This document defines how a user's display, or parts thereof, can be used as the source of a media stream using getDisplayMedia, an extension to the Media Capture API [[!GETUSERMEDIA]].

This document is not complete. It is subject to major changes and, while early experimentations are encouraged, it is therefore not intended for implementation.

Introduction

This document describes an extension to the Media Capture API [[GETUSERMEDIA]] that enables the acquisition of a user's display, or part thereof, in the form of a video stream. This enables a number of applications, including screen sharing using WebRTC [[WEBRTC]].

This feature has signficant security implications. Applications that use this API to access information that is displayed to users could access confidential information from other origins if that information is under the control of the application. This includes content that would otherwise be inaccessible due to the protections offered by the user agent sandbox.

This document concerns itself primarily with the capture of video, but the general mechanisms defined here could be extended to other types of media, of which audio [[GETUSERMEDIA]] and depth [[MEDIACAPTURE-DEPTH]] are currently defined.

This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.

Implementations that use ECMAScript [[ECMA-262]] to implement the APIs defined in this specification must implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]], as this specification uses that specification and terminology.

Example

The following example demonstrates a request for display capture using the navigator.mediaDevices.getDisplayMedia method defined in this document.

navigator.mediaDevices.getDisplayMedia({ video: true })
  .then(stream => {
    // we have a stream, attach it to a feedback video element
    videoElement.srcObject = stream;
  }, error => {
    console.log("Unable to acquire screen capture", error);
  });

Terminology

This document uses the definition of NavigatorUserMedia, MediaStreamTrack, and ConstrainablePattern from [[!GETUSERMEDIA]].

Screen capture encompasses the capture of several different types of screen-based surfaces. Collectively, these are referred to as display surfaces, of which this document defines the following types:

This document draws a distinction between two variants of each type of display surface:

Some operating systems permit windows from different applications to occlude other windows, in whole or part, so the visible display surface is a strict subset of the logical display surface.

Capturing Displayed Media

Capture of displayed media is enabled through the addition of a new getDisplayMedia method on the NavigatorUserMedia interface, that is similar to getUserMedia [[!GETUSERMEDIA]].

Though all constraints are supported for display device capture, not all constraints will have an effect. Usually, this is because a display device cannot alter its behavior in a way that the constraint implies, just as a video camera cannot alter volume.

For instance, it is unlikely that setting the following constraints will have any useful effect (other than an overconstrained error): aspectRatio, facingMode, and echoCancellation.

See for information on the deviceId and groupId constraints.

NavigatorUserMedia Additions

partial interface NavigatorUserMedia {
    Promise<MediaStream> getDisplayMedia (optional MediaStreamConstraints constraints);
};
getDisplayMedia

This method operates identically to getUserMedia, except that it acquires media from display devices, and the constraints argument does not accept MediaTrackConstraints values. This prevents an application from influencing the selection of sources, see for details.

In addition to drawing from a different set of sources and requiring user selection, getDisplayMedia also differs from getUserMedia in that permissions cannot be persisted.

If the constraints argument is omitted, a default value containing a single video attribute set to true is assumed.

Parameter Type Nullable Optional Description
constraints MediaStreamConstraints
Return type: Promise<MediaStream>

Constraining Display Surface Selection

The getDisplayMedia function does not permit the use of constraints for selection of a source as described in the getUserMedia() algorithm. Prior to invoking the getUserMedia() algorithm, if either of the video and audio attributes are set to a MediaTrackConstraints value (as opposed to being absent or set to a Boolean value), reject the promise with a InvalidAccessError and abort.

Not accepting constraints for source selection means that getDisplayMedia only provides fingerprinting surface that exposes whether audio, video or audio and video sources are present. (This is a fingerprinting vector.)

New Constraints for Captured Display Surfaces

Two constraints are defined that allow an application to observe properties of the selected display surface. Since the source of media cannot be changed after a MediaStreamTrack has been returned and constraints do not affect the selection of display surfaces, these constraints cannot be changed by an application.

The displaySurface constraint allows an application to observe the type of display surface that is being captured.

The logicalSurface constraint allows an application to observe whether the surface that is captured is a logical display surface, rather than the visible display surface.

Extensions to MediaTrackConstraintSet

partial dictionary MediaTrackConstraintSet {
             ConstrainDOMString displaySurface;
             ConstrainBoolean   logicalSurface;
};
displaySurface of type ConstrainDOMString

The type of display surface that is being captured. This assumes values from the DisplayCaptureSurfaceType enumeration.

logicalSurface of type ConstrainBoolean

A value of true indicates capture of a logical display surface; a value of false indicates a capture capture of a visible display surface.

DisplayCaptureSurfaceType

The DisplayCaptureSurfaceType enumeration describes the different types of display surface.

enum DisplayCaptureSurfaceType {
    "monitor",
    "window",
    "application",
    "browser"
};
Enumeration description
monitor a monitor display surface, physical display, or collection of physical displays
window a window display surface, or single application window
application an application display surface, or entire collection of windows for an application
browser a browser display surface, or single browser window

Device Identifiers

Each potential source of capture is treated by this API as a discrete media source. However, display capture sources MUST NOT be enumerated by enumerateDevices, since this would reveal too much information about the host system.

Display capture sources therefore cannot be selected with the deviceId constraint, since this would allow applications to influence selection; setting deviceId constraint can only cause the resulting MediaStreamTrack to become overconstrained.

