`document.domain` Setting is Deprecated

Setting document.domain is deprecated. document.domain setting can be used to relax the same-origin restrictions between different frames, hosted within the same site but on different origins. Doing so make themselves effectively same-origin for the purpose of synchronous access.

For example, https.//www.example.test/ might host a media player on media.example.org. If both the main page and the frame execute document.domain = "example.org"; they may then access each others' DOM tree, which they normally couldn't. (A better way would be to cooperate by postMessage().)

This usage is now being deprecated. (More information can be found here and there).

What is happening, and when?

M100: Chrome will show deprecation warnings when document.domain is set.
M106: Chrome will disable document.domain by default.

Note that the second milestone is tentative: When the time comes, we will examine how many pages will be impacted by this change, and will start a separate discussion (intent to remove) on the blink-dev mailing list.

`document.domain` and Origin-keyed Agent Clusters

Most documentation on this change is phrased in terms of origin-keyed agent clusters. This is a concept in the HTML specification. Here we focus on the behaviour of the document.domain setter, which is the visible effect.

A web browser can cluster sites (in order to assign them to operating system processes) and sites can be clustered by origin, or by site. Origin-keyed agent clustering is preferable for security reasons. However when sites are clustered by origin, synchronous access to frames outside of that origin (but within the same site) is no longer possible. Thus sites in origin-keyed agent clusters disable the document.domain setter. This is the mechanism underlying this change: From M106 on pages will be assigned to origin-keyed agent clusters by default and therefore document.domain will no longer be settable by default, either.

This also gives us an opt-out mechanism for pages who do not wish to follow this change: By setting the Origin-Agent-Cluster: ?0 http header, a site can request assignment to a site-keyed agent cluster, and document.domain will continue to work for them as it currently does. Note that adding this header has no other observable effect and thus retains the current (pre-M106) behaviour. This makes it an easy and safe way to opt-out.

Setting this header is a no-op in current versions of Chromium since it matches the default setting, and will preserve this behaviour in the future. It is also a no-op in other browsers, since they either match Chromium's current default or have not implemented the Origin-Agent-Cluster header at all.

Where are the deprecation warnings found?

The deprecation warnings are found in the issues tab.

What does the deprecation warning tell me?

There are two deprecation warnings: One for setting the document.domain accessors, which modifies the security behaviour. And from M101 on, a second warning when a cross-domain access is made that is facilitated by the modified document.domain property. The first warning tells you where the setup happens, and the second one tells you where it is being used (and thus likely why this is being done in the first place).

How Can I Test This?

In the DevTools console, for a page www.example.test:

document.domain = "example.test";
document.domain;  // "example.test" in a site-keyed agent cluster.
                  // "www.example.test" in an origin-keyed agent cluster.

One can also directly query whether a page is assigned to an origin-keyed agent cluster, by querying window.originAgentCluster.

window.originAgentCluster;  // true, if page is assigned to an origin-keyed
                            // agent cluster.

How to enable/disable the deprecation:

Enable the Warning (Before M100)

Start Chrome with --enable-features=OriginAgentClusterDefaultWarning

Enable the Deprecation (Scheduled for M106)

In chrome://flags, go to the “Origin Agent Cluster Default” setting and enable it.
Or start Chrome with --enable-features=OriginAgentClusterDefaultEnable
Or add the Origin-Agent-Cluster: ?1 header to your pages and frames. (E.g. in a testing instance of your site.)

Testing at Scale / Reporting API

The deprecation warnings are delivered through the Reporting API. They can be pragrammatically processed using ReportingObserver. For example, the first code snippet in https://developers.google.com/web/updates/2018/07/reportingobserver will report these warnings. The message object delivered by this API is a DeprecationReportBody instance which offers information about the source code location that triggered the warning.

What can I do?

If your site does not use document.domain setting you don't have to do anything. You could explicitly set the Origin-Agent-Cluster: ?1 header. But after M106 this would be the default behaviour anyhow.

If your site uses document.domain setting to enable cross-origin Javascript access, you should refactor the code to instead use window.postMessage() (or any other mechanism) to cooperate across origins. Or alternatively reduce the need for cross-origin cooperation by moving the cooperating pieces onto the same origin.

What if I don't have the time right now, or want to continue setting `document.domain`?

You can add the Origin-Agent-Cluster: ?0 HTTP header to your site. Note that the header must be set both for the main page, as well as for the embedded frame(s) that wish to use document.domain setting.

Enterprise Users

Users of Chrome for Enterprise can set the OriginAgentClusterDefaultEnabled policy to False to retain the current (pre-M106) default for all of their users, until all their internal sites and customers have migrated off of document.domain usage.

document.domain Setting is Deprecated