W3C Recommendation

This version:
Latest published version:
Latest editor's draft:
Test suite:
Implementation report:
Previous version:
Julien Genestoux, Invited Expert
Aaron Parecki, Invited Expert
Julien Genestoux
Past Authors:
Brad Fitzpatrick
Brett Slatkin
Martin Atkins

Please check the errata for any errors or issues reported since publication.

See also translations.


WebSub provides a common mechanism for communication between publishers of any kind of Web content and their subscribers, based on HTTP web hooks. Subscription requests are relayed through hubs, which validate and verify the request. Hubs then distribute new and updated content to subscribers when it becomes available. WebSub was previously known as PubSubHubbub.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. 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 document was published by the Social Web Working Group as a Recommendation. Comments regarding this document are welcome. All interested parties are invited to provide implementation and bug reports and other comments through the Working Group's Issue tracker. These will be discussed by the Social Web Community Group and considered in any future versions of this specification.

Please see the Working Group's implementation report.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

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 1 March 2017 W3C Process Document.

1. Definitions

An HTTP [RFC7230] (or HTTPS [RFC2818]) resource URL. The unit to which one can subscribe to changes.
Hub ("the hub")
The server (URL [URL]) which implements both sides of this protocol. Any hub MAY implement its own policies on who can use it.
An owner of a topic. Notifies the hub when the topic feed has been updated. As in almost all pubsub systems, the publisher is unaware of the subscribers, if any. Other pubsub systems might call the publisher the "source".
An entity (person or program) that wants to be notified of changes on a topic. The subscriber must be directly network-accessible and is identified by its Subscriber Callback URL.
A unique relation to a topic by a subscriber that indicates it should receive updates for that topic. A subscription's unique key is the tuple (Topic URL, Subscriber Callback URL). Subscriptions may (at the hub's decision) have expiration times akin to DHCP leases which must be periodically renewed.
Subscriber Callback URL
The URL [URL] at which a subscriber wishes to receive content distribution requests.
An event that causes updates to multiple topics. For each event that happens (e.g. "Brad posted to the Linux Community."), multiple topics could be affected (e.g. "Brad posted." and "Linux community has new post"). Publisher events cause topics to be updated and the hub looks up all subscriptions for affected topics, delivering the content to subscribers.
Content Distribution Notification / (Content Distribution Request)
A payload describing how a topic's contents have changed, or the full updated content. Depending on the topic's content type, the difference (or "delta") may be computed by the hub and sent to all subscribers.

2. High-level protocol flow

(This section is non-normative.)

WebSub Protocol Flow Diagram

Earlier versions of this protocol were called PubSubHubbub:

3. Conformance

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3.1 Conformance Classes

WebSub describes three roles: publishers, subscribers and hubs. This section describes the conformance criteria for each role.



A conforming subscriber:


A conforming hub:

3.2 Candidate Recommendation Exit Criteria

This specification exited the CR stage with at least two independent, interoperable implementations of each feature. Each feature may have been implemented by a different set of products. There was no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

3.2.1 Publisher

A WebSub Publisher is an implementation that advertises a topic and hub URL on one or more resource URLs. The conformance criteria are described in Conformance Classes above.

3.2.2 Subscriber

A WebSub Subscriber is an implementation that discovers the hub and topic URL given a resource URL, subscribes to updates at the hub, and accepts content distribution requests from the hub. The subscriber MAY support authenticated content distribution. The conformance criteria are described in Conformance Classes above.

3.2.3 Hub

A WebSub Hub is an implementation that handles subscription requests and distributes the content to subscribers when the corresponding topic URL has been updated. Hubs MUST support subscription requests with a secret and deliver authenticated requests when requested. Hubs MUST deliver the full contents of the topic URL in the request, and MAY reduce the payload to a diff if the content type supports it. The conformance criteria are described in Conformance Classes above.

3.2.4 Independent

Each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

3.2.5 Interoperable

A Subscriber and Hub implementation are considered interoperable for a specific feature when the Hub takes the defined action that the Subscriber requests, the Subscriber gets the expected response from a Hub according to the feature, and the Hub sends the expected response to the Subscriber.

