HSTS Check: a beginner's guide

HSTS stands for HTTP Strict Transport Security. It is a small HTTP response header — `Strict-Transport-Security: max-age=31536000; includeSubDomains; preload` — that tells the browser, "from now on, never connect to this domain over plain HTTP. Always force HTTPS, no exceptions, even if the user typed `http://` or clicked an old `http://` link." The browser remembers the instruction for the duration specified by the `max-age` directive (typically one or two years), and if a request to the HTTP version of the site ever comes up during that time, the browser silently rewrites it to HTTPS before it goes out on the wire.

You should care because **without HSTS, your visitors are still vulnerable to a class of attack called the first-visit hijack or *SSL stripping***. If a user types `bank.example.com` into their browser without specifying `https://`, the browser starts by sending an HTTP request — and an attacker on the same network (an open Wi-Fi hotspot, a compromised router, a state-level adversary on a national ISP) can intercept that initial plaintext request and never let it upgrade to HTTPS. The user thinks they're on a normal site; they are actually being silently man-in-the-middled. HSTS closes this window by making the browser remember, after the very first HTTPS visit, to never speak HTTP to that domain again.

The five things every HSTS check looks at:

• Is the HSTS header present? The bare minimum.

• Is `max-age` long enough? Modern best practice is at least one year (31536000 seconds), and two years for sites that want to be on the preload list.

• Is `includeSubDomains` set? Without it, only the apex domain is protected — every subdomain is still hijackable.

• Is `preload` set, and is the domain on the [HSTS preload list](https://hstspreload.org/)? Preloading bakes the HSTS rule into the browser itself, so even the very first visit is protected.

• Are there any HTTP-only subdomains that would break under `includeSubDomains`? This is the main reason teams hesitate — turning on `includeSubDomains` and discovering that an internal subdomain wasn't HTTPS-ready.

Three questions an HSTS check answers:

• Are my visitors protected against SSL-stripping attacks during their first visit?

• Is my entire domain — including all subdomains — covered, or just the root?

• Should I be on the HSTS preload list, and what would it take to qualify?

The cost of skipping HSTS is the small but persistent risk of session hijacking on insecure networks. The fix is one HTTP header. For sites that handle payments, logins, or any sensitive data, HSTS is no longer optional; it is the baseline. The protocol is defined in RFC 6797.

In one sentence: Check HSTS (HTTP Strict Transport Security) policy and preload status

Validates HSTS (HTTP Strict Transport Security) (HTTP (HyperText Transfer Protocol) Strict Transport Security, the official internet standard) configuration including max-age duration, includeSubDomains directive, preload directive, and preload list inclusion status. Checks against hstspreload.org requirements — the shared preload list used by Chrome, Firefox, Safari, and Edge.

Don't worry if some of the words above are still unfamiliar — there's a plain-language glossary at the bottom of this page, and most of the terms link to their own beginner guides if you want to learn more.

Here's what's actually happening behind the scenes when you call this endpoint:

Fetches the Strict-Transport-Security header from the domain, parses all directives (max-age, includeSubDomains, preload), validates against preload requirements (max-age ≥ 31536000, includeSubDomains required, preload directive required), and queries the hstspreload.org API (Application Programming Interface) to check if the domain is already on the browser preload list. Returns human-readable max-age formatting and specific remediation guidance for any issues found.

If you're using an AI assistant through MCP, you don't need to understand any of the technical details — the assistant calls the tool and translates the result for you.

Let's skip the marketing fluff and answer the only question that actually matters: why should you, a real human with a real to-do list, care about the HSTS Check tool? Here's the plain-English version, written the way you'd hear it from a friend who happens to do this for a living.

HSTS (HTTP Strict Transport Security) (the official internet standard) protects against SSL (Secure Sockets Layer) stripping and protocol downgrade attacks by ensuring browsers only connect via HTTPS (secure HyperText Transfer Protocol). Without HSTS, an attacker on the same network can intercept the initial HTTP (HyperText Transfer Protocol) request and redirect to a malicious site. Preload list inclusion provides protection from the very first visit — critical for high-security applications. HSTS is required by PCI DSS 4.0.1 and recommended by OWASP and NIST SP 800-52r2 (Guidelines for TLS (Transport Layer Security) Implementations).

Picture this in real life. Imagine a devops engineer. Here's the situation they're walking into: After migrating to HTTPS (secure HyperText Transfer Protocol), verify HSTS (HTTP Strict Transport Security) is properly configured to prevent downgrade attacks. Without the right tool, that person would be stuck copy-pasting between five browser tabs, reading documentation written for engineers, and crossing their fingers that the answer they cobble together is correct. With the HSTS Check tool, the same person gets a clear answer in seconds — no spreadsheets, no guessing, no waiting for someone on the infrastructure team to free up.

Three questions this tool answers in plain English. If any of these have ever crossed your mind, the HSTS Check tool is built for you:

• Is my website encrypted properly, or are visitors going to see a scary browser warning?

• Am I missing any of the security headers that modern browsers expect?

• Could a known weakness on my site quietly be costing me trust, traffic, or compliance?

