can a Bitcoin transaction have 2 confirmations and still be reorged?

[coinlary]

How possible it is for a Bitcoin transaction to have 2 confirmations despite ending up on a stale block before being dropped in a typical chain reorganization scenario, or once a transaction has 2 confirmations, does that generally mean it is safe to release funds even if there is a fork.

Trying to understand whether 2 confirmations is enough for every transaction without worrying about a potential chain reorganization. I mean how much protection does 2 confirmations provides against reorg.

Also, how likely is it for a block explorer to miss a fork perhaps just a few seconds or minutes?

[Karl_3000]

The probability that a transaction that has two confirmations will be reversed is very rare but it is possible. 51% attack is the worse where transactions that have more than 3 confirmations can be reversed because of chain reorg. If there is no 51% attack, I prefer 3 to 6 confirmations. But the higher the confirmations the lower the risks of transaction reversion.

[Zaguru12]

How possible it is for a Bitcoin transaction to have 2 confirmations despite ending up on a stale block before being dropped in a typical chain reorganization scenario, or once a transaction has 2 confirmations, does that generally mean it is safe to release funds even if there is a fork.

There is a possibility because the main chain that is considered isn’t based on the number of blocks rather it is base on the number of most accumulated chain work so two blocks can still end up not been part of the most chain work and a chain reorg can possibly happen.

Although the probability increases as the number of blocks increases. The probability has been calculated and explained by Satoshi on the bitcoin white paper page 8

Trying to understand whether 2 confirmations is enough for every transaction without worrying about a potential chain reorganization. I mean how much protection does 2 confirmations provides against reorg.

Also, how likely is it for a block explorer to miss a fork perhaps just a few seconds or minutes?

For a smaller amount transaction I will recommend that 1-3 confirmation is ok but a very higher amount can take six confirmation to erase the doubt totally.

One block chain reorg is common but even with that it’s usually understood that once a stale block is formed after a chain reorg majority of the transactions in this stale block are still added to the replacement block or the next block

[Upgrade00]

One block chain reorg is common but even with that it’s usually understood that once a stale block is formed after a chain reorg majority of the transactions in this stale block are still added to the replacement block or the next block

Also, more often than not, transactions will be present on both the stale block and the active one not needing any recycling back into the mempool to get confirmed. In the few case that it's not added, it will be added to another block.
Transactions from the stale block would have also not gotten to 2 confirmations.

Trying to understand whether 2 confirmations is enough for every transaction without worrying about a potential chain reorganization. I mean how much protection does 2 confirmations provides against reorg.

For almost every 1-2 confirmations is secure. Except you're transacting an amount so high it motivates the receiver to try to directly manipulate the network and still be profitable, then you can wait for about 3-6 to make sure.

[Danish Ali]

How likely is it for a block explorer to miss a fork perhaps just a few seconds or minutes?

Explorers simply present what the latest best-chain-work the backing-node is observing from its perspective. A reorg means there's a slight delay (several seconds, possibly longer) between a node/explorer reporting the stale block as "tip" and the new best chain getting reported by the node/explorer. So maybe a user might flash on "2 confirmations" for a bit until it goes back to "0 confirmations" that's just a user experience thing that has nothing to do with the actual risk of reorg.

Satoshi's paper (Section 11) provides the actual probability of an attacker catching up using a Poisson process, rather than just a simplistic estimate:

Code:

λ = z * (q/p)
P = 1 - Σ (k=0 to z) [ (λ^k * e^-λ / k!) * (1 - (q/p)^(z-k)) ]

Plugging in q=0.10 (10% attacker hashrate):

Code:

z=0  ->  P = 1.0000000
z=1  ->  P = 0.2045873
z=2  ->  P = 0.0509779
z=5  ->  P = 0.0009137
z=6  ->  P = 0.0002428

This directly matches the whitepaper's C-code output. It proves that risk drops exponentially per additional confirmation, which is why 1-2 confirmations are mathematically fine for small transactions, while 6 confirmations became the baseline standard for high value transfers.

[BlackHatCoiner]

The probability of mining a stale block is roughly 0.1%, considering there have been around 50 stale blocks in the past 52,500. That means the that mining a stale block on top of another stale block is about 1 in a million.

I'd be pretty confident with that chance, but you can wait for another 10 minutes if you want to be extra safe.

[coinlary]

Great

Thank you all for your comments

I'd be pretty confident with that chance, but you can wait for another 10 minutes if you want to be extra safe.

That's it, 3 confirmation on another average 10 mins.
Thanks  for the link, i found it  useful.

Btw, does anyone have a link to a news where someone lost some Bitcoin due to a chain reorganization, or has that never happened? Just trying to figure something out. I’m not asserting that a reorg itself directly causes losses(I believe the question is straightforward enough)

[Karl_3000]

Btw, does anyone have a link to a news where someone lost some Bitcoin due to a chain reorganization, or has that never happened? Just trying to figure something out. I’m not asserting that a reorg itself directly causes losses(I believe the question is straightforward enough)

It has actually happened before but the one that I know about was not for a bad intention but an experiment. It was a successful experiment as the coin was double spent.

https://bitcointalk.org/index.php?topic=152348.msg1616747#msg1616747

[coinlary]

https://bitcointalk.org/index.php?topic=152348.msg1616747#msg1616747

Thanks .