What is HTTP strict transport security (HSTS)?

Delivering web pages and data over HTTP connections exposes users to security risks.

Because servers are usually not connected directly to each other, they must pass their requests and responses through a series of network routers. These routers, located in-between servers, have complete access to requests sent via HTTP connections. Since the data is transferred as unencrypted plain text, the routers can act as a “man in the middle” and read or manipulate data in transit.

This can cause users to receive manipulated information or be directed to hostile servers used by attackers to steal information like passwords and credit card info. This kind of interception can go undetected since a compromised HTTP response looks the same as a genuine response.

The HSTS policy forces all responses to pass through HTTPS connections instead of plain text HTTP. This ensures that the entire channel is encrypted before any data is sent, making it impossible for attackers to read or modify the data in transit.

How HSTS works

First, you must add the HSTS response header to the server. Enabling HSTS on a server involves adding the following HSTS response header in an HTTPS reply:

Strict-Transport-Security: max-age=expireTime [; includeSubdomains]

For example:

Strict-Transport-Security: max-age=16070400; includeSubDomains

The minimum parameter is the max-age in seconds. This specifies the time the browser should connect to the server using the HTTPS connection. However, it’s recommended to include the subdirectories so that the browser uses the HTTPS connection for existing and future subdomains.

When the browser accesses the website, the server replies with the HSTS header.

This instructs the browser to only connect to the server and the entire domain through HTTPS. The browser will then remember to use the HTTPS connection for the specified max-age.

Even if a user types http://www.domain.com, types the domain name without http, uses a bookmark, or a third party HTTP link, the browser will automatically upgrade the request to HTTPS. Once the max-age expires, the browser starts accessing the server through HTTP unless the user specifies HTTPS.

After receiving the HSTS header, the browser sends an HTTPS request.

HSTS is supported by most browsers. Chrome and Mozilla Firefox maintain an HSTS preload list that automatically informs the browser that the website can only be accessed through HTTPS. A webmaster can add a website to the preloaded HSTS list by adding the “preload” parameter to the header and then submitting the domain to the list.

For example:

Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

Example of HSTS

Say a user tries to connect to an online banking platform through public WiFi and the access point is a hacker’s computer instead. The hacker intercepts the original HTTP request and redirects the user to a clone of the bank’s site. This exposes all the user’s private data such as credit card information and password.

The HSTS policy resolves this, and as long as the user had accessed the bank’s website using an HTTPS connection befoer, the browser will automatically use HTTPS, preventing such man-in-the middle attacks.

Here are a few sample headers:

Strict-Transport-Security: max-age=631138519; includeSubdomains

This enforces the use of HTTPS for 20 years, including present and future subdomains.

Strict-Transport-Security: max-age=31536000

This enforces the use of HTTPS for one year, but does not include subdomains.

Benefits of HSTS

  1. Protection against HTTP downgrade attacks (SSL stripping attacks) by requiring all traffic to utilize HTTPS. It rewrites requests that do not point to encrypted sources.
  2. Mixed content defense. HSTS automatically upgrades fetches to HTTPS in situations where a domain has mixed content.
  3. Better security in general: An HSTS-compliant browser aborts the connection to an HSTS-compliant server whenever the security of a certificate can’t be confirmed. Plus, users can’t click through self-signed certificates.