From blog.malwarebytes.com
Despite advance warnings that a root certificate provided by Let’s Encrypt would expire on September 30, users reported issues with a variety of services and websites once that deadline hit. So what happened?
The problem
A number of high profile tech and security companies noticed their products and services were affected by the certificate expiration, such as cloud computing services for Amazon, Google, and Microsoft, IT and cloud security services for Cisco, as well as sellers that were unable to log in on Shopify.
When a user’s browser arrives at your website one of the first things it checks for is the validity of the SSL certificate. An SSL certificate is a digital certificate that authenticates a website’s identity and enables an encrypted connection. SSL certificates are issued by a Certificate Authority (CA). Most browsers will accept certificates issued by hundreds of different CAs. Let’s Encrypt is a CA that provides digital certificates as a free non-profit and millions of websites rely on Let’s Encrypt services.