Web Share API

W3C Working Draft

More details about this document
This version:
https://www.w3.org/TR/2022/WD-web-share-20220707/
Latest published version:
https://www.w3.org/TR/web-share/
Latest editor's draft:
https://w3c.github.io/web-share/
History:
https://www.w3.org/standards/history/web-share
Commit history
Test suite:
https://wpt.live/web-share/
Editors:
Matt Giuca (Google Inc.)
Eric Willigers (Google Inc.)
Marcos Cáceres (Apple Inc.)
Feedback:
GitHub w3c/web-share (pull requests, new issue, open issues)
Browser support:
caniuse.com

Copyright © 2022 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and permissive document license rules apply.

Abstract

This specification defines an API for sharing text, links and other content to an arbitrary destination of the user's choice.

The available share targets are not specified here; they are provided by the user agent. They could, for example, be apps, websites or contacts.

Status of This Document

This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This is a work in progress. Wide review and feedback welcome.

This document was published by the Web Applications Working Group as a Working Draft using the Recommendation track.

Publication as a Working Draft does not imply endorsement by W3C and its Members.

This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 2 November 2021 W3C Process Document.

1. Usage Examples

This section is non-normative.

1.1 Sharing text and links

This example shows a basic share operation. In response to a button click, this JavaScript code shares the current page's URL.

Example 1: Sharing text and URL 
shareButton.addEventListener("click", async () => {
  try {
    await navigator.share({ title: "Example Page", url: "" });
    console.log("Data was shared successfully");
  } catch (err) {
    console.error("Share failed:", err.message);
  }
});

Note that a url of '' refers to the current page URL, just as it would in a link. Any other absolute or relative URL can also be used.

In response to this call to share(), the user agent would display a picker or chooser dialog, allowing the user to select a target to share this title and the page URL to.

1.2 Sharing a file

This example shows how to share a file. Note that the files member is an array, allowing for multiple files to be shared.

Example 2: Sharing a file 
shareButton.addEventListener("click", async () => {
  const file = new File(data, "some.png", { type: "image/png" });
  try {
    await navigator.share({
      title: "Example File",
      files: [file]
    });
  } catch (err) {
    console.error("Share failed:", err.message);
  }
});

1.3 Validating a share

Calling canShare() method with a ShareData dictionary validates the shared data. Unlike share(), it can be called without transient activation.

Example 3 
const file = new File([], "some.png", { type: "image/png" });

// Check if files are supported
if (navigates.canShare({files: [file]})) {
  // Sharing a png file would probably be ok...
}

// Check if a URL is ok to share...
if (navigates.canShare({ url: someURL })) {
  // The URL is valid and can probably be shared...
}

1.4 Checking if members are supported

Because of how WebIDL dictionaries work, members passed to share(()) that are unknown to the user agent are ignored. This can be a problem when sharing multiple members, but the user agent doesn't support sharing one of those members. To be sure that every member being passed is supported by the user agent, you can pass them to canShare() individually to check if they are supported.

Example 4: Future-proofing shares 
const data = {
  title: "Example Page",
  url: "https://example.com",
  text: "This is a text to share",
  someFutureThing: "some future thing",
};
const allSupported = Object.entries(data).every(([key, value]) => {
  return navigator.canShare({ [key]: value });
});
if (allSupported) {
  await navigator.share(data);
}

Alternatively, you can adjust application's UI to not show UI components for unsupported members.

Example 5: Filtering out unsupported members 
const data = {
  title: "Example Page",
  url: "https://example.com",
  text: "This is a text to share",
  someFutureThing: "some future thing",
};

// Things that are not supported...
const unsupported = Object.entries(data).filter(([key, value]) => {
  return !navigator.canShare({ [key]: value });
});

2. API definition

2.1 Extensions to the Navigator interface 

WebIDLpartial interface Navigator {
  [SecureContext] Promise<undefined> share(optional ShareData data = {});
  [SecureContext] boolean canShare(optional ShareData data = {});
};

2.1.1 Internal Slots

This API adds the following internal slot to the Navigator interface.

Promise? [[sharePromise]]
The this.[[sharePromise]] is a promise that represents a user's current intent to share some data with a share target. It is initialized to null.

2.1.2 share() method

When the share() method is called with argument data, run the listed steps listed below while taking into consideration the following security implications.

Web Share enables data to be sent from websites to a share target, which can be a native applications. While this ability is not unique to Web Share, it does come with a number of potential security risks that can vary in severity (depending on the underlying platform).

The data passed to share() might be used to exploit buffer overflow or other remote code execution vulnerabilities in the share target that receive shares. There is no general way to guard against this, but implementors will want to be aware that it is a possibility (particularly when sharing files).