3.2.6 Feature

For the purposes of evaluating exit criteria, each of the following is considered a feature:

  • Discovering the hub and topic URLs by looking at the HTTP headers of the resource URL.
  • Discovering the hub and topic URLs by looking at the contents of the resource URL as an XML document.
  • Discovering the hub and topic URLs by looking at the contents of the resource URL as an HTML document.
  • Subscribing to the hub with a callback URL.
  • Subscribing to the hub and requesting a specific lease duration.
  • Subscribing to the hub with a secret and handling authenticated content distribution.
  • Requesting that a subscription is deactivated by sending an unsubscribe request.
  • The Subscriber acknowledges a pending subscription on a validation request.
  • The Subscriber rejects a subscription validation request for an invalid topic URL.
  • The Subscriber returns an HTTP 2xx response when the payload is delivered.
  • The Subscriber verifies the signature for authenticated content distribution requests.
  • The Subscriber rejects the distribution request if the signature does not validate.
  • The Subscriber rejects the distribution request when no signature is present if the subscription was made with a secret.
  • The Hub respects the requested lease duration during a subscription request.
  • The Hub allows Subscribers to re-request already active subscriptions, extending the lease duration.
  • The Hub sends the full contents of the topic URL in the distribution request.
  • The Hub sends a diff of the topic URL for the formats that support it.
  • The Hub sends a valid signature for subscriptions that were made with a secret.

4. Discovery

The discovery mechanism aims at identifying at least 2 URLs.


Publishers may wish to advertise and publish to more than one hub for fault tolerance and redundancy. If one hub fails to propagate an update to the document, then using multiple independent hub is a way to increase the liklihood of delivery to subscribers. As such, subscribers may subscribe to one or more of the advertised hubs.

The protocol currently supports the following discovery mechanisms. Publishers MUST implement at least one of them:


Since <link> has been limited to being placed in the <head> for many years, some consuming code might only check the <head>. Therefore it is more robust to place the <link> tags only in the HTML <head> rather than in the <body>.

Example 1
GET /feed HTTP/1.1
Host: example.com

HTTP/1.1 200 Ok
Content-type: text/html
Link: <https://hub.example.com/>; rel="hub"
Link: <http://example.com/feed>; rel="self"

<!doctype html>
    <link rel="hub" href="https://hub.example.com/">
    <link rel="self" href="http://example.com/feed">

When perfoming discovery, subscribers MUST implement all three discovery mechanisms in the following order, stopping at the first match:

  1. Issue a GET or HEAD request to retrieve the topic URL. Subscribers MUST check for HTTP Link headers first.
  2. In the absence of HTTP Link headers, and if the topic is an XML based feed or an HTML page, subscribers MUST check for embedded link elements.

4.1 Content Negotiation

For practical purposes, it is important that the rel=self URL only offers a single representation. As the hub has no way of knowing what Media Type ([RFC6838]) or language may have been requested by the subscriber upon discovery, it would not be able to deliver the content using the appropriate representation of the document.

It is, however, possible to perform content negotiation by returning an appropriate rel=self URL according to the HTTP headers used in the initial discovery request. For example, a request to /feed with an Accept header containing application/json could return a rel=self value of /feed.json.

The example below illustrates how a topic URL can return different Link headers depending on the Accept header that was sent.

Example 2
GET /feed HTTP/1.1
Host: example.com
Accept: application/json

HTTP/1.1 200 Ok
Content-type: application/json
Link: </feed.json>; rel="self"
Link: <https://hub.example.com/>; rel="hub"

  "items": [...]
Example 3
GET /feed HTTP/1.1
Host: example.com
Accept: text/html

HTTP/1.1 200 Ok
Content-type: text/html
Link: </feed.html>; rel="self"
Link: <https://hub.example.com/>; rel="hub"


Similarly, the technique can also be used to return a different rel=self URL depending on the language requested by the Accept-Language header.

