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How to add Strict-Transport-Security on IIS

Last updated · Reviewed by a CISSP-certified security practitioner

To add Strict-Transport-Security on IIS, set the header in web.config with the value max-age=63072000; includeSubDomains; preload. Forces browsers to only use HTTPS for your domain. Save web.config; IIS applies it on the next request. Recycle the app pool if needed. 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 IIS configuration?

Add this to web.config:

web.config
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Strict-Transport-Security" value="max-age=63072000; includeSubDomains; preload" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

How do I add it step by step?

  1. Open web.config.
  2. Paste the Strict-Transport-Security configuration shown below.
  3. Save web.config; IIS applies it on the next request. Recycle the app pool if needed.
  4. Run curl -I https://yourdomain.com and look for the strict-transport-security line.
  5. 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

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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.
  • Headers in web.config apply to every response from that site.
  • Removing the Server header requires removeServerHeader (IIS 10+) or URL Rewrite.

Quick reference

HeaderStrict-Transport-Security
Recommended valuemax-age=63072000; includeSubDomains; preload
StackIIS
Whereweb.config
CompliancePCI 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 IIS?

Headers in web.config apply to every response from that site. 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.

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