Share targets that dereference a shared URL and forward that information on might inadvertently forward information that might be otherwise confidential. This can lead to unexpected information leakage if shares reference content that is only accessible by that application, the host on which it runs, or its network location.

Malicious sites might exploit share targets that leak information by providing URLs that ultimately resolve to local resources, including, but not limited to, "file:" URLs or local services that might otherwise be inaccessible. Even though this API limits shared URLS to a restricted set of sharable schemes, use of redirects to other URLs or tweaks to DNS records for hosts in those URLs might be used to cause applications to acquire content.

To avoid being used in these attacks, share targets can consume the URL, retrieve the content, and process that information without sharing it. For instance, a photo editing application might retrieve an image that is "shared" with it. A share target can also share the URL without fetching any of the referenced content.

Share targets that fetch content for the purposes of offering a preview or for sharing content risk information leakage. Content that is previewed and authorized by a user might be safe to forward, however it is not always possible for a person to identify when information should be confidential, so forwarding any content presents a risk. In particular, the title might be used by an attacker to trick a user into misinterpreting the nature of the content.

As with any user of DOMException, implementors need to carefully consider what information is revealed in the error message when share() is rejected. Even distinguishing between the case where no share targets are available and user cancellation could reveal information about which share targets are installed on the user's device.

  1. Let document be the current settings object's relevant global object's associated Document.
  2. If document is not fully active, return a promise rejected with an "InvalidStateError" DOMException.
  3. If document is not allowed to use "web-share", return a promise rejected with a "NotAllowedError" DOMException.
  4. If this.[[sharePromise]] is not null, return a promise rejected with an "InvalidStateError" DOMException.
  5. Let global be this's relevant global object.
  6. If global does not have transient activation, return a promise rejected with a "NotAllowedError" DOMException.
  7. Consume user activation of global.
  8. Let base be this's relevant settings object's API base URL.
  9. If validate share data with data and base returns false, then return a promise rejected with a TypeError.
  10. If data's url member is present:
    1. Let url be the result of running the URL parser on data's url with base.
    2. Assert: url is URL.
    3. Set data to a copy of data, with its url member set to the result of running the URL serializer on url.
  11. If a file type is being blocked due to security considerations, return a promise rejected with a "NotAllowedError" DOMException.
  12. Set this.[[sharePromise]] to be a new promise.
  13. Return this.[[sharePromise]] and in parallel:
    1. If there are no share targets available, queue a global task on the user interaction task source using global to:
      1. Reject this.[[sharePromise]] with an "AbortError" DOMException.
      2. Set this.[[sharePromise]] to null.
      3. Abort these steps.
    2. Present the user with a choice of one more share targets and the ability abort the operation. This UI surface serves as a security confirmation, ensuring that websites cannot silently send data to native applications. The user agent SHOULD show intermediary UI through which the user can verify the shared content (if the OS-level UI does not provide this functionality).
    3. Wait for the user's choice.
    4. If the user chose to abort the share operation, queue a global task on the user interaction task source using global to:
      1. Reject this.[[sharePromise]] with an "AbortError" DOMException,
      2. Set this.[[sharePromise]] to null.
      3. Abort these steps.
    5. Activate the chosen share target, convert data to a format suitable for ingestion into the target, and transmit the converted data to the target.
    6. If an error occurs starting the target or transmitting the data, queue a global task on the user interaction task source using global to:
      1. Reject this.[[sharePromise]] with an "DataError" DOMException.
      2. Set this.[[sharePromise]] to null.
      3. Abort these steps.
    7. Once the data has either been successfully transmitted to the share target, or successfully transmitted to the OS (if the transmission to the share target cannot be confirmed), queue a global task on the user interaction task source using global to:
      Note
      1. Resolve this.[[sharePromise]] with undefined.
      2. Set this.[[sharePromise]] to null.

2.1.3 canShare(data) method

When the canShare() method is called with argument ShareData data, run the following steps:

  1. Let document be the current settings object's relevant global object's associated Document.
  2. If document is not fully active, return false.
  3. If document is not allowed to use "web-share", return false.
  4. Return the result of validate share data with data and this's relevant settings object's API base URL.

2.1.4 Validate share data

A sharable scheme is any of the following URL schemes:

  • http
  • https
  • Any safelisted scheme that the user agent supports for the purpose of sharing.

