SSL Certificate Expiry (SSL Expiry)
Why SSL Expiry Matters
Expired SSL certificates are one of the most common causes of website outages and security warnings. When a certificate expires, every visitor to your site sees a full-page warning that the connection is not secure. Most users will not click through this warning, effectively making your site inaccessible.
For sites with HSTS enabled (which instructs browsers to always use HTTPS), an expired certificate is even more damaging. The browser will refuse to connect at all - there is no option to bypass the warning. This means an HSTS-enabled site with an expired certificate is completely down until the certificate is renewed.
Certificate expiry monitoring is essential. Set up automated alerts well before expiration (30 days, 14 days, 7 days). Better yet, use automated renewal with ACME/Certbot for Let's Encrypt certificates, which handles renewal automatically when the certificate is within 30 days of expiry. For manually managed certificates, add calendar reminders and monitoring checks.
How to Test for SSL Expiry
An SSL checker shows your certificate's exact expiration date and how many days remain. Set up monitoring to check this regularly. Many monitoring services can alert you weeks before expiration. If your certificate is from Let's Encrypt, verify that your ACME client's automatic renewal is functioning.
Real-World Example
In 2020, Microsoft Teams experienced a multi-hour global outage because an authentication certificate expired without being renewed. The expired certificate prevented users worldwide from signing in. The root cause was a manual renewal process with no automated monitoring.