Example 4
GET /feed HTTP/1.1
Host: example.com
Accept-Language: de-DE

HTTP/1.1 200 Ok
Content-type: text/html

Link: </feed-de.json>; rel="self"
Link: <https://hub.example.com/>; rel="hub"

  "items": [...]

5. Subscribing and Unsubscribing

Subscribing to a topic URL consists of four parts that may occur immediately in sequence or have a delay.

Unsubscribing works in the same way, except with a single parameter changed to indicate the desire to unsubscribe. Also, the Hub will not validate unsubscription requests with the publisher.

5.1 Subscriber Sends Subscription Request

Subscription is initiated by the subscriber making an HTTPS or HTTP POST [RFC7231] request to the hub URL. This request MUST have a Content-Type header of application/x-www-form-urlencoded (described in Section [HTML5]), MUST use UTF-8 [Encoding] as the document character encoding, and MUST use the following parameters in its body, formatted accordingly:

REQUIRED. The subscriber's callback URL where content distribution notifications should be delivered. The callback URL SHOULD be an unguessable URL that is unique per subscription. ([capability-urls])
REQUIRED. The literal string "subscribe" or "unsubscribe", depending on the goal of the request.
REQUIRED. The topic URL that the subscriber wishes to subscribe to or unsubscribe from. Note that this MUST be the "self" URL found during the discovery step, which may be different from the URL that was used to make the discovery request.
OPTIONAL. Number of seconds for which the subscriber would like to have the subscription active, given as a positive decimal integer. Hubs MAY choose to respect this value or not, depending on their own policies, and MAY set a default value if the subscriber omits the parameter. This parameter MAY be present for unsubscription requests and MUST be ignored by the hub in that case.
OPTIONAL. A subscriber-provided cryptographically random unique secret string that will be used to compute an HMAC digest for authorized content distribution. If not supplied, the HMAC digest will not be present for content distribution requests. This parameter SHOULD only be specified when the request was made over HTTPS [RFC2818]. This parameter MUST be less than 200 bytes in length.

Subscribers MAY also include additional HTTP [RFC7230] request parameters, as well as HTTP [RFC7230] Headers if they are required by the hub.

Hubs MUST ignore additional request parameters they do not understand.

Hubs MUST allow subscribers to re-request subscriptions that are already activated. Each subsequent request to a hub to subscribe or unsubscribe MUST override the previous subscription state for a specific topic URL and callback URL combination, but only once the action is verified (Section 4.3). If verification fails, the subscription state MUST be left unchanged. This is required so subscribers can renew their subscriptions before the lease seconds period is over without any interruption. The subscriber MAY use a new hub.secret value in a future subscription, and MAY make a new subscription without a hub.secret.

5.1.1 Subscription Parameter Details

The topic and callback URLs MAY use HTTP [RFC7230] or HTTPS [RFC2818] schemes. The topic URL MUST be the one advertised by the publisher in a Self Link Header during the discovery phase. (See Section 3 ). Hubs MAY refuse subscriptions if the topic URL does not correspond to the one advertised by the publisher. The topic URL can otherwise be free-form following the URL spec [URL]. Hubs MUST always decode non-reserved characters for these URL parameters; see section 1.2 on "Percent-encoded bytes" in [URL].

The callback URL SHOULD be an unguessable unique URL ([capability-urls]) and SHOULD use HTTPS [RFC7230]. The callback URL acts as authentication from the hub to the subscriber when confirming subscriptions and delivering the content. Additionally, the callback SHOULD be unique (not re-used for multiple hubs) and changed when subscriptions are renewed.

The callback URL MAY contain arbitrary query string parameters (e.g., ?foo=bar&red=fish). Hubs MUST preserve the query string during subscription verification by appending new parameters to the end of the list using the & (ampersand) character to join. Existing parameters with names that overlap with those used by verification requests will not be overwritten. When sending the content distribution request, the hub will make a POST request to the callback URL including any query string parameters in the URL portion of the request, not as POST body parameters.

