GeezerVison

Social Web Working Group

Specification Profiles

This specification defines two closely related and interacting protocols:

A client to server protocol, or "Social API"

This protocol permits a client to act on behalf of a user. For example, this protocol is used by a mobile phone application to interact with a social stream of the user's actor.

A server to server protocol, or "Federation Protocol"

This protocol is used to distribute activities between actors on different servers, tying them into the same social graph.

The ActivityPub specification is designed so that once either of these protocols are implemented, supporting the other is of very little additional effort. However, servers may still implement one without the other. This gives three conformance classes:

ActivityPub conformant Client

This designation applies to any implementation of the entirety of the client portion of the client to server protocol.

ActivityPub conformant Server

This designation applies to any implementation of the entirety of the server portion of the client to server protocol.

ActivityPub conformant Federated Server

This designation applies to any implementation of the entirety of the federation protocols.

It is called out whenever a portion of the specification only applies to implementation of the federation protocol. In addition, whenever requirements are specified, it is called out whether they apply to the client or server (for the client-to-server protocol) or whether referring to a sending or receiving server in the server-to-server protocol.

Object Identifiers

All Objects in [[!activitystreams-core]] should have unique global identifiers. ActivityPub extends this requirement; all objects distributed by the ActivityPub protocol MUST have unique global identifiers, unless they are intentionally transient or anonymous, in which case the identifier MAY be omitted. If present, the identifiers MUST be publicly dereferencable URIs, such as HTTPS URIs, with their authority belonging to that of their originating server. (Publicly facing content SHOULD use HTTPS URIs).

Identifiers MUST be provided for activities posted in server to server communication, unless the activity is intentionally transient. However, for client to server communication, a server receiving an object posted to the outbox with no specified id SHOULD allocate an object ID and attach it to the posted object.

All objects have the following properties:

id

The object's unique global identifier (unless the object is transient, in which case the id MAY be omitted).

type

The type of the object.

Retrieving objects

The HTTP GET method may be used to dereference an object's id property to retrieve the object. Servers MAY use HTTP content negotiation as defined in [[!RFC7231]] to select the type of data to return in response to a request, but MUST present the Activity Streams object representation in response to application/ld+json; profile="https://www.w3.org/ns/activitystreams", and SHOULD also present the Activity Streams representation in response to application/activity+json as well. The client MUST specify an Accept header with the application/ld+json; profile="https://www.w3.org/ns/activitystreams" media type in order to retrieve the object.

Servers MAY implement other behavior for requests which do not comply with the above requirement. (For example, servers may implement additional legacy protocols, or may use the same URI for both HTML and Activity Streams representations of a resource).

Servers MAY require authorization as specified in , and may additionally implement their own authorization rules. Servers SHOULD fail requests which do not pass their authorization checks with the appropriate HTTP error code, or the 403 Forbidden error code where the existence of the object is considered private. An origin server which does not wish to disclose the existence of a private target MAY instead respond with a status code of 404 Not Found.

The source property

In addition to all the properties defined by the [[!activitystreams-vocabulary]], ActivityPub extends the Object by supplying the source property. The source property is intended to convey some sort of source from which the content markup was derived, as a form of provenance, or to support future editing by clients. In general, clients do the conversion from source to content, not the other way around.

The value of source is itself an object which uses its own content and mediaType fields to supply source information.

          {
            "@context": ["https://www.w3.org/ns/activitystreams",
                         {"@language": "en"}],
            "type": "Note",
            "id": "http://postparty.example/p/2415",
            "content": "<p>I <em>really</em> like strawberries!</p>",
            "source": {
              "content": "I *really* like strawberries!",
              "mediaType": "text/markdown"}
          }
        

Actor objects

Actor objects MUST have, in addition to the properties mandated by , the following properties:

inbox

An OrderedCollection comprised of all the messages received by the actor; see .

outbox

An [[!activitystreams-core]] OrderedCollection comprised of all the messages produced by the actor; see .

Implementations SHOULD, in addition, provide the following properties:

following

A link to an [[!activitystreams-core]] collection of the actors that this actor is following; see .

followers

A link to an [[!activitystreams-core]] collection of the actors that follow this actor; see .

Implementations MAY provide the following properties:

liked

A link to an [[!activitystreams-core]] collection of objects this actor has liked; see .

