whose copy of the blockchain will be the true one now in such a case where we have majority of tampered full nodes accepting invalid blocks

If they continue to follow the invalid chain, then the malicious nodes would create a *new* network, due to an incompatible (hard forking) rule change. The original network would continue to operate, otherwise unaffected by this.

> **Comments:**
> **username_0**: is this called a Sybil attack or a 51% attack? (my guess is it cannot be called a 51% attack because we don't have the majority hash rate but only influence majority of full nodes)  
> **username_1**: @username_0 A majority attack (51% attack) involves controlling a majority of the hashrate. What you've described isn't really a sybil attack either, as the dishonest nodes will just be ignored/banned by other nodes if they continuously misbehave. What you're imagining is perhaps most similar to an eclipse attack (which can overcome by making a connection to only one single honest peer).  
> **username_0**: so basically even if there's a single working neutral node in the network (assuming it's also a miner node) the blockchain will work just fine and secure.  
> **username_1**: @username_0 well, if there were zero honest nodes, then there wouldn't really be a network to speak of in the first place. No number of dishonest nodes (or miners) can convince an honest node to accept an invalid block. For eclipse attacks, see: https://bitcoin.stackexchange.com/questions/tagged/eclipse-attack?tab=Votes  
> **username_0**: okay but I'm asking about a single honest full miner node, will the network work then?  
> **username_1**: @username_0 if there were no miners on the network, then the network would not progress (no new blocks being added). If there were only one miner, the network could progress, but users of that network would likely have censorship (majority attack) concerns. Having many competing miners on the network alleviates this concern.  

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