CloudFlare error 1013: HTTP hostname and TLS SNI hostname mismatch

CloudFlare's error 1023 page can appear while

  • attempting to access dashboard.getclef.com
  • or attempting to perform a Clef login on a Clef-enabled web site.

What is the cause of this error, and how do I fix it?

According to CloudFlare:

The most common reason for seeing this message is if your local machine or network is proxying SSL traffic. Some SSL proxies have implementations that can cause an inconsistency between the SNI and the Host Header.

To test whether your local machine and/or network is proxying TLS/SSL traffic in such a manner that causes this mismatched hostname, CloudFlare recommends performing the following test:

  1. Go to https://cc.dcsec.uni-hannover.de
  2. Find the line for SNI information. If the value returned on this line is anything other than cc.dcsec.uni-hannover.de, then you need either
    1. to adjust your local machine's TLS/SSL connection settings,
    2. to follow up with your networking provider regarding your network's TLS/SSL settings,
    3. or to contact CloudFlare support for assistance (see the directions at the bottom of this support document).