{
  "@context": ["https://www.w3.org/ns/activitystreams",
               {"@language": "ja"}],
  "type": "Person",
  "id": "https://kenzoishii.example.com/",
  "following": "https://kenzoishii.example.com/following.json",
  "followers": "https://kenzoishii.example.com/followers.json",
  "liked": "https://kenzoishii.example.com/liked.json",
  "inbox": "https://kenzoishii.example.com/inbox.json",
  "outbox": "https://kenzoishii.example.com/feed.json",
  "preferredUsername": "kenzoishii",
  "name": "石井健蔵",
  "summary": "この方はただの例です",
  "icon": [
    "https://kenzoishii.example.com/image/165987aklre4"
  ]
}
        

Implementations MAY, in addition, provide the following properties:

streams

A list of supplementary Collections which may be of interest.

preferredUsername

A short username which may be used to refer to the actor, with no uniqueness guarantees.

endpoints

A JSON object which maps additional (typically server/domain-wide) endpoints which may be useful either for this actor or someone referencing this actor. This mapping may be nested inside the actor document as the value or may be a link to a JSON-LD document with these properties.

The endpoints mapping MAY include the following properties:

proxyUrl

Endpoint URI so this actor's clients may access remote Activity Streams objects which require authentication to access. To use this endpoint, the client posts an x-www-form-urlencoded id parameter with the value being the id of the requested Activity Streams object.

oauthAuthorizationEndpoint

If OAuth 2.0 bearer tokens [[RFC6749]] [[RFC6750]] are being used for authenticating client to server interactions, this endpoint specifies a URI at which a browser-authenticated user may obtain a new authorization grant.

oauthTokenEndpoint

If OAuth 2.0 bearer tokens [[RFC6749]] [[RFC6750]] are being used for authenticating client to server interactions, this endpoint specifies a URI at which a client may acquire an access token.

provideClientKey

If Linked Data Signatures and HTTP Signatures are being used for authentication and authorization, this endpoint specifies a URI at which browser-authenticated users may authorize a client's public key for client to server interactions.

signClientKey

If Linked Data Signatures and HTTP Signatures are being used for authentication and authorization, this endpoint specifies a URI at which a client key may be signed by the actor's key for a time window to act on behalf of the actor in interacting with remote servers.

sharedInbox

An optional endpoint used for wide delivery of publicly addressed activities and activities sent to followers. sharedInbox endpoints SHOULD also be publicly readable OrderedCollection objects containing objects addressed to the Public special collection. Reading from the sharedInbox endpoint MUST NOT present objects which are not addressed to the Public endpoint.

Properties containing natural language values, such as name or summary, make use of natural language support defined in Activity Streams.

Outbox

The outbox is discovered through the outbox property of an actor object. The outbox MUST be an OrderedCollection.

The outbox stream contains activities the user has published, subject to the ability of the requestor to retrieve the activity (that is, the contents of the outbox are filtered by the permissions of the person reading it). If a user submits a request without Authorization the server SHOULD respond with all of the Public posts. This could potentially be all relevant objects published by the user, though the number of available items is left to the discretion of those implementing and deploying the server.

The outbox accepts HTTP POST requests, with behaviour described in Client to Server Interactions.

Inbox

The inbox is discovered through the inbox property of an actor's profile. The inbox MUST be an OrderedCollection.

The inbox stream contains all activities received by the actor. The server SHOULD filter content according to the requester's permission. In general, the owner of an inbox is likely to be able to access all of their inbox contents. Depending on access control, some other content may be public, whereas other content may require authentication for non-owner users, if they can access the inbox at all.

The server MUST perform de-duplication of activities returned by the inbox. Duplication can occur if an activity is addressed both to an actor's followers, and a specific actor who also follows the recipient actor, and the server has failed to de-duplicate the recipients list. Such deduplication MUST be performed by comparing the id of the activities and dropping any activities already seen.

The inboxes of actors on federated servers accepts HTTP POST requests, with behaviour described in Delivery. Non-federated servers SHOULD return a 405 Method Not Allowed upon receipt of a POST request.

Followers Collection

Every actor SHOULD have a followers collection. This is where one would find a list of all the actors that are following the actor. The followers collection MUST be either an OrderedCollection or a Collection and MAY be filtered on privileges of an authenticated user or as appropriate when no authentication is given.

