Plain-English explanations of browser security warnings and email bounces. Each page tells you what it means if you were just trying to buy something, and what to do if it is your own site. Want a straight answer about a specific site? Run a free check.
The full page warnings a browser shows before it will load a site.
Chrome's full page certificate warning, and what it actually means.
The certificate has expired, or your device clock is wrong.
The certificate is not signed by an authority the browser trusts.
The certificate does not cover the address you visited.
The certificate was cancelled by the authority that issued it.
Firefox's certificate warning, and how it differs from Chrome's.
Firefox does not trust the certificate issuer. Often a missing intermediate.
Firefox's version of an expired certificate.
Firefox's version of a certificate that does not match the address.
Safari's certificate warning, which shows no error code.
When the page will not load at all, or loads without protection.
The secure connection failed before it started.
The server is too old to agree on a secure connection.
A redirect loop, very often caused by an HTTPS misconfiguration.
The address bar label, and why the padlock disappeared.
An HTTPS page loading parts of itself insecurely.
The red warnings from Google Safe Browsing.
Chrome's current red warning, which replaced three older ones.
The classic phishing warning, and what Chrome shows now instead.
The malware warning, and how an owner gets it lifted.
When your messages are rejected instead of delivered.
Errors from the proxy sitting in front of a site.
Cloudflare could not complete a secure connection to your server.
Cloudflare reached your server but rejected its certificate.
Your DNS points somewhere Cloudflare will not proxy.
Paste any address and we will tell you which checks it passes and which it fails, in seconds.