Chrome is really tightening up the security game here. In Chrome 66 it will untrust Symantec-issued SSL/TLS certificates, after Symantec has repeatedly screwed up by wrongly issuing certificates for domains, including google.com itself.
Thanks to a decision in September by Google to stop trusting Symantec-issued SSL/TLS certs, from mid-April Chrome browser users visiting websites using a certificate from the security biz issued before June 1, 2016 or after December 1, 2017 will be warned that their connection is not private and someone may be trying to steal their information. They will have to click past the warning to get to the website.
This will also affect certs that use Symantec as their root of trust even if they were issued by an intermediate organization. For example, certificates handed out by Thawte, GeoTrust, and RapidSSL that rely on Symantec will be hit by Google’s crackdown. If in doubt, check your cert’s root certificate authority to see if it’s Symantec or not.
Arkadiy Tetelman has recently done an experiment and made an inventory of how many sites in the Alexa Top 1 Million that will be affected by this.
Included in the 100,000 affected sites we find/found (some have gotten a new certificate by now)