title: Let's encrypt websites with massive errors of CERTIFICATE_VERIFY_FAILED
body: <p>I monitor a lot of sites and I found out that almost every site using Let's encrypt now has an error of: CERTIFICATE_VERIFY_FAILED</p>
  <p>Some examples:</p>
  <ol>
  <li><a href="https://www.telefonica.com/es/" rel="nofollow noreferrer">https://www.telefonica.com/es/</a></li>
  <li><a href="https://www.anchorage.com/" rel="nofollow noreferrer">https://www.anchorage.com/</a></li>
  <li><a href="https://www.stanford.edu/" rel="nofollow noreferrer">https://www.stanford.edu/</a></li>
  <li><a href="https://www.nsa.gov/" rel="nofollow noreferrer">https://www.nsa.gov/</a></li>
  <li><a href="https://stackoverflow.com/">https://stackoverflow.com/</a></li>
  </ol>
  <p>My own site has the same problem.</p>
  <p>This is easy to test:</p>
  <pre><code>requests.get('https://www.nsa.gov/', timeout=3, allow_redirects=False, headers={'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0'})
  </code></pre>
  <p>Any idea what is going on? And how to fix it?</p>
  <p>Edit. Even stackoverflow.com</p>