The follow activity generally is a request to see the objects an actor creates. This makes the Followers collection an appropriate default target for delivery of notifications.

Following Collection

Every actor SHOULD have a following collection. This is a list of everybody that the actor has followed, added as a side effect. The following collection MUST be either an OrderedCollection or a Collection and MAY be filtered on privileges of an authenticated user or as appropriate when no authentication is given.

Liked Collection

Every actor MAY have a liked collection. This is a list of every object from all of the actor's Like activities, added as a side effect. The liked collection MUST be either an OrderedCollection or a Collection and MAY be filtered on privileges of an authenticated user or as appropriate when no authentication is given.

Public Addressing

In addition to [[!activitystreams-core]] collections and objects, Activities may additionally be addressed to the special "public" collection, with the identifier https://www.w3.org/ns/activitystreams#Public. For example:

          {
            "@context": "https://www.w3.org/ns/activitystreams",
            "id": "as:Public",
            "type": "Collection"
          }
        

Activities addressed to this special URI shall be accessible to all users, without authentication. Implementations MUST NOT deliver to the "public" special collection; it is not capable of receiving actual activities. However, actors MAY have a sharedInbox endpoint which is available for efficient shared delivery of public posts (as well as posts to followers-only); see .

Likes Collection

Every object MAY have a likes collection. This is a list of all Like activities with this object as the object property, added as a side effect. The likes collection MUST be either an OrderedCollection or a Collection and MAY be filtered on privileges of an authenticated user or as appropriate when no authentication is given.

Shares Collection

Every object MAY have a shares collection. This is a list of all Announce activities with this object as the object property, added as a side effect. The shares collection MUST be either an OrderedCollection or a Collection and MAY be filtered on privileges of an authenticated user or as appropriate when no authentication is given.

Client Addressing

Clients are responsible for addressing new Activities appropriately. To some extent, this is dependent upon the particular client implementation, but clients must be aware that the server will only forward new Activities to addressees in the to, bto, cc, bcc, and audience fields.

The Followers Collection and/or the Public Collection are good choices for the default addressing of new Activities.

Clients SHOULD look at any objects attached to the new Activity via the object, target, inReplyTo and/or tag fields, retrieve their actor or attributedTo properties, and MAY also retrieve their addressing properties, and add these to the to or cc fields of the new Activity being created. Clients MAY recurse through attached objects, but if doing so, SHOULD set a limit for this recursion. (Note that this does not suggest that the client should "unpack" collections of actors being addressed as individual recipients).

Clients MAY give the user the chance to amend this addressing in the UI.

For example, when Christine likes the following article by Amy:

  {
    "@context": ["https://www.w3.org/ns/activitystreams",
                 {"@language": "en-GB"}],
    "id": "https://rhiaro.co.uk/2016/05/minimal-activitypub",
    "type": "Article",
    "name": "Minimal ActivityPub update client",
    "content": "Today I finished morph, a client for posting Activity Streams2...",
    "attributedTo": "https://rhiaro.co.uk/#amy",
    "to": "https://rhiaro.co.uk/followers/",
    "cc": "https://e14n.com/evan"
  }
        

the like is generated by the client as:

  {
    "@context": ["https://www.w3.org/ns/activitystreams",
                 {"@language": "en"}],
    "type": "Like",
    "actor": "https://dustycloud.org/christine/",
    "summary": "Christine liked 'Minimal ActivityPub update client'",
    "object": "https://rhiaro.co.uk/2016/05/minimal-activitypub",
    "to": ["https://rhiaro.co.uk/#amy",
           "https://dustycloud.org/followers",
           "https://rhiaro.co.uk/followers/"],
    "cc": "https://e14n.com/evan"
  }
        

The receiving outbox can then perform delivery to not only the followers of Christine (the liker), but also to Amy, and Amy's followers and Evan, both of whom received the original article.

Clients submitting the following activities to an outbox MUST provide the object property in the activity: Create, Update, Delete, Follow, Add, Remove, Like, Block, Undo. Additionally, clients submitting the following activities to an outbox MUST also provide the target property: Add, Remove.

Create Activity

The Create activity is used when posting a new object. This has the side effect that the object embedded within the Activity (in the object property) is created.

