How to add Strict-Transport-Security on WordPress

Last updated November 11, 2026 · Reviewed by a CISSP-certified security practitioner

To add Strict-Transport-Security on WordPress, set the header in functions.php (child theme) or a headers plugin with the value max-age=63072000; includeSubDomains; preload. Forces browsers to only use HTTPS for your domain. Clear page caches (plugin + CDN) and curl -I your homepage. Then confirm it with curl -I or a free FixSecurityHeaders scan.

What does Strict-Transport-Security do?

HSTS tells the browser to always connect over HTTPS for a set time, even if someone types http:// or clicks an old link.

Risk if missing: Without it, the first request can be downgraded to plain HTTP, letting attackers on public Wi‑Fi intercept or rewrite traffic (SSL stripping).

What is the WordPress configuration?

Add this to functions.php (child theme) or a headers plugin:

functions.php (child theme) or a headers plugin

<?php
// Child theme functions.php (or a small mu-plugin)
add_action('send_headers', function () {
    header('Strict-Transport-Security: max-age=63072000; includeSubDomains; preload');
});

How do I add it step by step?

Open functions.php (child theme) or a headers plugin.
Paste the Strict-Transport-Security configuration shown below.
Clear page caches (plugin + CDN) and curl -I your homepage.
Run curl -I https://yourdomain.com and look for the strict-transport-security line.
Scan your site on FixSecurityHeaders to confirm the grade improved.

How do I verify it worked?

terminal

curl -sI https://yourdomain.com | grep -i "strict-transport-security"
# expected:
# strict-transport-security: max-age=63072000; includeSubDomains; preload

Check your site now

Free scan, instant grade, exact fixes for your stack.

What are the common mistakes?

The preload directive is hard to undo: removal from browser preload lists takes months.
includeSubDomains breaks any subdomain that still serves plain HTTP. Audit subdomains first.
Start with a short max-age (e.g. 300) and ramp up to 1–2 years once verified.
Browsers ignore HSTS sent over plain HTTP; it must be sent on the HTTPS response.
Prefer setting headers at the web server (nginx/Apache) — PHP headers don't apply to static files.
Page-cache plugins may serve cached responses without your headers until purged.

Quick reference

Header	`Strict-Transport-Security`
Recommended value	`max-age=63072000; includeSubDomains; preload`
Stack	WordPress
Where	functions.php (child theme) or a headers plugin
Compliance	PCI DSS 4.0 4.2.1, NIST 800-171 3.13.8, CMMC SC.L2-3.13.8, OWASP ASVS V14.4.5, SOC 2 CC6.7

FAQ

What value should I use for Strict-Transport-Security?

A safe default is max-age=63072000; includeSubDomains; preload. The preload directive is hard to undo: removal from browser preload lists takes months.

Why isn't Strict-Transport-Security showing up on WordPress?

Prefer setting headers at the web server (nginx/Apache) — PHP headers don't apply to static files. Also clear any CDN or page cache, and check you edited the config that actually serves your domain.

Does Strict-Transport-Security affect SEO or performance?

No measurable performance cost — it's a few bytes per response. It doesn't affect rankings directly, but a misconfigured policy can break page resources, so test before enforcing.

Which compliance frameworks reference Strict-Transport-Security?

PCI DSS 4.0 4.2.1, NIST 800-171 3.13.8, CMMC SC.L2-3.13.8, OWASP ASVS V14.4.5, SOC 2 CC6.7.