5.1.2 Subscription Response Details

If the hub URL supports WebSub and is able to handle the subscription or unsubscription request, it MUST respond to a subscription request with an HTTP [RFC7231] 202 "Accepted" response to indicate that the request was received and will now be verified (Section 4.3 ) and validated (Section 4.2 ) by the hub. The hub SHOULD perform the verification and validation of intent as soon as possible.

If a hub finds any errors in the subscription request, an appropriate HTTP [RFC7231] error response code (4xx or 5xx) MUST be returned. In the event of an error, hubs SHOULD return a description of the error in the response body as plain text, used to assist the client developer in understanding the error. This is not meant to be shown to the end user. Hubs MAY decide to reject some callback URLs or topic URLs based on their own policies (e.g., domain authorization, topic URL port numbers). However, since verification and validation of intent are asynchronous steps that logically begin after the HTTP response has been returned, the HTTP response MUST NOT depend on the process or outcome of verification or validation.

If the hub URL is not able to handle subscription or unsubscription requests, it MAY redirect to another hub which supports WebSub. It does so by yielding an HTTP [RFC7231] 307 (temporary redirect) or 308 (permanent redirect) response. It MUST also include at least a HTTP [RFC7230] Location Header containing a preferred URL reference for the hub to use by the subscriber. The subscriber is expected to retry the subscription or unsubscription at the new hub URL.

5.2 Subscription Validation

Subscriptions MAY be validated by the Hubs who may require more details to accept or refuse a subscription. The Hub MAY also check with the publisher whether the subscription should be accepted.

If (and when) the subscription is accepted, the hub MUST perform the verification of intent of the subscriber.

If (and when) the subscription is denied, the hub MUST inform the subscriber by sending an HTTP [RFC7231] (or HTTPS [RFC2818]) GET request to the subscriber's callback URL as given in the subscription request. This request has the following query string arguments appended (format described in Section 4 of [URL]):

REQUIRED. The literal string "denied".
REQUIRED. The topic URL given in the corresponding subscription request.
OPTIONAL. The hub may include a reason for which the subscription has been denied.

The subscription MAY be denied by the hub at any point (even if it was previously accepted). The Subscriber SHOULD then consider that the subscription is not possible anymore.

5.3 Hub Verifies Intent of the Subscriber

In order to prevent an attacker from creating unwanted subscriptions on behalf of a subscriber (or unsubscribing desired ones), a hub must ensure that the subscriber did indeed send the subscription request.

The hub verifies a subscription request by sending an HTTP [RFC7231] (or HTTPS [RFC2818]) GET request to the subscriber's callback URL as given in the subscription request. This request has the following query string arguments appended (format described in Section 4 of [URL]):

REQUIRED. The literal string "subscribe" or "unsubscribe", which matches the original request to the hub from the subscriber.
REQUIRED. The topic URL given in the corresponding subscription request.
REQUIRED. A hub-generated, random string that MUST be echoed by the subscriber to verify the subscription.
REQUIRED/OPTIONAL. The hub-determined number of seconds that the subscription will stay active before expiring, measured from the time the verification request was made from the hub to the subscriber. Hubs MUST supply this parameter when hub.mode is set to "subscribe". This parameter MAY be present when hub.mode is "unsubscribe" and MUST be ignored by subscribers in that case.

5.3.1 Verification Details

The subscriber MUST confirm that the hub.topic corresponds to a pending subscription or unsubscription that it wishes to carry out. If so, the subscriber MUST respond with an HTTP success (2xx) code with a response body equal to the hub.challenge parameter. If the subscriber does not agree with the action, the subscriber MUST respond with a 404 "Not Found" response.

The hub MUST consider other server response codes (3xx, 4xx, 5xx) to mean that the verification request has failed. If the subscriber returns an HTTP [RFC7231] success (2xx) but the content body does not match the hub.challenge parameter, the hub MUST also consider verification to have failed.

Hubs MAY make the hub.lease_seconds equal to the value the subscriber passed in their subscription request but MAY change the value depending on the hub's policies. To sustain a subscription, the subscriber MUST re-request the subscription on the hub before hub.lease_seconds seconds has elapsed.

