Chrome 4 went stable yesterday. One of the many new things in this release is the addition of Strict Transport Security. STS allows a site to request that it always be contacted over HTTPS. So far, only Chrome supports it. However, the popular NoScript Firefox extension also supports it and hopefully support will appear in Firefox proper at some point.

The issue that STS addresses is that users tend to type http:// at best, and omit the scheme entirely most of the time. In the latter case, browsers will insert http:// for them.

However, HTTP is insecure. An attacker can grab that connection, manipulate it and only the most eagle eyed users might notice that it redirected to https://www.bank0famerica.com or some such. From then on, the user is under the control of the attacker, who can intercept passwords etc at will.

An STS enabled server can include the following header in an HTTPS reply:

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

When the browser sees this, it will remember, for the given number of seconds, that the current domain should only be contacted over HTTPS. In the future, if the user types http:// or omits the scheme, HTTPS is the default. In fact, all requests for URLs in the current domain will be redirected to HTTPS. (So you have to make sure that you can serve them all!).

For more details, see the specification.

There is still a window where a user who has a fresh install, or who wipes out their local state, is vulnerable. Because of that, we'll be starting a "Preloaded STS" list. These domains will be configured for STS out of the box. In the beginning, this will be hardcoded into the binary. As it (hopefully) grows, it can change into a list this is shared across browsers, like the safe-browsing database is today.

If you own a site that you would like to see included in the preloaded STS list, contact me at .