SSL Certificate Checker

What is SSL Certificate Checker?

An SSL/TLS certificate checker connects to a host on its HTTPS port and reports everything about the certificate it presents — issuer, subject, Subject Alternative Names (SANs), validity dates, signature algorithm — plus which TLS protocol versions the server supports and the full certificate chain back to a trusted root.

It catches exactly the kind of problem that's invisible until a browser throws a scary warning at a real visitor: an expiring certificate, a missing intermediate in the chain, a hostname mismatch, or support for a deprecated TLS version.

How to use the SSL Certificate Checker tool

1 Enter the hostname you want to check, e.g. example.com (without https://).
2 Leave the port as 443 unless the service runs HTTPS on a custom port.
3 Run the check and review the issuer, validity dates, and SANs to confirm the certificate matches the domain and hasn't expired.
4 Check the supported TLS protocol versions — flag anything still relying on TLS 1.0/1.1, which modern browsers and compliance standards (like PCI DSS) now reject or discourage.

When you need this tool

Catching an SSL certificate that's about to expire before it takes a site offline with browser warnings.
Verifying a newly issued certificate installed correctly and the chain includes all required intermediates.
Confirming a multi-domain or wildcard certificate's SAN list actually covers every subdomain it needs to.
Auditing which TLS protocol versions and cipher suites a server still supports for a security review.
Diagnosing "certificate not trusted" errors by checking whether the chain is incomplete or self-signed.

How it works

The tool opens a TLS connection to the host and port you specify, performs the TLS handshake, and reads back the certificate and chain the server presents, along with the negotiated and supported protocol versions. It parses the certificate's issuer, subject, SAN list, validity window and signature algorithm directly from the X.509 certificate data returned during the handshake.

Frequently asked questions

What happens if my SSL certificate expires?

Every browser will show a hard, un-skippable security warning to visitors ("Your connection is not private") instead of your site, and most API clients and app backends will refuse the connection outright. There's no grace period — the moment the validity window closes, the certificate is untrusted everywhere.

What's the difference between a certificate error and an incomplete chain?

A certificate error (expired, wrong hostname, self-signed) is a problem with the leaf certificate itself. An incomplete chain means the leaf certificate is valid, but the server isn't sending the intermediate certificate(s) needed to link it back to a trusted root — some browsers tolerate this by caching intermediates, which is why a site can look "fine" in one browser and broken in another.

Is TLS 1.0 or 1.1 support a real problem?

Yes — both are deprecated by all major browsers and are non-compliant with PCI DSS and most modern security standards. Supporting them doesn't break anything for current users, but it's flagged by security scanners and should be disabled in favor of TLS 1.2 and 1.3.

What are SANs and why do they matter?

Subject Alternative Names (SANs) are the full list of hostnames a certificate is valid for — modern browsers check the SAN list, not the older "Common Name" field, to decide whether a certificate matches the domain being visited. A certificate missing a subdomain from its SAN list will fail validation for that subdomain even if the main domain works fine.