Hubs MUST enforce lease expirations, and MUST NOT issue perpetual lease durations.


The spec uses GET vs POST to differentiate between the confirmation/denial of the subscription request and delivering the content. While this is not considered "best practice" from a web architecture perspective, it does make implementation of the callback URL simpler. Since the POST body of the content distribution request may be any arbitrary content type and only includes the actual content of the document, using the GET vs POST distinction to switch between handling these two modes makes implementations simpler.

6. Publishing

The publisher MUST inform the hubs it previously designated when a topic has been updated. The hub and the publisher can agree on any mechanism, as long as the hub is eventually able send the updated payload to the subscribers.


The specific mechanism for the publisher to inform the hub is left unspecified. For example, some existing public hubs [1] [2] [3] ask publishers to send a POST request with the keys hub.mode="publish" and hub.url=(the URL of the resource that was updated).

6.1 Subscription Migration

If the publisher wishes to migrate existing subscriptions to a new topic URL, it can do so using HTTP redirects.

This does not require any participation on the part of the previous hub, and works whether or not the publisher changes hubs as well.

7. Content Distribution

A content distribution request is sent from the Hub to the Subscriber when new content is available for a topic URL. The request is an HTTP [RFC7231] (or HTTPS [RFC2818]) POST request from the hub to the subscriber's callback URL. The HTTP body of the POST request MUST include the payload of the content distribution notification. The content distribution request MUST have a Content-Type Header corresponding to the Content-Type of the topic, and MUST contain the full contents of the topic URL, with an exception allowed as described below.

For Atom ([RFC4287]) and RSS ([RSS-2.0]) feeds, the hub MAY remove already-delivered atom:entry or rss:item elements from the feed.

The request MUST include at least one Link Header [RFC5988] with rel=hub pointing to a Hub associated with the topic being updated. It MUST also include one Link Header [RFC5988] with rel=self set to the canonical URL of the topic being updated. The Hub SHOULD combine these headers into a single Link Header [RFC5988]. All these URLs are those resulting from the discovery process (Section 3). The subscriber MUST NOT use these Link headers to identify the subscription corresponding to the content distribution request, because the Link headers are metadata associated with the topic content, not with any particular subscription. For example, the topic URL in the content distribution request may be different from the topic URL that was originally subscribed to.

The subscriber's callback URL MUST return an HTTP [RFC7231] 2xx response code to indicate a success. The subscriber's callback URL MAY return an HTTP 410 code to indicate that the subscription has been deleted, and the hub MAY terminate the subscription if it receives that code as a response. The hub MUST consider all other subscriber response codes as failures; that means subscribers MUST NOT use HTTP redirects for moving subscriptions. Subscribers SHOULD respond to content distribution requests as quickly as possible; their success response code SHOULD only indicate receipt of the message, not acknowledgment that it was successfully processed by the subscriber. The response body from the subscriber MUST be ignored by the hub. Hubs SHOULD retry content distribution requests up to self-imposed limits on the number of times and the overall time period to retry. When the failing delivery exceeds the hub's limits, the hub stops attempting to deliver that nofication. The hub MUST keep the subscription active until the end of the lease duration, and if a new update is published to the topic, MUST continue to retry delivery to the previously-failing subscriber.

7.1 Authenticated Content Distribution

If the subscriber supplied a value for hub.secret in their subscription request, the hub MUST generate an HMAC signature of the payload and include that signature in the request headers of the content distribution request. The X-Hub-Signature header's value MUST be in the form method=signature where method is one of the recognized algorithm names and signature is the hexadecimal representation of the signature. The signature MUST be computed using the HMAC algorithm [RFC6151] with the request body as the data and the hub.secret as the key.

7.1.1 Recognized algorithm names

The following algorithms are the initially registered algorithm names, based on the contents of the [FIPS-PUB-180-4] registry at the time of publishing.

