What is a block in Bitcoin?
This document is being updated as new information arrives. Last update: 2015-07-15 13:00. All times are UTC.
Note: this situation has not been fully resolved, and it does not appear that it will be fully resolved anytime soon. Users of the affected wallets listed below are still advised to wait additional confirmations or to switch to a safer wallet.
Summary
Your bitcoins are safe if you received them in transactions confirmed before 2015-07-15 12:00 UTC.
However, there has been a problem with a planned upgrade. For bitcoins received later than the time above, confirmation scores are significantly less reliable then they usually are for users of certain software:
- should wait an additional 30 confirmations more than you would normally wait. Electrum users, please see this note.
- Bitcoin Core 0.9.4 or earlier users should wait an additional 30 confirmations more than you would normally wait or upgrade to Bitcoin Core 0.10.2.
- Web wallet users should wait an additional 30 confirmations more than you would normally wait, unless you know for sure that your wallet is secured by Bitcoin Core 0.9.5 or later.
- Bitcoin Core 0.9.5 or later users are unaffected. (Note: upgrade to 0.10.2 is recommended due to denial-of-service vulnerabilities unrelated to this alert.)
Miners
If you solo mine, please switch to Bitcoin Core 0.10.2.
When Will Things Go Back To Normal?
The problem is miners creating invalid blocks. Some software can detect that those blocks are invalid and reject them; other software can’t detect that blocks are invalid, so they show confirmations that aren’t real.
- 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.
What's Happening
Summary: Some miners are currently generating invalid blocks. Almost all software (besides Bitcoin Core 0.9.5 and later) will accept these invalid blocks under certain conditions.
So far, the following forks of two or more blocks have occurred:
| Start date | End time | Blocks | Double Spends |
|---|---|---|---|
| 4 July @ 02:10 | 03:50 | ||
| 5 July @ 21:50 | 23:40 | Not yet known |
The paragraphs that follow explain the cause more throughly.
For several months, an increasing amount of mining hash rate has been signaling its intent to begin enforcing BIP66 strict DER signatures. As part of the BIP66 rules, once 950 of the last 1, 000 blocks were version 3 (v3) blocks, all upgraded miners would reject version 2 (v2) blocks.
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.
All software that assumes blocks are valid (because invalid blocks cost miners money) is at risk of showing transactions as confirmed when they really aren’t. This particularly affects lightweight (SPV) wallets and software such as old versions of Bitcoin Core which have been downgraded to SPV-level security by the new BIP66 consensus rules.
The recommended fix, which was attempted, was to get all miners off of SPV mining and back to full validation (at least temporarily). If this happens, Bitcoin.org will reduce its current recommendation of waiting 30 extra confirmations to a lower number.
Updates
- 6 July 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 . Reports that the situation has passed are not correct. Please continue to wait 30 more confirmations than you usually would wait before accepting a transaction.