When a Create activity is posted, the actor of the activity SHOULD be copied onto the object's attributedTo field.

A mismatch between addressing of the Create activity and its object is likely to lead to confusion. As such, a server SHOULD copy any recipients of the Create activity to its object upon initial distribution, and likewise with copying recipients from the object to the wrapping Create activity. Note that it is acceptable for the object's addressing to be changed later without changing the Create's addressing (for example via an Update activity).

        {
          "@context": "https://www.w3.org/ns/activitystreams",
          "type": "Note",
          "content": "This is a note",
          "published": "2015-02-10T15:04:55Z",
          "to": ["https://example.org/~john/"],
          "cc": ["https://example.com/~erik/followers",
                 "as:Public"]
        }
        

        {
          "@context": "https://www.w3.org/ns/activitystreams",
          "type": "Create",
          "id": "https://example.net/~mallory/87374",
          "actor": "https://example.net/~mallory",
          "object": {
            "id": "https://example.com/~mallory/note/72",
            "type": "Note",
            "attributedTo": "https://example.net/~mallory",
            "content": "This is a note",
            "published": "2015-02-10T15:04:55Z",
            "to": ["https://example.org/~john/"],
            "cc": ["https://example.com/~erik/followers",
                   "as:Public"]
          },
          "published": "2015-02-10T15:04:55Z",
          "to": ["https://example.org/~john/"],
          "cc": ["https://example.com/~erik/followers",
                 "as:Public"]
        }
        

Update Activity

The Update activity is used when updating an already existing object. The side effect of this is that the object MUST be modified to reflect the new structure as defined in the update activity, assuming the actor has permission to update this object.

Delete Activity

The Delete activity is used to delete an already existing object. The side effect of this is that the server MAY replace the object with a Tombstone of the object that will be displayed in activities which reference the deleted object. If the deleted object is requested the server SHOULD respond with either the HTTP 410 Gone status code if a Tombstone object is presented as the response body, otherwise respond with a HTTP 404 Not Found.

A deleted object:

          {
            "@context": "https://www.w3.org/ns/activitystreams",
            "id": "https://example.com/~alice/note/72",
            "type": "Tombstone",
            "published": "2015-02-10T15:04:55Z",
            "updated": "2015-02-10T15:04:55Z",
            "deleted": "2015-02-10T15:04:55Z"
          }
        

Follow Activity

The Follow activity is used to subscribe to the activities of another actor.

The side effect of receiving this in an outbox is that the server SHOULD add the object to the actor's following Collection when and only if an Accept activity is subsequently received with this Follow activity as its object.

Add Activity

Upon receipt of an Add activity into the outbox, the server SHOULD add the object to the collection specified in the target property, unless:

• the target is not owned by the receiving server, and thus they are not authorized to update it.
• the object is not allowed to be added to the target collection for some other reason, at the receiving server's discretion.

Remove Activity

Upon receipt of a Remove activity into the outbox, the server SHOULD remove the object from the collection specified in the target property, unless:

• the target is not owned by the receiving server, and thus they are not authorized to update it.
• the object is not allowed to be removed from the target collection for some other reason, at the receiving server's discretion.

Like Activity

The Like activity indicates the actor likes the object.

The side effect of receiving this in an outbox is that the server SHOULD add the object to the actor's liked Collection.

Block Activity

The Block activity is used to indicate that the posting actor does not want another actor (defined in the object property) to be able to interact with objects posted by the actor posting the Block activity. The server SHOULD prevent the blocked user from interacting with any object posted by the actor.

Servers SHOULD NOT deliver Block Activities to their object.

Undo Activity

The Undo activity is used to undo a previous activity. See the Activity Vocabulary documentation on Inverse Activities and "Undo". For example, Undo may be used to undo a previous Like, Follow, or Block. The undo activity and the activity being undone MUST both have the same actor. Side effects should be undone, to the extent possible. For example, if undoing a Like, any counter that had been incremented previously should be decremented appropriately.

There are some exceptions where there is an existing and explicit "inverse activity" which should be used instead. Create based activities should instead use Delete, and Add activities should use Remove.

Delivery

Federated servers SHOULD perform delivery on all Activities posted to the outbox according to outbox delivery. Servers MAY filter activities for privacy, abuse mitigation, or other reasons.