The SHA-1 algorithm as specified in Section 6.1 of [FIPS-PUB-180-4]
The SHA-256 algorithm as specified in Section 6.2 of [FIPS-PUB-180-4]
The SHA-384 algorithm as specified in Section 6.5 of [FIPS-PUB-180-4]
The SHA-512 algorithm as specified in Section 6.4 of [FIPS-PUB-180-4]

In the future, an extension may be specified allowing subscribers to indicate which algorithms they can use for validation. As of this writing, most hubs sign with SHA-1, despite its known cryptographic weakness, in order to be interoperable with older subscribers.

7.1.2 Signature validation

When subscribers receive a content distribution request with the X-Hub-Signature header specified, they SHOULD recompute the signature with the shared secret using the same method (provided in the X-Hub-Signature header) as the hub. If the signature does not match, subscribers MUST locally ignore the message as invalid. Subscribers MAY still acknowledge this request with a 2xx response code in order to be able to process the message asynchronously and/or prevent brute-force attempts of the signature. Using this technique along with HTTPS [RFC2818] for subscription requests enables simple subscribers to receive authenticated content distribution requests from hubs without the need for subscribers to run an HTTPS [RFC2818] server.

Please note however that this signature only ensures that the payload was not forged. Since the request also includes headers, these should not be considered as safe by the subscriber, unless of course the subscriber uses HTTPS [RFC2818] callbacks.

8. Security Considerations

Here is a summary of security considerations. It is important to note that WebSub is a server-to-server protocol which relies only on HTTP. It is strongly recommended to use HTTPS for all requests.

8.1 Discovery

The decision about whether a subscriber should look for <link> elements inside a page's <body> (as well as the <head>) is not straightforward, and there is currently no clear consensus. One reason to ignore the <body> during discovery is that some web sites might (perhaps accidentally) allow users to post content containing <link> elements, though the working group does not know of any specific examples of such sites. If WebSub discovery uses such <link> elements, a user contributing to such sites could potentially maliciously cause all subscribers to use an alternate hub which later delivers malicious content. Given this potential attack, it may be prudent to do discovery only in the <head> of HTML documents.

8.2 Subscriptions

First, subscribers SHOULD always favor the HTTPS URL for hubs (even if the URL is advertised as HTTP). Second, subscribers SHOULD use unique unguessable capability URLs for the callbacks, as well as make them available via HTTPS. Finally, subscribers SHOULD use a hub.secret when subscribing to allow signature of the content distribution.

Hubs SHOULD enforce short lived hub.lease_seconds (10 days is a good default). When performing intent verification, the hub SHOULD use a random, single-use hub.challenge.

8.3 Distribution

The Hub MUST use the exact callback used by the subscriber (including the use of HTTPS). Hubs MUST sign their requests using the hub.secret supplied by subscribers if requested.

If the subscriber included a hub.secret in the subscription request, the subscriber SHOULD validate the hub's provided signature, and if they do so, they MUST use the server's stated signature mechanism, and discard requests which fail the test.

If a subscriber does not use a secure callback URL (HTTPS), or if it is suspected that the TLS transport between the hub and subscriber may be compromised, then the integrity of the content delivery notification is only protected by the hub.secret and the hashing algorithm used. In this case, an appropriate hashing algorithm should be used based on the security requirements of the application. As SHA-1 has been demonstrated to be compromised as of the date of this publication, a minimum of SHA-256 should be used.

8.4 Security and Privacy Review

These questions provide an overview of security and privacy considerations for this specification as guided by Self-Review Questionnaire: Security and Privacy ([security-privacy-questionnaire]).

Does this specification deal with personally-identifiable information?
The only potentially personally-identifiable information involved are topic and callback URLs.
Does this specification deal with high-value data?
No, there is no authentication or other credentials involved.
Does this specification introduce new state for an origin that persists across browsing sessions?
Does this specification expose persistent, cross-origin state to the web?
The WebSub subscriber should create a resource with information about the topic to which it subscribes.
Does this specification expose any other data to an origin that it doesn't currently have access to?
Does this specification enable new script execution/loading mechanisms?
Does this specification allow an origin access to a user's location?
Does this specification allow an origin access to sensors on a user's device?
Does this specification allow an origin access to aspects of a user's local computing environment?
Does this specification allow an origin access to other devices?
Does this specification allow an origin some measure of control over a user agent's native UI?
Does this specification expose temporary identifiers to the web?
Does this specification distinguish between behavior in first-party and third-party contexts?
How should this specification work in the context of a user agent's "incognito" mode?
WebSub is a server to server protocol, in which "incognito" mode does not have a meaning.
Does this specification persist data to a user's local device?
Does this specification allow downgrading default security characteristics?

