---
title: "Some Miners Generating Invalid Blocks"
alias: "spv-mining"
active: true
banner: "WARNING: many wallets currently vulnerable to double-spending of confirmed transactions (click here to read)"
---
<p><em>This document is being updated as new information arrives.  Last
update: 2015-07-04 09:00 UTC</em></p>

{% assign confs="15" %}

<h2 id="summary">Summary</h2>

<p>Your bitcoins are safe if you received them in transactions confirmed before 2015-07-04 08:00 UTC.</p>

<p>After that time, confirmation scores are not as reliable as they
usually are for users of certain software:</p>

<ul>
  <li><b>Lightweight (SPV) wallet users</b> should wait an additional {{confs}}
  confirmations more than you would normally wait.</li>

  <li><b>Bitcoin Core 0.9.4 or earlier users</b> should wait an
  additional {{confs}} confirmations more than you would normally
  wait or upgrade to <a href="/en/download">Bitcoin Core 0.10.2</a>.</b></li>

  <li><b>Web wallet users</b> should wait an additional {{confs}} 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.</li>

  <li><b>Bitcoin Core 0.9.5 or later users are unaffected.</b></li>
</ul>

<h2 id="miners">Miners</h2>

If you pool mine, please switch to a pool that properly validates
blocks.  (If you solo mine, please switch to Bitcoin Core 0.10.2.)

<p><b>Bad pools:</b> these pools are not correctly validating, and are
losing money.</p>

<ul>
 <li>BTC Nuggets</li>
</ul>

<p><b>Good pools:</b> these pools properly validate blocks.  Please
switch to them, at least until the bad pools have fixed their
systems.</p>

<ul>
  <li><a href="http://eligius.st/">Eligius</a></li>
  <li><a href="https://www.kano.is">kano/ckpool</a></li>
  <li><a href="http://p2pool.in/">P2Pool</a></li>
  <li><a href="https://www.f2pool.com/">F2Pool</a></li>
</ul>

<h2 id="solution">When Will Things Go Back To Normal?</h2>

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.

<ul>
  <li><b>Bitcoin Core 0.9.5 and later</b> never had any problems because
      it could detect which blocks were invalid.</li>

  <li><b>Bitcoin Core 0.9.4 and earlier</b> will never provide as much
      security as later versions of Bitcoin Core because it doesn't know
      about the additional <a
      href="https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki">BIP66</a>
      consensus rules. <a href="/en/download">Upgrade</a> is recommended
      to return to full node security.</li>

  <li><b>Lightweight (SPV) wallets</b> are not safe until all the major
      pools switch to full validation. After all the major pools fix
      their problems, lightweight wallets will remain vulnerable to one,
      two, or rarely three confirmation double spends until near 100% of
      total hash rate has upgraded to a BIP66-compliant version of
      Bitcoin Core.</li>

  <li><b>Web wallets</b> 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.</li>
</ul>

<h2 id="cause">What's Happening</h2>

<p>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.  The paragraphs that follow
   explain the cause more throughly.</p>

<p>For several months, an increasing amount of mining hash rate has been
   signaling its intent to begin enforcing <a
   href="https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki">BIP66</a>
   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.</p>

<p>Early morning UTC 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.</p>

<p>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.</p>

<p>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.</p>

<p>The immediate fix, which is well underway as of this writing, is to
   get all miners off of SPV mining and back to full validation (at
   least temporarily). As this progresses, we will reduce our
   current recommendation of waiting {{confs}} extra confirmations to a
   lower number.</p>

<p>However, the BIP66 soft fork implementation method of waiting for
   only 95% of miners to upgrade does leave miner-trusting software such
   as lightweight wallets at increased risk of seeing invalid single
   confirmations (10% risk), invalid double confirmations (1% risk), and
   maybe even invalid triple confirmations (0.1% risk) until more of
   the 5% non-upgraded miners do finally upgrade.  So for the next
   several weeks (maybe months), lightweight wallet users, web wallet
   users, and users of old versions of Bitcoin Core should wait an extra
   two to three confirmations.</p>

<!--
<div style="text-align:right">
  <i>This notice last updated: 2015-07-04 06:00 UTC</i>
</div>
-->