Missing HTTP Strict Transport Security

Virtually every site runs on HTTPS these days, and current browsers try HTTPS first even when someone types an address without a protocol, clicks an old link, or follows a bookmark from years ago. But when that attempt fails, they fall back to an unencrypted request, at most after a warning the user can click past. That fallback is exactly what an attacker waits for, because they can make the attempt fail themselves. This article covers how HSTS closes that window, and which settings go wrong.

What is HTTP Strict Transport Security?

HTTP Strict Transport Security (HSTS) is an HTTP header with which a server instructs the browser to reach this domain over HTTPS only from now on. The browser remembers that instruction for the given period and from then on converts every http:// address for this domain to https://, before a single byte goes onto the network.

The difference from an ordinary redirect is subtle but decisive. A redirect is a response from the server: the browser therefore has to send an unencrypted request in order to hear that it should switch to HTTPS. HSTS is an instruction the browser remembers: no unencrypted request is sent to find out that it should not have been allowed.

Picture a reception desk where visitors have to show their passport. A redirect is the receptionist saying: “you are inside now, but please walk back and check in properly.” HSTS is the sign outside that visitors had already read before they walked in. In the first case, someone has already stood in the hall; in the second, they have not.

How does a downgrade attack work?

The attack takes place on the network: a public wifi network, a compromised router, or an intermediate provider. The attacker does not have to break encryption, only prevent it from ever being established.

Vulnerable:

HTTP/1.1 301 Moved Permanently
Location: https://portal.example/login

On the face of it this is correct: the server neatly forwards visitors to HTTPS. The problem is that in order to receive this response, the user first had to send the following:

GET /login HTTP/1.1
Host: portal.example

A current browser first tries HTTPS on its own, but an attacker sitting in between simply blocks that attempt, and the browser then falls back to this request, which travels across the network in readable form. The attacker never lets the redirect arrive. Instead they maintain the connection with the user over HTTP themselves, speak HTTPS with your server at the back, and read along with everything the victim types: username, password, session cookie. The user sees a working login page and at most notices that the padlock is missing.

Safe:

HTTP/1.1 200 OK
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

After one successful HTTPS visit the browser remembers this for a year. If the user then types portal.example in the address bar, the browser sends an HTTPS request straight away; there is no unencrypted request left to intercept. includeSubDomains extends that rule to every subdomain, which matters because a forgotten test.portal.example would otherwise remain an open side door. With preload, and once you submit the domain at hstspreload.org, it joins a list included in the browser itself, so even the very first visit is protected.

What is the impact of missing HSTS?

The severity is usually rated low to medium, because the attacker needs a position in the network path. That condition is less demanding than it sounds: a public network on a train, in a hotel or a café, a spoofed wifi access point, or a compromised home device will do.

If the attack succeeds, the damage is complete. The attacker reads everything that crosses the connection: credentials, session cookies, personal data and the contents of forms. They can also alter the response, which opens the way to a fake login screen or injected script code. Because the session cookie is captured in that scenario, the attacker does not even need to know the password to continue.

A max-age that is too short weakens the protection without removing it entirely. If the value is 30 days, a user who has not visited for six weeks is vulnerable again on their next visit, which is precisely the moment they type the address once more.

How do you detect missing HSTS?

The check starts with the HTTPS response: is there a Strict-Transport-Security header, and if so, how long is the max-age? A value under a year is a point of attention, and a value of zero actively disables the protection.

Then comes the investigation that scanners usually skip. Is the header set on subdomains too, or does includeSubDomains cover domains that offer no HTTPS themselves? Does the HTTP version of the site forward correctly to HTTPS, and does it land on the right path? Is the header accidentally set only on the home page and not on the API or the login endpoint? And is the domain on the preload list while a subdomain without a certificate still exists? During a penetration test AssistSec checks the full certificate chain and redirect structure, and assesses whether any path remains along which a user can still speak to your application unencrypted.

How do you prevent missing HSTS?

• Send Strict-Transport-Security on every HTTPS response, with max-age=31536000 as a guide value.
• Add includeSubDomains once you are sure all subdomains support HTTPS.
• Build up the lifetime in phases: start short, verify that everything works, then raise it to a year.
• Keep forwarding HTTP to HTTPS; HSTS complements that redirect rather than replacing it.
• Consider joining the preload list for domains that run permanently and fully on HTTPS.
• Never set the header on an unencrypted response; browsers ignore it there, which gives a false sense of security.
• Automate certificate renewal, for example through ACME, and monitor expiry: with HSTS visitors can no longer click past a certificate error, and publicly trusted certificates have been valid for at most 200 days since 15 March 2026, falling to 100 days in 2027 and 47 days in 2029.
• Configure the header centrally in your web server or proxy, so new hosts and routes inherit it automatically.