A. Acknowledgements

This section is non-normative.

The editors wish to thank the authors of PubSubHubbub, the IndieWeb community, and other implementers for their support, encouragement and enthusiasm. In particular, the editors wish to thank Brad Fitzpatrick, Brett Slatkin, Martin Atkins, Amy Guy, Barry Frost, Benjamin Roberts, Eugen Rochko, Jordan Potter, Matthias Pfefferle, Malcolm Blaney, Marten de Vries, Sandro Hawke, Tantek Çelik, and Tony Garnock-Jones.

B. Change Log

This section is non-normative.

B.1 Changes from 03 October 2017 PR to this version

B.2 Changes from 11 April 2017 CR to 03 October 2017 PR

B.3 Changes from 24 November WD to 11 April 2017 CR

B.4 Changes from 20 October FPWD to 24 November 2016

C. References

C.1 Normative references

Encoding Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://encoding.spec.whatwg.org/
HTML5. Ian Hickson; Robin Berjon; Steve Faulkner; Travis Leithead; Erika Doyle Navara; Theresa O'Connor; Silvia Pfeiffer. W3C. 28 October 2014. W3C Recommendation. URL: https://www.w3.org/TR/html5/
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
HTTP Over TLS. E. Rescorla. IETF. May 2000. Informational. URL: https://tools.ietf.org/html/rfc2818
Web Linking. M. Nottingham. IETF. October 2010. Proposed Standard. URL: https://tools.ietf.org/html/rfc5988
Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms. S. Turner; L. Chen. IETF. March 2011. Informational. URL: https://tools.ietf.org/html/rfc6151
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing. R. Fielding, Ed.; J. Reschke, Ed.. IETF. June 2014. Proposed Standard. URL: https://tools.ietf.org/html/rfc7230
Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content. R. Fielding, Ed.; J. Reschke, Ed.. IETF. June 2014. Proposed Standard. URL: https://tools.ietf.org/html/rfc7231
URL Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://url.spec.whatwg.org/

C.2 Informative references

Good Practices for Capability URLs. Jeni Tennison. W3C. 18 February 2014. W3C Working Draft. URL: https://www.w3.org/TR/capability-urls/
Secure Hash Standard (SHS). National Institute of Standards and Technology. U.S. Department of Commerce. URL: http://dx.doi.org/10.6028/NIST.FIPS.180-4
PubSubHubbub Core 0.3 -- Working Draft. B. Fitzpatrick; B. Slatkin; M. Atkins.URL: https://pubsubhubbub.github.io/PubSubHubbub/pubsubhubbub-core-0.3.html
PubSubHubbub Core 0.4 -- Working Draft. B. Fitzpatrick; B. Slatkin; M. Atkins; J. Genestoux.URL: https://pubsubhubbub.github.io/PubSubHubbub/pubsubhubbub-core-0.4.html
The Atom Syndication Format. M. Nottingham, Ed.; R. Sayre, Ed.. IETF. December 2005. Proposed Standard. URL: https://tools.ietf.org/html/rfc4287
Media Type Specifications and Registration Procedures. N. Freed; J. Klensin; T. Hansen. IETF. January 2013. Best Current Practice. URL: https://tools.ietf.org/html/rfc6838
RSS 2.0. Dave Winer. RSS Board. Stable. URL: http://www.rssboard.org/rss-specification
Self-Review Questionnaire: Security and Privacy. Mike West. W3C. 10 December 2015. W3C Note. URL: https://www.w3.org/TR/security-privacy-questionnaire/