View source for SecurityEngineering/HTTP Strict Transport Security (HSTS) Preload List

Firefox ships with a list of hosts that are considered HTTP Strict Transport Security (HSTS - [https://tools.ietf.org/html/rfc6797 see RFC 6797]) by default. This list is based on [https://www.chromium.org/hsts/ a list Chromium maintains]. The versions of the list as it exists in the various channels of Firefox are available here: [https://hg.mozilla.org/mozilla-central/file/tip/security/manager/ssl/nsSTSPreloadList.inc mozilla-central] [https://hg.mozilla.org/releases/mozilla-beta/file/tip/security/manager/ssl/nsSTSPreloadList.inc mozilla-beta] [https://hg.mozilla.org/releases/mozilla-release/file/tip/security/manager/ssl/nsSTSPreloadList.inc mozilla-release] [https://hg.mozilla.org/releases/mozilla-esr60/file/tip/security/manager/ssl/nsSTSPreloadList.inc mozilla-esr60].

Every day, an automated job attempts to update the preload list in mozilla-central and mozilla-esr. This involves running an xpcshell script that makes an https request to each candidate host on the list. If xpcshell can connect successfully to a host and receives a "Strict-Transport-Security" header with a max-age value of at least 10886400 (18 weeks in seconds), that host is included in the list (the "preload" directive is ignored). If xpcshell cannot connect successfully to a host or does not receive an appropriate header, that host is not included in the preload list.

The xpcshell script is [https://hg.mozilla.org/mozilla-central/file/tip/taskcluster/docker/periodic-updates/scripts/getHSTSPreloadList.js here]. Output from the automated job as run on each branch is available here: [https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&searchStr=pfu mozilla-central] [https://treeherder.mozilla.org/#/jobs?repo=mozilla-esr60&searchStr=pfu esr60] (scroll down until there's a line containing "pfu", click on that, then click on "live.log" in the pane that pops up).

To guard against accidentally dropping a host from the list due to intermittent network issues or an active attacker, if a host is already on the preload list in Firefox but cannot be reached, the script keeps it on the preload list. For a host to be removed from Firefox's preload list, it must be accessible when the update script runs and it must either not send a Strict-Transport-Security header or it must send the header with a max-age less than 10886400.

The preload list has a built-in expiration time that is 18 weeks from when the list was most recently updated.