July 2015 chain forks

The July 2015 forks were two (so far) block chain forks with two or more invalid blocks. They were caused by miners who didn't properly validate blocks before mining on top of them.

Wallet Advice

• If you use Bitcoin Core, upgrade to 0.10.2 to ensure you don't see invalid confirmations.

• If you use a peer-to-peer lightweight wallet (also called a P2P SPV wallet), you must wait for additional confirmations. See the Bitcoin.org alert for the currently recommended number of additional confirmations.

• If you use Electrum, see the Electrum section below.

• If you use web wallet or an exchange, you should wait for additional confirmations. See the Bitcoin.org alert for the currently recommended number of additional confirmations.

Electrum

Electrum gets data from a Bitcoin Core-backed server. If the server is running a recent version of Bitcoin Core, it may be safe---but you must take action to be sure.

1. Go to Tools → Network and unclick Auto-connect. This prevents Electrum from automatically changing to servers that have the longest block chain (which, during these invalid chain forks, may be the wrong chain). Note that Electrum will warn you when you're not on the longest chain—you must ignore these warnings until this fork event is over or use the plugin mentioned at the bottom of this section to determine whether you're on the valid chain.
2. Go to the console tab
3. It should display a message (called a banner) from the server operator.
4. If the message includes a line that says "Bitcoin Core 0.10.2" or "bitcoind 0.10.2", then confirmations reported by the server are likely to be accurate. The only other safe versions are 0.9.5, 0.10.0, and 0.10.1.
5. If the message does not indicate that the server is running Bitcoin Core 0.10.2, you can change servers by going to Tools → Network, unclicking Auto-connect, and choosing a server from the list. After connecting, a new message will be displayed; use the instructions in step #3 again.

Please note that this information is provided voluntarily by the server administrators. It is not validated.

The following Electrum servers have a safe Bitcoin Core version in their banner:

• electrum.no-ip.org (version 0.10.0)
• electrum.jdubya.info (version 0.10.2)
• vps.hsmiths.com (version 0.10.2)
• erbium1.sytes.net (version 0.10.0)

During a fork, it is possible to use the Get Block Header custom plugin to authoratatively determine which side of the fork an Electrum server is on. Bitcoin Core 0.10.x servers should always be on the valid side of the fork.