You can either click the tool and get the answer yourself, or ask your AI assistant — connected through MCP (Model Context Protocol) — to ask the question for you and translate the answer into something you can paste into Slack.

Who gets the most out of this. Founders and freelancers running their own sites, agencies handing off projects to clients, security and compliance teams chasing audit findings, and developers hardening login pages. If you see yourself in that list, this is one of the EdgeDNS tools you should bookmark today.

What happens if you skip this entirely. Skip it and visitors get browser warnings, search engines lose trust in your site, and a single missed setting can become a public security incident. That's why running this check — even once a month — is one of the cheapest forms of insurance you can give your domain.

If you're still on the fence about whether the HSTS Check tool belongs in your toolbox, this section is for you. Below you'll meet three real people — a devops engineer, a security engineer, and a security auditor — facing three real situations where this tool turns a stressful afternoon into a five-minute task. Read whichever story sounds closest to your week.

Story 1: HTTPS Migration Verification

Imagine you're a devops engineer. After migrating to HTTPS (secure HyperText Transfer Protocol), verify HSTS (HTTP Strict Transport Security) is properly configured to prevent downgrade attacks.

Why it matters: Ensure HTTPS (secure HyperText Transfer Protocol) is enforced and users can't be redirected to HTTP (HyperText Transfer Protocol).

Story 2: Preload List Submission Prep

Imagine you're a security engineer. Before submitting to HSTS (HTTP Strict Transport Security) preload list, verify all requirements are met (max-age, includeSubDomains, preload).

Why it matters: Avoid preload submission rejection by validating configuration first.

Story 3: Security Assessment

Imagine you're a security auditor. Evaluate HSTS (HTTP Strict Transport Security) configuration as part of web application security assessment.

Why it matters: Document HTTPS (secure HyperText Transfer Protocol) enforcement controls for security audits.

Common situations across teams. Beyond the three stories above, here are the everyday workplace moments when people across the company reach for the HSTS Check tool — or one of the tools right next to it in this category. If any of these are on your calendar this month, that's your sign:

• After every site redesign or platform migration.

• Before a penetration test, security review, or vendor questionnaire.

• When your SSL certificate is about to expire and you want to confirm the renewal worked.

• On a recurring monthly schedule, so you catch new issues before attackers do.

If you can see yourself in even one of those bullets, the HSTS Check tool will pay for itself the first time you use it.

Still not sure? Here's the easiest test in the world. Open Claude, ChatGPT, Gemini, or any other AI assistant connected to the EdgeDNS MCP server and ask, in your own words: "Is the HSTS Check tool useful for my job?" The assistant will look at the tool, ask you a couple of follow-up questions about what you're trying to accomplish, and give you a straight answer in plain English. No commitment, no signup forms, no jargon.

If you've connected the EdgeDNS MCP server to Claude, ChatGPT, Gemini, Cursor, or any other AI assistant, you don't need to write any code. Just ask in plain English:

"Use the HSTS Check tool to check google.com and explain anything that looks wrong in plain language."

The AI will figure out which tool to call, fill in the right parameters, run it, and then explain the result back to you. No copy-pasting between tabs. No reading raw JSON. No memorizing endpoint names.

If you're a developer and want to call the endpoint from a script or your own application, here's the simplest possible example. Replace the placeholder API key with the real one from your dashboard.

There's just one piece of information you need to provide. The table below explains exactly what it is and what a real value looks like.

When you call this tool, you'll get back a JSON object with the fields below. If you're talking to it through an AI assistant, the assistant reads these for you and explains them in plain language — you don't need to memorize them.

These tools pair naturally with HSTS Check. If you're already running this check, the related ones often answer the next question that comes up:

• Security Headers — Analyze HTTP security headers, score, and compliance

• SSL Certificates — Analyze active SSL/TLS certificate details

• Security Score — Composite security score from 8 weighted factors

If you're using an AI assistant through MCP (Model Context Protocol), you can ask it to run several of these in one go: "Check HSTS Check and the related tools for example.com and tell me what looks off."

If any words on this page felt like jargon, here's a plain-language version. Click any linked term to read a full beginner-friendly guide.

API (Application Programming Interface) — A way for one program to ask another program for something — like a waiter taking your order to the kitchen.

HTTP (HyperText Transfer Protocol) — The language web browsers and websites use to talk to each other.

HTTPS (secure HyperText Transfer Protocol) — HTTP with encryption — the little padlock in your browser. It means nobody between you and the website can read what you're sending.

SSL (Secure Sockets Layer) — The original encryption used by HTTPS. The name stuck even though every modern site actually uses TLS, the newer replacement.

TLS (Transport Layer Security) — The encryption that puts the 'S' in HTTPS. It scrambles data so nobody between you and a website can read it.

HSTS (HTTP Strict Transport Security) — A header that tells browsers "always use HTTPS for this site, never plain HTTP, even if the user types it." Prevents downgrade attacks.

RFC (Request for Comments) — The official internet standards documents. When someone says 'RFC 8484' they mean a specific numbered standards document — in that case, the one defining DNS over HTTPS.