SSL Certificate Inspection & Debugging
Decode PEM certificates, check SSL configuration, and validate security headers in one browser-based workflow.
Overview
SSL/TLS issues are among the most common causes of production outages and security vulnerabilities. This workflow helps you decode and inspect certificate contents, verify your SSL configuration, and audit your HTTP security headers, all without sending data to third-party tools.
Step-by-Step Implementation
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Quick Summary
Three-step TLS audit: decode the PEM to see what's actually in the cert, query the live endpoint to verify chain validity, then check the response headers (HSTS, CSP, X-Frame) that backstop TLS. Catches 90% of 'why is the padlock red?' incidents.
Key Takeaways
- A cert can be 'valid' (signature OK, not expired) but still cause errors if the SAN doesn't include the requested hostname.
- Most browser trust failures are missing intermediate certificates, the leaf is fine, the chain is incomplete.
- HSTS with `preload` requires submission to the browser preload list; without it, the first request is still HTTP-attackable.
- Wildcard certs (`*.example.com`) match one level only, `*.example.com` does NOT cover `a.b.example.com`.
- Let's Encrypt certs expire every 90 days, automate renewal or set a 60-day calendar alert.
When to use it
- Diagnosing 'NET::ERR_CERT_AUTHORITY_INVALID' or similar browser warnings.
- Auditing security headers ahead of a SOC2 / ISO 27001 review.
- Post-deployment verification that a wildcard or SAN cert was installed correctly.
- Investigating why curl works but a browser fails on the same URL (often: missing intermediates).
Common Mistakes
- Installing only the leaf certificate and forgetting the intermediate chain, the leaf alone won't validate.
- Setting HSTS `max-age` to a small value during testing and forgetting to raise it for production.
- Forgetting that HSTS is sticky, once a browser caches it, you can't easily roll back without preload removal.
- Trusting a 'green padlock' as security, TLS encrypts transport, it doesn't validate the server's intent.
SSL Certificate Inspection & Debugging, Frequently Asked
Why does my cert fail in mobile browsers but work on desktop?
Usually a missing intermediate certificate. Desktop browsers sometimes cache intermediates from past sessions; mobile doesn't. Always serve the full chain.
How long should `Strict-Transport-Security max-age` be?
Two years (`max-age=63072000`) is the preload-list requirement. Start with one month during testing, raise to two years once you're confident.
What's the difference between SSL and TLS?
SSL is the old name; TLS is what every modern cert actually uses. 'SSL certificate' has stuck as marketing terminology, but you should configure TLS 1.2+ only.