Troubleshooting SSL/TLS Certificate Errors
SSL/TLS certificate errors prevent secure connections and scare away visitors. This guide explains common certificate warnings, their causes, and step-by-step fixes for website operators and visitors.
Key Takeaways
- Browsers display different warnings depending on the specific certificate problem.
- Warning: "Your connection is not private" / NET::ERR_CERT_DATE_INVALID
- Warning: NET::ERR_CERT_COMMON_NAME_INVALID
- Warning: NET::ERR_CERT_AUTHORITY_INVALID
- Warning: Padlock shows warning, some resources blocked.
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Certificate Error Types
Browsers display different warnings depending on the specific certificate problem. Understanding the error type speeds up diagnosis.
Expired Certificate
Warning: "Your connection is not private" / NET::ERR_CERT_DATE_INVALID
Cause: The certificate's validity period has passed. Certificates are typically valid for 90 days (Let's Encrypt) to 1 year.
Fix (site owner): Renew the certificate. Set up automatic renewal with your certificate authority. Check that your renewal cron job is working.
Name Mismatch
Warning: NET::ERR_CERT_COMMON_NAME_INVALID
Cause: The certificate was issued for a different domain than the one being accessed. Common when accessing a site by IP or an alternate domain.
Fix: Ensure the certificate covers all domains and subdomains. Use a wildcard certificate (*.example.com) or a SAN (Subject Alternative Name) certificate listing all domains.
Self-Signed Certificate
Warning: NET::ERR_CERT_AUTHORITY_INVALID
Cause: The certificate wasn't issued by a trusted Certificate Authority. Common in development environments.
Fix: Use a proper CA certificate for production. Let's Encrypt provides free, trusted certificates.
Mixed Content
Warning: Padlock shows warning, some resources blocked.
Cause: An HTTPS page loads some resources (images, scripts, stylesheets) over HTTP.
Fix: Update all resource URLs to use HTTPS or protocol-relative URLs. Set Content-Security-Policy: upgrade-insecure-requests as a safety net.
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