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.
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.
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);
});
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:
MediaStreamTrack.MediaStreamTrack.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.
Capture of displayed media is enabled through the addition of a
new getDisplayMedia method on the
NavigatorUserMedia interface, that is similar
to getUserMedia
[[!GETUSERMEDIA]].
getDisplayMedia supports all constraints that are
defined for use with getUserMedia, however it does not
use these constraints to select a specific source or to narrow the set of
options for source selection (see ). Two new
constraints allow an application to observe what type of information was
acquired.
NavigatorUserMedia AdditionsThis method operates identically to getUserMedia,
except that it acquires media from output devices.
The getDisplayMedia function permits the use of
constraints in the same way that constraints are defined
for getUserMedia. However, these constraints MUST NOT
constrain the selection of choices that are presented to users in the same
way.
Once a display surface has been selected, constraints apply to the MediaStreamTrack instances that are returned. This allows for changes such as adjustments to frame rate or resolution, or other constraints.
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, 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 express a preference or requirement
to capture the logical display surface, rather than
the visible display surface.
The type of display surface that is being captured. This
assumes values from
the DisplayCaptureSurfaceType enumeration.
A value of true indicates capture of a
logical display surface; a value of false
indicates a capture capture of a visible display
surface.
DisplayCaptureSurfaceTypeThe DisplayCaptureSurfaceType enumeration
describes the different types of display surface.
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 MUST NOT be selected with the
deviceId constraint, since this would allow applications to
influence selection. A display capture source is represented in the
MediaStreamTrack API as having a deviceId
parameter that is randomized each time a MediaStreamTrack is
connected. The only other constraint on this value is that it cannot
duplicate any existing values for deviceId.
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. Though 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.
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.
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 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.
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.
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.
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.
This section will be removed before publication.
Changed from getOuputMedia to getDisplayMedia
Using monitor to describe scren/display share.
Changed getUserMedia to new method getOutputMedia.
The editors wish to thank ....