How to continuously monitor SSL certificates

▸ Why Scheduled Audits Beat Snapshot Checks

A one-time SSL check tells you the current state but nothing about what changes between now and tomorrow. Certificates get renewed, replaced, or accidentally downgraded. Let's Encrypt issues 90-day certificates; once 47-day lifetimes land (as planned), each domain needs a fresh renewal every 6–7 weeks. A continuous monitoring loop catches every state transition — not just the final expiry — giving you lead time to fix problems before they become outages.

▸ How Live Handshake Verification Works

Continuous monitoring works by re-probing the live TLS handshake on a fixed schedule and comparing the result against the last known certificate. The comparison checks the serial number, notAfter timestamp, issuer, and Subject Alternative Names. Any difference triggers a change event. This catches: a certificate that was replaced mid-validity (potentially a security issue), a renewal that changed the issuer (e.g. switching from DigiCert to Let's Encrypt), and a cert that was truncated to a shorter validity window.

▸ Determining Your Monitoring Cadence

Check frequency depends on your risk tolerance and plan. Free-tier monitoring runs on a slower schedule; paid plans can check as frequently as every minute. For most domains, hourly checks are more than sufficient — certificate problems do not appear silently within a one-hour window in ways that matter. The exception is high-value production APIs where a deployment could push a broken certificate at any moment: there, minute-level checks catch failures faster than your on-call rotation would notice.

▸ Monitoring Certificate Transparency Logs

Detect changes in Certificate Transparency logs in addition to live probing. CT logs record every certificate a CA issues, with a timestamp and a Signed Certificate Timestamp (SCT). Watching CT for your domains surfaces new issuances the moment they happen — before the certificate reaches port 443. This is how you catch mis-issuance: a CA incorrectly issues a certificate for your domain without your knowledge. You see it in CT and can request revocation within hours rather than discovering it months later.

▸ Audit Logging & Historical Fingerprints

Store every check result with a timestamp and keep an audit history. When an incident happens — 'why was the cert different last Tuesday at 3am?' — you want a queryable record of every probe result. History also powers uptime-style reporting: what percentage of checks in the last 30 days returned 'valid' vs 'expiring' or 'unreachable'. This data is useful for SLA reporting and post-mortems.

▸ Protecting Every Secondary Hostname

Scale the monitoring loop to your full fleet, not just the main domain. Most outages happen on secondary hostnames: the API subdomain, the admin panel, the staging environment someone forgot to renew, the internal service behind a load balancer. Building a monitoring config for each hostname manually is tedious; a good tool lets you add a domain in under a minute and have it join the same check-alert-history loop automatically.

▸ Integration with Webhooks and REST APIs

Continuous SSL monitoring integrates with your existing alerting infrastructure. Webhooks let you push certificate events into PagerDuty, Opsgenie, or a custom Slack bot. A REST API lets you pull certificate state into your own dashboards or feed it into a SIEM. For teams that have already invested in an incident management workflow, certificate alerts should join that workflow — not live in a separate tool that nobody checks.

▸ Infrastructure-Free Globally Distributed Probes

CertFleet runs this loop on Cloudflare Workers — globally distributed, no infrastructure to maintain. Add a domain, configure your alert channels, and the monitoring starts immediately. Start with the free instant checker to verify any domain right now, or read how domain-level monitoring works. Five domains and five uptime monitors are free forever.