user85767: I know Bitcoin Core downloads and processes the full chain much quicker now than it used to but what exactly is so bad about downloading a compressed blocks, chainstate and database folder from a third party?
In the event of any malicious changes to the chainstate, wouldn't Bitcoin Core at its startup reject the new blocks anyway?

user47827: The challenge is essentially who you are downloading from. If you already have a full node that has completed the IBD (initial block download) verification process you may choose to just transfer that over that blockchain to a second full node you are setting up to save on completing the process a second time.
However, if you download it from a malicious party they could send you an entirely independent blockchain with orders of magnitude less proof of work. If they also controlled some of the peers you are connected to they could continue to feed you new blocks built on top of this independent blockchain. Unless you did sanity checks against a third party block explorer or verified the accumulated proof of work you may struggle to work out what has happened.
The Bitcoin protocol is designed to be trust minimized. If you relax those assumptions and start trusting third parties you do start to open up "security holes" in Nick Szabo's parlance.

user24925: No, the chainstate is the nodes view of correctness. It would take many hours or days to re-verify the whole chain on most hardware, so an assumption is made that the state stored at the previous shutdown is most likely correct. There's some sanity checking of the previous couple of blocks to make sure there is no gross accidental corruption, but malicious modifications of the chainstate before that time will not be caught.
A maliciously modified chainstate can contain fake outputs that do not exist in the canonical one, meaning you could be duped into accepting a false transaction paying money to you that nobody else would consider valid. If any significant portion of the network was downloading unverified chainstate databases it would be a considerable issue.

85767: <p>I know Bitcoin Core downloads and processes the full chain much quicker now than it used to but what exactly is so bad about downloading a compressed <code>blocks, chainstate and database</code> folder from a third party?</p>
<p>In the event of any malicious changes to the chainstate, wouldn't Bitcoin Core at its startup reject the new blocks anyway?</p>
 47827: <p>The challenge is essentially who you are downloading from. If you already have a full node that has completed the IBD (initial block download) verification process you may choose to just transfer that over that blockchain to a second full node you are setting up to save on completing the process a second time.</p>
<p>However, if you download it from a malicious party they could send you an entirely independent blockchain with orders of magnitude less proof of work. If they also controlled some of the peers you are connected to they could continue to feed you new blocks built on top of this independent blockchain. Unless you did sanity checks against a third party block explorer or verified the accumulated proof of work you may struggle to work out what has happened.</p>
<p>The Bitcoin protocol is designed to be trust minimized. If you relax those assumptions and start trusting third parties you do start to open up &quot;security holes&quot; in Nick Szabo's <a href="https://nakamotoinstitute.org/trusted-third-parties/" rel="nofollow noreferrer">parlance</a>.</p>
 24925: <p>No, the <code>chainstate</code> is the nodes view of correctness. It would take many hours or days to re-verify the whole chain on most hardware, so an assumption is made that the state stored at the previous shutdown is most likely correct. There's some sanity checking of the previous couple of blocks to make sure there is no gross <em>accidental corruption</em>, but <em>malicious modifications</em> of the <code>chainstate</code> before that time will not be caught.</p>
<p>A maliciously modified chainstate can contain fake outputs that do not exist in the canonical one, meaning you could be duped into accepting a false transaction paying money to you that nobody else would consider valid. If any significant portion of the network was downloading unverified chainstate databases it would be a considerable issue.</p>
