FEP-2c59: Discovery of a Webfinger address from an ActivityPub actor

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Summary

[Webfinger][Webfinger] is used on the fediverse to abstract out variations in [ActivityPub][ActivityPub] actor URL formats, giving a uniform way of addressing an actor. With a Webfinger address, a client can discover the actor's ActivityPub actor URL. This specification defines an explicit way to reverse the process, and discover a preferred Webfinger address from an ActivityPub actor URL.

Motivation

[ActivityPub][ActivityPub] is a protocol for federated social networking. It uses HTTPS URLs to identify actors, that is, the people, organizations, applications and groups that use the protocol.

The URL format is not standardized, and there are many variations. For example, a user with the username evan on the activitypub.example domain could have an actor URL of:

  • A path on the domain, like https://activitypub.example/users/evan
  • Including an ID number, like https://activitypub.example/users/1234
  • A specific subdomain for social data, like https://social.activitypub.example/evan
  • One subdomain per user, like https://evan.activitypub.example

[Webfinger][Webfinger] is a discovery protocol for people on the Internet. A Webfinger address is an email-address-like identity in the form user@activitypub.example. Following the Webfinger protocol, a client can discover information about the person at that address, including their ActivityPub actor URL.

This discovery is unidirectional and can cross domain boundaries. This allows potentially misleading or harmful use of WebFinger to identify actors on the network without their consent. For example, the owner of idiot.example could create a Webfinger address pompous@idiot.example that points to https://activitypub.example/users/evan.

In addition, ActivityPub servers that display actors' Webfinger addresses in their UIs need a reliable way to know which Webfinger is preferred for a given actor ID.

To solve this problem, [Mastodon][Mastodon] constructs a canonical Webfinger address for an actor by extracting the preferredUsername from the actor's profile, and prepending it to the domain name of the actor's identity URL. It then does a Webfinger lookup on that address, and if the actor URL returned matches the identity URL, it uses that Webfinger address.

This technique can cause issues for certain actor URLs. First, preferredUsername is not a required property for an ActivityPub actor.

Second, it becomes inconvenient to use subdomains or alternate domains for actor URLs. If the original WebFinger is evan@organization.example, and the ActivityPub URL is https://social.organization.example/evan, the Webfinger will be canonicalized to the longer evan@social.organization.example. A similar problem arises if the actor URL uses one subdomain per user, like https://evan.organization.example/.

This specification defines a way to discover a Webfinger address from an ActivityPub actor without constructing it from properties and URL parsing. It gives domain owners the option to use a registered domain for their Webfinger addresses, and host their ActivityPub server on a subdomain or a different domain. The Mastodon method can be used as a fallback.

Specification

  • An ActivityPub actor SHOULD include a webfinger property, which is a canonical Webfinger address for the actor.
  • The webfinger property is functional, that is, there can be at most one webfinger property per actor.
  • If provided, the JRD link data for the Webfinger address in the webfinger property MUST link directly to the actor URL verbatim, without redirects or aliases.
  • The identity in the webfinger property SHOULD be a plain Webfinger address, like user@domain.example.
  • The identity in the webfinger property MAY be an acct: URL, like acct:user@domain.example.

Context

The context document for this specification is https://purl.archive.org/socialweb/webfinger. Its contents are as follows:

{
  "@context": {
    "wf": "https://purl.archive.org/socialweb/webfinger#",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "webfinger": {
      "@id": "wf:webfinger",
      "@type": "xsd:string"
    }
  }
}

例

A publisher can include the webfinger property for an actor.

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://purl.archive.org/socialweb/webfinger"
    ],
    "id": "https://social.example.com/evanp",
    "type": "Person",
    "name": "Evan Prodromou",
    "inbox": "https://social.example.com/evanp/inbox",
    "outbox": "https://social.example.com/evanp/outbox",
    "following": "https://social.example.com/evanp/following",
    "followers": "https://social.example.com/evanp/followers",
    "liked": "https://social.example.com/evanp/liked",
    "webfinger": "evanp@example.com"
}

A publisher can include the webfinger property for a Person object referenced in the actor property of an activity.

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://purl.archive.org/socialweb/webfinger"
    ],
    "id": "https://social.example.com/evanp/activity/775",
    "to": ["Public"],
    "type": "Like",
    "summary": "Evan liked a note",
    "actor": {
        "type": "Person",
        "name": "Evan Prodromou",
        "id": "https://social.example.com/evanp",
        "webfinger": "evanp@example.com"
    },
    "object": {
        "type": "Note",
        "id": "https://social.example.com/other/note/221",
        "to": ["Public"]
    }
}

The publisher can include the webfinger property for an actor with an acct: prefix.

{
    "@context": [
        "https://www.w3.org/ns/activitystreams",
        "https://purl.archive.org/socialweb/webfinger"
    ],
    "id": "https://prefix.example/urlfan",
    "type": "Person",
    "name": "URL Fan",
    "inbox": "https://prefix.example/urlfan/inbox",
    "outbox": "https://prefix.example/urlfan/outbox",
    "following": "https://prefix.example/urlfan/following",
    "followers": "https://prefix.example/urlfan/followers",
    "liked": "https://prefix.example/urlfan/liked",
    "webfinger": "acct:urlfan@prefix.example"
}

Security Considerations

The source of the webfinger property is important. A third party may spoof, intentionally or unintentionally, the webfinger property of another actor. The property should only be treated as the preferred Webfinger address for the actor if it comes from the actor id URL, or if it is delivered with the actor's [HTTP Signature][HTTP Signature] via the ActivityPub protocol.

参考文献

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