SSL/TLS Monitoring and Observability: Preventing Certificate Outages Before They Happen

Every major certificate outage follows the same pattern: "We didn't know it was expiring." With proper monitoring infrastructure, this is entirely preventable. Here's how to build TLS observability that catches issues before they become incidents.

What to Monitor

Certificate Health

• Expiration — Days until expiry for every certificate, with graduated alerting
• Chain validity — Complete chain from leaf to root, intermediate certificates present and valid
• Revocation status — CRL and OCSP checks for all certificates
• Algorithm strength — Flag certificates using weak keys (RSA < 2048, SHA-1)
• SAN coverage — Verify all required hostnames are covered

TLS Configuration

• Protocol versions offered (flag TLS 1.0/1.1)
• Cipher suite ordering and strength
• HSTS header presence and configuration
• OCSP stapling status

Certificate Transparency

• Monitor CT logs for certificates issued for your domains
• Alert on unexpected issuance (potential compromise or misconfiguration)
• Track all CAs issuing certificates for your domains

Monitoring Infrastructure

Prometheus + blackbox_exporter

The standard open-source approach: blackbox_exporter probes TLS endpoints, exposes certificate metrics (expiry, issuer, serial), Prometheus scrapes and stores, Grafana dashboards visualize, Alertmanager sends notifications.

Dedicated Certificate Monitoring

For large-scale deployments: commercial certificate monitoring platforms (Keyfactor, Venafi, DigiCert), custom certificate scanners running on schedule, integration with CMDB for asset correlation.

Alerting Strategy

• 90 days — Informational: renewal should be scheduled
• 30 days — Warning: renewal overdue, investigate
• 7 days — Critical: escalate to on-call, immediate action required
• 1 day — Emergency: all-hands, prevent imminent outage
• Renewal failure — Alert immediately when automated renewal fails

The Dashboard Every SRE Needs

One dashboard showing: total certificates managed, certificates expiring within 30/60/90 days, recently renewed certificates, failed renewal attempts, TLS configuration compliance score, and CT log anomalies. If this dashboard is green, your TLS infrastructure is healthy.