Update published by Bitcoin.org on July 6th, 2015 at 04:00: “A new fork occurred starting 5 July at 21:30 with three blocks before the valid chain again became the strongest chain. See the recently-added list of forks. Reports that the situation has past are not correct. Please continue to wait 30 more confirmations than you usually would wait before accepting a transaction.”
On Saturday, some problems created by the planned upgrade affected transactions on the Bitcoin network. According to the Bitcoin community website, transactions confirmed after 8 AM UTC were affected due to the generation of invalid blocks, which were generated after the implementation of BIP66 strict DER signatures.
In fact, it depends on the software whether invalid blocks can be detected and rejected or not. If the software can’t cope with it, the confirmations displayed aren’t real.
Let’s consider the list provided by Bitcoin.org:
- Bitcoin Core 0.9.5 and later never had any problems because it could detect which blocks were invalid.
- Bitcoin Core 0.9.4 and earlier will never provide as much security as later versions of Bitcoin Core because it doesn’t know about the additional BIP66 consensus rules.Upgrade is recommended to return to full node security.
- Lightweight (SPV) wallets are not safe for less than 30 confirmations until all the major pools switch to full validation.
- Web wallets are very diverse in what infrastructure they run and how they handle double spends, so unless you know for sure that they use Bitcoin Core 0.9.5 or later for full validation, you should assume they have the same security as the lightweight wallets described above.
“Early morning on 4 July 2015, the 950/1000 (95%) threshold was reached. Shortly thereafter, a small miner (part of the non-upgraded 5%) mined an invalid block–as was an expected occurrence. Unfortunately, it turned out that roughly half the network hash rate was mining without fully validating blocks (called SPV mining), and built new blocks on top of that invalid block. Note that the roughly 50% of the network that was SPV mining had explicitly indicated that they would enforce the BIP66 rules. By not doing so, several large miners have lost over $50,000 dollars worth of mining income so far,” reads the Bitcoin community website.
Moreover, according to the above mentioned site, software that detects valid blocks can show transactions as confirmed when they really aren’t. Old versions of Bitcoin Core and lightweight (SPV) wallets are particularly affected.
The issue is being fixed making all miners fully validate their coins. If this happens, Bitcoin.org is going to decrease its recent recommendation of waiting 30 extra confirmations to a lower number.