Internet DRAFT - draft-soni-protocol-handler-well-known-uri
draft-soni-protocol-handler-well-known-uri
Internet Engineering Task Force S. Lasso
Internet-Draft 10 July 2023
Intended status: Standards Track
Expires: 11 January 2024
Discovery of Well-Known Web-Based Protocol Handlers
draft-soni-protocol-handler-well-known-uri-00
Abstract
This document defines a well-known URI for opening URLs in arbitrary,
user-provided web domains, as if the domain were a web-based protocol
handler.
Status of This Memo
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This Internet-Draft will expire on 11 January 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Example Uses of Well-Known Web-Based Protocol Handlers . . . 2
3.1. Fediverse Instance Interactions . . . . . . . . . . . . . 2
3.2. Web-Based Feed Readers . . . . . . . . . . . . . . . . . 4
3.3. Webmail Prompt as an Alternative to Bare mailto: . . . . 5
4. Semantics of the Well-Known Web-Based Protocol Handler . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Open Redirect . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Improper Authorization in Handler for Custom URL
Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The recent growth of the Fediverse [W3C.activitypub] has highlighted
a need for a quick and simple mechanism for opening URLs [WHATWG-URL]
across instances. This specification defines a well-known URI for
such purpose with the desired properties: low bandwidth cost, and low
latency cost.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" 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.
The use of URL throughout this document refers to the WHATWG URL
Standard [WHATWG-URL]. The use of URI refers to BCP 190 [RFC8820].
3. Example Uses of Well-Known Web-Based Protocol Handlers
3.1. Fediverse Instance Interactions
Suppose a Fediverse instance has the ability to display public posts
to visiting users. Someone might share a link to a public post from
said instance, say https://public.example/post/foo. Visiting users
from other instances can click a "reply" or other interaction button,
and when doing so, they are greeted with a dialog asking them for
their instance domain. Once they fill out the dialog, they get
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navigated to the well-known web-based protocol handler for their
instance, say https://users.example/.well-known/protocol-
handler?target=https%3A%2F%2Fpublic.example%2Fpost%2Ffoo, where they
can interact from their instance with the post. This workflow is
demonstrated in Figure 1 through Figure 3.
+-------------------------------------------------------------+
| https://public.example/post/foo |
+-------------------------------------------------------------+
| Public User said: |
| Hi there! |
| |
| [Reply] |Boost| |Like| |
+-------------------------------------------------------------+
Figure 1: Browsing the public post directly
+-------------------------------------------------------------+
| https://public.example/post/foo |
+-------------------------------------------------------------+
| Public +-----------------------------------------+ |
| Hi ther | To reply to this post, please type | |
| | your fediverse instance domain below: | |
| |Reply| | Domain: [users.example____] | |
| | |Confirm| |Cancel| | |
| +-----------------------------------------+ |
+-------------------------------------------------------------+
Figure 2: The interaction dialog, which shows up when clicking
"reply", already filled out
+-------------------------------------------------------------+
| https://users.example/.well-known/protocol-handler?target=h |
| ttps%3A%2F%2Fpublic.example%2Fpost%2Ffoo |
+-------------------------------------------------------------+
| Public User from public.example said: |
| Hi there! |
| |
| [Reply] |Boost| |Like| |
+-------------------------------------------------------------+
Figure 3: The post, now seen from users.example
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3.2. Web-Based Feed Readers
Suppose a website provides an Atom feed [RFC4287]. A visiting user
wants to open the Atom feed with their favorite web-based feed
reader. The user clicks the "Atom feed" button and is prompted for
their web-based feed reader. They fill out the prompt and get
navigated to the well-known web-based protocol handler for their feed
reader, say, https://feedreader.example/.well-known/protocol-
handler?target=feed%3A%2F%2Fwebsite.example%2Findex.atom, where they
can browse and/or subscribe to the Atom feed. This workflow is
demonstrated in Figure 4 through Figure 6.
+-------------------------------------------------------------+
| https://website.example/index.html |
+-------------------------------------------------------------+
| Recent posts: |
| - Hello world! |
| |
| [Atom Feed] |
+-------------------------------------------------------------+
Figure 4: Browsing the website
+-------------------------------------------------------------+
| https://website.example/index.html |
+-------------------------------------------------------------+
| Recent +-----------------------------------------+ |
| - Hello | To view this feed, please type your | |
| | feed reader's domain below: | |
| |Atom f | Domain: [feedreader.example____] | |
| | |Confirm| |Cancel| | |
| +-----------------------------------------+ |
+-------------------------------------------------------------+
Figure 5: The feed dialog, which shows up when clicking "Atom
Feed", already filled out
+-------------------------------------------------------------+
| https://feedreader.example/.well-known/protocol-handler?tar |
| get=feed%3A%2F%2Fwebsite.example%2Findex.atom |
+-------------------------------------------------------------+
| Posts from https://website.example/index.atom |
| - Hello world! |
+-------------------------------------------------------------+
Figure 6: The Atom feed, now seen from feedreader.example
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3.3. Webmail Prompt as an Alternative to Bare mailto:
Suppose a website wants to allow users to contact the website author.