Uploading Media

Servers MAY support uploading document types to be referenced in activites, such as images, video or other binary data, but the precise mechanism is out of scope for this version of ActivityPub. The Social Web Community Group is refining the protocol in the ActivityPub Media Upload report.

Delivery

The following is required by federated servers communicating with other federated servers only.

An activity is delivered to its targets (which are actors) by first looking up the targets' inboxes and then posting the activity to those inboxes. Targets for delivery are determined by checking the Activity Streams audience targeting; namely, the to, bto, cc, bcc, and audience fields of the activity.

The inbox is determined by first retrieving the target actor's JSON-LD representation and then looking up the inbox property. If a recipient is a Collection or OrderedCollection, then the server MUST dereference the collection (with the user's credentials) and discover inboxes for each item in the collection. Servers MUST limit the number of layers of indirections through collections which will be performed, which MAY be one.

Servers MUST de-duplicate the final recipient list. Servers MUST also exclude actors from the list which are the same as the actor of the Activity being notified about. That is, actors shouldn't have their own activities delivered to themselves.

What to do when there are no recipients specified is not defined, however it's recommended that if no recipients are specified the object remains completely private and access controls restrict the access to object. If the object is addressed exclusively to the "public" collection, the object may be publicly viewable in the actor's outbox without being delivered to any actors.

An HTTP POST request (with authorization of the submitting user) is then made to the inbox, with the Activity as the body of the request. This Activity is added by the receiver as an item in the inbox OrderedCollection. Attempts to deliver to an inbox on a non-federated server SHOULD result in a 405 Method Not Allowed response.

For federated servers performing delivery to a third party server, delivery SHOULD be performed asynchronously, and SHOULD additionally retry delivery to recipients if it fails due to network error.

Note: Activities being distributed between actors on the same origin may use any internal mechanism, and are not required to use HTTP.

While it is not required reading to understand this specification, it is worth noting that ActivityPub's targeting and delivery mechanism overlaps with the Linked Data Notifications specification, and the two specifications may interoperably combined. In particular, the inbox property is the same between ActivityPub and Linked Data Notifications, and the targeting and delivery systems described in this document are supported by Linked Data Notifications. In addition to JSON-LD compacted Activity Streams documents, Linked Data Notifications also supports a number of RDF serializations which are not required for ActivityPub implementations. However, ActivityPub implementations which wish to be more broadly compatible with Linked Data Notifications implementations may wish to support other RDF representations.

Create Activity

Receiving a Create activity in an inbox has surprisingly few side effects; the activity should appear in the actor's inbox and it is likely that the server will want to locally store a representation of this activity and its accompanying object. However, this mostly happens in general with processing activities delivered to an inbox anyway.

Update Activity

For server to server interactions, an Update activity means that the receiving server SHOULD update its copy of the object of the same id to the copy supplied in the Update activity. Unlike the client to server handling of the Update activity, this is not a partial update but a complete replacement of the object.

The receiving server MUST take care to be sure that the Update is authorized to modify its object. At minimum, this may be done by ensuring that the Update and its object are of same origin.

Delete Activity

The side effect of receiving this is that (assuming the object is owned by the sending actor / server) the server receiving the delete activity SHOULD remove its representation of the object with the same id, and MAY replace that representation with a Tombstone object.

(Note that after an activity has been transmitted from an origin server to a remote server, there is nothing in the ActivityPub protocol that can enforce remote deletion of an object's representation).

Follow Activity

The side effect of receiving this in an inbox is that the server SHOULD generate either an Accept or Reject activity with the Follow as the object and deliver it to the actor of the Follow. The Accept or Reject MAY be generated automatically, or MAY be the result of user input (possibly after some delay in which the user reviews). Servers MAY choose to not explicitly send a Reject in response to a Follow, though implementors ought to be aware that the server sending the request could be left in an intermediate state. For example, a server might not send a Reject to protect a user's privacy.

Accept Activity

The side effect of receiving this in an inbox is determined by the type of the object received, and it is possible to accept types not described in this document (for example, an Offer).

If the object of an Accept received to an inbox is a Follow activity previously sent by the receiver, the server SHOULD add the actor to the receiver's Following Collection.

Reject Activity

The side effect of receiving this in an inbox is determined by the type of the object received, and it is possible to reject types not described in this document (for example, an Offer).