To validate share data with data and base, run the following steps:

  1. If none of data's members title, text, or url or files are present, return false.
  2. Let titleTextOrUrl be true if any of title, or text, or url is present.
  3. If data's files member is present:
    1. If titleTextOrUrl is false and data's files member is empty, return false.
      Note

      This causes a { files: [] } dictionary to be treated as an empty dictionary. However, passing a dictionary like {text: "text", files: []} is fine, as files is just ignored.

    2. If the implementation does not support file sharing, return false.
    3. If the user agent believes sharing any of the files in files would result in a potentially hostile share (i.e., the user agent determines a file is malicious in some way, because of its contents, size, or other characteristic), return false.
  4. If data's url member is present:
    1. Let url be the result of running the URL parser on data's url member, with base, and no encoding override.
    2. If url is failure, return false.
    3. If the url's scheme is a local scheme, or file, or javascript, or ws, or wss, return false.
    4. If url's scheme is not a sharable scheme, return false.
  5. Return true.

2.2 ShareData dictionary 

WebIDLdictionary ShareData {
  sequence<File> files;
  USVString title;
  USVString text;
  USVString url;
};

The ShareData dictionary consists of several optional members:

files member
Files to be shared.
text member
Arbitrary text that forms the body of the message being shared.
title member
The title of the document being shared. May be ignored by the target.
url member
A URL string referring to a resource being shared.
Note

3. Share targets

A share target is the abstract concept of a destination that the user agent will transmit the share data to. What constitutes a share target is at the discretion of the user agent.

A share target might not be directly able to accept a ShareData (due to not having been written with this API in mind). However, it MUST have the ability to receive data that matches some or all of the concepts exposed in ShareData. To convert data to a format suitable for ingestion into the target, the user agent SHOULD map the members of ShareData onto equivalent concepts in the target. It MAY discard or combine members if necessary. The meaning of each member of the payload is at the discretion of the share target.

Note
Mapping the ShareData to the share target's (or operating system's) native format can be tricky as some platforms will not have an equivalent set of members. For example, if the target has a "text" member but not a "URL" member (as is the case on Android), one solution is to concatenate both the text and url members of ShareData and pass the result in the "text" member of the target.

Each share target MAY be made conditionally available depending on the ShareData payload delivered to the share() method.

3.1 Examples of share targets

This section is non-normative.

The list of share targets can be populated from a variety of sources, depending on the user agent and host operating system. For example:

Note
There is an attempt to standardize the registration of websites to receive share data for that final use case; see Web Share Target.

In some cases, the host operating system will provide a sharing or intent system similar to Web Share. In these cases, the user agent can simply forward the share data to the operating system and not talk directly to native applications.

4. Permissions Policy integration

This specification defines a policy-controlled permission identified by the string "web-share". Its default allowlist is 'self'.

Developers can use the means afforded by the Permissions Policy specification to control if and when a third-party context is allowed to use this API.

Note

A document’s permission policy determines whether a share() call immediately rejects with a "NotAllowedError" DOMException.

5. Accessibility considerations

This section is non-normative.

When this specification is used to present information in the user interface, implementors will want to follow the OS level accessibility guidelines for the platform.

6. Privacy considerations

7. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

8. IDL Index

WebIDLpartial interface Navigator {
  [SecureContext] Promise<undefined> share(optional ShareData data = {});
  [SecureContext] boolean canShare(optional ShareData data = {});
};

dictionary ShareData {
  sequence<File> files;
  USVString title;
  USVString text;
  USVString url;
};

9. Changlog

This section is non-normative.

A. Acknowledgments

Thanks to the Web Intents team, who laid the groundwork for the web app interoperability use cases. In particular, Paul Kinlan, who did a lot of early advocacy for Web Share.

B. References

B.1 Normative references

[fetch]
Fetch Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://fetch.spec.whatwg.org/
[fileapi]
File API. Marijn Kruisselbrink; Arun Ranganathan. W3C. 4 June 2021. W3C Working Draft. URL: https://www.w3.org/TR/FileAPI/
[html]
HTML Standard. Anne van Kesteren; Domenic Denicola; Ian Hickson; Philip Jägenstedt; Simon Pieters. WHATWG. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[permissions-policy]
Permissions Policy. Ian Clelland. W3C. 16 July 2020. W3C Working Draft. URL: https://www.w3.org/TR/permissions-policy-1/
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc2119
[RFC8174]
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc8174
[url]
URL Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://url.spec.whatwg.org/
[WEBIDL]
Web IDL Standard. Edgar Chen; Timothy Gu. WHATWG. Living Standard. URL: https://webidl.spec.whatwg.org/

B.2 Informative references

[dom]
DOM Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://dom.spec.whatwg.org/
[encoding]
Encoding Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://encoding.spec.whatwg.org/