A display capture source is represented in the MediaStreamTrack API as having deviceId and groupId attributes that are randomized each time a MediaStreamTrack is connected. These values cannot duplicate any existing values.

Security and Permissions

This section is informative; however, it notes some serious risks to platform security if the advice it contains are not adhered to.

This is consistent with other documents, but the absence of strong normative language here is a little worrying.

The risks to user privacy and security posed by capture of displayed content are twofold. The immediate and obvious risk is that users inadvertently share content that they did not wish to share, or might not have realized would be shared.

Display capture presents a less obvious risk to the cross site request forgery protections offered by the browser sandbox. Display and capture of information that is also under the control of an application, even indirectly, can allow that application to access information that would otherwise by inaccessible to it directly. For example, the canvas API does not permit sampling of a canvas, or conversion to an accessible form if it is not origin-clean [[2DCONTEXT]].

This issue is discussed in further detail in [[!RTCWEB-SECURITY-ARCH]] and [[!RTCWEB-SECURITY]].

Display capture that includes browser windows, particularly those that are under any form of control by the application, risks violation of these basic security protections. This risk is not entirely contained to browser windows, since control channels between browser applications and other applications, depending on the operating system. The key consideration is whether the captured display surface could be somehow induced to present information that would otherwise be secret from the application that is receiving the resulting media.

Capturing Logical or Visible Display Surfaces

Capture of logical display surfaces causes there to be a potential for content to be shared that a user is not made aware of. A logical display surface might render information that a user did not intend to expose. This can be more easily recognized if this information is visible. Such means are likely ineffectual against a machine, but a human recipient is less able to process content that appears only briefly.

Information that is not currently rendered to the screen SHOULD be obscured in captures unless the application has been specifically authorized to access that content (this might require elevated permissions).

How obscured areas of the logical display surface are captured to produce a visible display surface capture MAY vary. Some applications, like presentation software, benefit from having obscured portions of the screen render the image that appeared prior to being obscured. Freezing images can cause visual artifacts for changing content, or hide the fact that content is being obscured. Note that frozen portions of a capture can be incorrectly perceived as a bug. Alternatively, obscured areas might be replaced with content that marks them as being obscured, such as a grey color or hatching.

Some systems MAY only capture the logical display surface. Devices with small screens, for instance, do not typically have the concept of a window, and render applications in full screen modes only. These systems might provide a capture of an application that is not currently visible, which could be unusable without capturing the logical display surface.

An important consideration when capturing a window or other display surface that is partially transparent is that content from the background might be shared. A user agent MUST NOT capture content from the background of a captured display surface.

Authorizing Display Capture

This document provides recommends that implementations provide additional limitations on the mechanisms used to affirm user consent. These limitations are designed to mitigate the security and privacy risks that the API poses.

Two forms of consent interaction are described: active user consent and a range of elevated permissions. These are non-normative recommandations only.

Active User Consent

Active user consent is sufficient where there is little or no risk of an application gaining information that the user did not intend to share. These cases can be identified by those where the application that requests capture has no control over what is rendered to the captured display surface.

To prevent applications from limiting the available choices presented to a user with the goal of promoting a particular choice, the getDisplayMedia API does not permit the use of constraints to narrow the set of options presented.

Elevated Permissions

It is strongly advised that elevated permissions be required to access any display surface that might be used to circumvent cross-origin protections for content. The key goal of this consent process is not just to demonstrate that a user intends to share content, but to also to determine that the user exhibits an elevated level of trust in the application that is being granted access.

Several different controls might be provided to grant elevated permissions. This section describes several different capabilities that could be independently granted. A user agent might opt to prohibit access to any capability that requires elevated permissions.

If access to these surfaces is supported, it is strongly advised that any mechanism to acquire elevated permissions not rely solely on simple prompts for user consent. Any action needs to ensure that a decision to authorize an application with elevated privileges is deliberate. For instance, a user agent might require a process equivalent to software installation to signify that user consent for elevated permissions is granted.

An elevated permissions experience could allow the user agent to communicate the risks associated with enabling this feature, or at least to convey the need for augmented trust in the application.

Note that elevated permissions are not a substitute for active user consent. It is advised that user agents still present users with the ability to select what is shared, even for applications that have elevated permissions.

Capabilities Depending on Elevated Permissions

Elevated permissions are recommended as a prerequisite for access to capture of monitor or browser display surfaces. Note that capture of a complete monitor is included because this could include a window from the user agent.

Similarly, elevated permissions are a recommended prerequisite for access to logical display surfaces, where that would not ordinarily be provided.

A user agent SHOULD persist any elevated permissions that are granted to an origin. An elevated permissions process in part relies on its novelty to ensure that it correctly captures user intent.

Feedback and Interface During Capture

Implementations are advised to provide user feedback and control mechanisms similar to those offered users when sharing a camera or microphone, as recommended in [[GETUSERMEDIA]].

It is important that a user be aware that content is being shared when content is actively being captured. User agents are advised to display a prominent indicator while content is being captured. In addition to an indicator, a user agent is advised to provide a means to learn precisely what is being shared; while this capability is trivially provided by an application by rendering the captured content, this information allows a user to accurately assess what is being shared.

In addition to feedback mechanisms, a means to for the user to stop any active capture is advisable.