A visiting user clicks the "email" button and is prompted for their
email address. After filling it out, the user gets navigated to the
well-known web-based protocol handler for their email address, say,
https://webmail.example/.well-known/protocol-handler?target=mailto%3A
author%40website.example%3Ffrom%3Dvisitinguser%40webmail.example.
The webmail uses the embedded from value to automatically select the
correct user account from the multi-account switcher. This workflow
is demonstrated in Figure 7 through Figure 9.
| This example has the webmail interpreting the from value in a
| mailto URI, in apparent contradiction to the requirements in
| the mailto specification [RFC6068]. However, it is not being
| interpreted as a header value, but as a suggestion to the
| webmail's account switcher.
+-------------------------------------------------------------+
| https://website.example/index.html |
+-------------------------------------------------------------+
| Recent posts: |
| - Hello world! |
| |
| [Contact the Author] |
+-------------------------------------------------------------+
Figure 7: Browsing the website
+-------------------------------------------------------------+
| https://website.example/index.html |
+-------------------------------------------------------------+
| Recent +-----------------------------------------+ |
| - Hello | To contact the author, please type your | |
| | email: [visitinguser@webmail.example__] | |
| [Contac | |Confirm| |Cancel| | |
| +-----------------------------------------+ |
+-------------------------------------------------------------+
Figure 8: The contact dialog, which shows up when clicking
"Contact the Author", already filled out
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+-------------------------------------------------------------+
| https://webmail.example/.well-known/protocol-handler?target |
| =mailto%3Aauthor%40website.example%3Ffrom%3Dvisitinguser%40 |
| webmail.example |
+-------------------------------------------------------------+
| To: |author@website.example | |
| Subject: [______________] |
| Message: |
| +--------------------------------------+ |
| | | |
| | | |
| | | |
| +--------------------------------------+ |
| |Send| |Save Draft| |Cancel| |
+-------------------------------------------------------------+
Figure 9: The user's webmail
4. Semantics of the Well-Known Web-Based Protocol Handler
The well-known web-based protocol handler SHALL accept a target query
parameter. This query parameter SHALL carry an absolute-URL-with-
fragment-string; see Section 4.3 of [WHATWG-URL]. No restrictions
are placed on the URL's scheme. The choice to use the WHATWG URL
specification allows the well-known web-based protocol handler to
optionally be registered with the browser, should the website choose
to do so, as per Section 8.9.1.4 of [WHATWG-HTML].
The well-known web-based protocol handler is explicitly intended for
navigation: a web browser will navigate to it when using it. It is
up to the server to decide how to handle the given URL, but note
Security Considerations.
To support probing, a server MAY opt to return 4xx-responses for URL
schemes it doesn't recognize or doesn't accept. It is NOT
RECOMMENDED to use generic schemes (http, https, and the like), as
they convey less information when probing.
5. IANA Considerations
This specification registers the "protocol-handler" well-known URI in
the "well-known URIs" registry as defined by RFC 8615 [RFC8615].
URI suffix: protocol-handler
Change controller: IETF
Specification document(s): TBD
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Status: permanent
Related information: Not applicable.
6. Security Considerations
The specified well-known URI is a simple navigable resource, and the
security considerations relevant to any navigable resource apply.
Nevertheless, we highlight the following two vulnerabilities and
recommendations as they are particularly relevant.
6.1. Open Redirect
An open redirect attack occurs when an attacker uses an open redirect
on a legitimate website in order to trick an unsuspecting user into
visiting the attacker's phishing website.
Due to the nature of the well-known URI, common mitigations such as
checking Referer, Origin, and other headers, are not available here.
Where possible, the provided URL SHOULD be opened within the scope of
the well-known resource. Otherwise, the user MUST be notified of
scope changes and be given the option to confirm or deny them.
6.2. Improper Authorization in Handler for Custom URL Scheme
Some URLs may be used to carry actions. An attacker may include
malicious actions in such URLs. Since this endpoint is navigable, an
attacker can simply open it, sometimes even without user
intervention.
Servers implementing this well-known URI MUST NOT automatically
execute actions based on the target URL. Ideally, servers SHOULD
reject/ignore these actions altogether, and let the user execute the
action in the interface they are presented with, as it's not uncommon
for users to simply click "yes" or "confirm" in an attempt to dismiss
prompts. In some cases, for example for message composition URLs
(mailto, etc), this may simply not be possible; in these cases the
server can prompt the user before executing the action.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 8820, DOI 10.17487/RFC8820, June 2020,
<https://www.rfc-editor.org/info/rfc8820>.
[WHATWG-URL]
van Kesteren, A., "URL Standard", Living Standard (24 May
2023), <https://uri.spec.whatwg.org/>.
7.2. Informative References
[W3C.activitypub]
"ActivityPub", W3C REC activitypub, W3C activitypub,
<https://www.w3.org/TR/activitypub/>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC6068] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
<https://www.rfc-editor.org/info/rfc6068>.
[WHATWG-HTML]
van Kesteren, A., "HTML Standard", Living Standard (29
June 2023), <https://html.spec.whatwg.org/multipage/>.
Author's Address
Soni Lasso Terense
Email: fakedme@gmail.com
URI: https://soniex2.autistic.space/
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