If the object of a Reject received to an inbox is a Follow activity previously sent by the receiver, this means the recipient did not approve the Follow request. The server MUST NOT add the actor to the receiver's Following Collection.

Add Activity

Upon receipt of an Add activity into the inbox, the server SHOULD add the object to the collection specified in the target property, unless:

• the target is not owned by the receiving server, and thus they can't update it.
• the object is not allowed to be added to the target collection for some other reason, at the receiver's discretion.

Remove Activity

Upon receipt of a Remove activity into the inbox, the server SHOULD remove the object from the collection specified in the target property, unless:

• the target is not owned by the receiving server, and thus they can't update it.
• the object is not allowed to be removed to the target collection for some other reason, at the receiver's discretion.

Like Activity

The side effect of receiving this in an inbox is that the server SHOULD increment the object's count of likes by adding the received activity to the likes collection if this collection is present.

Announce Activity (sharing)

Upon receipt of an Announce activity in an inbox, a server SHOULD increment the object's count of shares by adding the received activity to the shares collection if this collection is present.

Undo Activity

The Undo activity is used to undo the side effects of previous activities. See the Activity Streams documentation on Inverse Activities and "Undo". The scope and restrictions of the Undo activity are the same as for the Undo activity in the context of client to server interactions, but applied to a federated context.

Authentication and Authorization

ActivityPub uses authentication for two purposes; first, to authenticate clients to servers, and secondly in federated implementations to authenticate servers to each other.

Unfortunately at the time of standardization, there are no strongly agreed upon mechanisms for authentication. Some possible directions for authentication are laid out in the Social Web Community Group Authentication and Authorization best practices report.

Verification

Servers should not trust client submitted content, and federated servers also should not trust content received from a server other than the content's origin without some form of verification.

Servers should be careful to verify that new content is really posted by the actor that claims to be posting it, and that the actor has permission to update the resources it claims to. See also and .

Accessing localhost URIs

It is often convenient while developing to test against a process running on localhost. However, permitting requests to localhost in a production client or server instance can be dangerous. Making requests to URIs on localhost which do not require authorization may unintentionally access or modify resources assumed to be protected to be usable by localhost-only.

If your ActivityPub server or client does permit making requests to localhost URIs for development purposes, consider making it a configuration option which defaults to off.

URI Schemes

There are many types of URIs aside from just http and https. Some libraries which handle fetching requests at various URI schemes may try to be smart and reference schemes which may be undesirable, such as file. Client and server authors should carefully check how their libraries handle requests, and potentially whitelist only certain safe URI types, such as http and https.

Recursive Objects

Servers should set a limit on how deep to recurse while resolving objects, or otherwise specially handle Activity Streams objects with recursive references. Failure to properly do so may result in denial-of-service security vulnerabilities.

Spam

Spam is a problem in any network, perhaps especially so in federated networks. While no specific mechanism for combating spam is provided in ActivityPub, it is recommended that servers filter incoming content both by local untrusted users and any remote users through some sort of spam filter.

Federation denial-of-service

Servers should implement protections against denial-of-service attacks from other, federated servers. This can be done using, for example, some kind of ratelimiting mechanism. Servers should be especially careful to implement this protection around activities that involve side effects. Servers SHOULD also take care not to overload servers with submissions, for example by using an exponential backoff strategy.

Client-to-server ratelimiting

Servers should ratelimit API client submissions. This serves two purposes:

1. It prevents malicious clients from conducting denial-of-service attacks on the server.
2. It ensures that the server will not distribute so many activities that it triggers another server's denial-of-service protections.

Client-to-server response denial-of-service

In order to prevent a client from being overloaded by oversized Collections, servers should take care to limit the size of Collection pages they return to clients. Clients should still be prepared to limit the size of responses they are willing to handle in case they connect to malicious or compromised servers, for example by timing out and generating an error.

Sanitizing Content

Any activity field being rendered for browsers (or other rich text enabled applications) should take care to sanitize fields containing markup to prevent cross site scripting attacks.

Not displaying bto and bcc properties

bto and bcc already must be removed for delivery, but servers are free to decide how to represent the object in their own storage systems. However, since bto and bcc are only intended to be known/seen by the original author of the object/activity, servers should omit these properties during display as well.