HSTS checker

What is an HSTS Checker?

The HSTS Checker verifies whether a website sends the Strict-Transport-Security response header, which instructs browsers to only ever connect to that domain over HTTPS, never plain HTTP, for a specified duration.

Why HSTS Matters

• Prevents downgrade attacks — Without HSTS, an attacker on the network can intercept the first HTTP request before a redirect to HTTPS happens (SSL stripping).
• Removes redirect overhead — Once a browser has seen the HSTS header, it upgrades all future requests to HTTPS internally, skipping the initial HTTP round trip entirely.
• max-age matters — A short or missing max-age value means the protection expires quickly and needs to be re-established.