Internet Engineering Task Force (IETF) P. Jones
Request for Comments: 7033 G. Salgueiro
Category: Standards Track Cisco Systems
ISSN: 2070-1721 M. Jones
Microsoft
J. Smarr
Google
September 2013
WebFinger
Abstract
This specification defines the WebFinger protocol, which can be used
to discover information about people or other entities on the
Internet using standard HTTP methods. WebFinger discovers
information for a URI that might not be usable as a locator
otherwise, such as account or email URIs.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7033.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Example Uses of WebFinger .......................................4
3.1. Identity Provider Discovery for OpenID Connect .............4
3.2. Getting Author and Copyright Information for a Web Page ....5
4. WebFinger Protocol ..............................................7
4.1. Constructing the Query Component of the Request URI.......7
4.2. Performing a WebFinger Query..............................8
4.3. The "rel" Parameter.......................................9
4.4. The JSON Resource Descriptor (JRD).......................11
4.4.1. subject.............................................11
4.4.2. aliases.............................................11
4.4.3. properties..........................................12
4.4.4. links...............................................12
4.5. WebFinger and URIs.......................................14
5. Cross-Origin Resource Sharing (CORS) ...........................14
6. Access Control .................................................15
7. Hosted WebFinger Services ......................................15
8. Definition of WebFinger Applications ...........................16
8.1. Specification of the URI Scheme and URI ...................17
8.2. Host Resolution ...........................................17
8.3. Specification of Properties ...............................17
8.4. Specification of Links ....................................18
8.5. One URI, Multiple Applications ............................18
8.6. Registration of Link Relation Types and Properties ........19
9. Security Considerations ........................................19
9.1. Transport-Related Issues ..................................19
9.2. User Privacy Considerations ...............................19
9.3. Abuse Potential ...........................................21
9.4. Information Reliability ...................................21
10. IANA Considerations ...........................................22
10.1. Well-Known URI ...........................................22
10.2. JSON Resource Descriptor (JRD) Media Type ................22
10.3. Registering Link Relation Types ..........................24
10.4. Establishment of the "WebFinger Properties" Registry .....24
10.4.1. The Registration Template .........................24
10.4.2. The Registration Procedures .......................25
11. Acknowledgments ...............................................26
12. References ....................................................26
12.1. Normative References .....................................26
12.2. Informative References ...................................27
1. Introduction
WebFinger is used to discover information about people or other
entities on the Internet that are identified by a URI [6] using
standard Hypertext Transfer Protocol (HTTP) [2] methods over a secure
transport [12]. A WebFinger resource returns a JavaScript Object
Notation (JSON) [5] object describing the entity that is queried.
The JSON object is referred to as the JSON Resource Descriptor (JRD).
For a person, the type of information that might be discoverable via
WebFinger includes a personal profile address, identity service,
telephone number, or preferred avatar. For other entities on the
Internet, a WebFinger resource might return JRDs containing link
relations [8] that enable a client to discover, for example, that a
printer can print in color on A4 paper, the physical location of a
server, or other static information.
Information returned via WebFinger might be for direct human
consumption (e.g., looking up someone's phone number), or it might be
used by systems to help carry out some operation (e.g., facilitating,
with additional security mechanisms, logging into a web site by
determining a user's identity service). The information is intended
to be static in nature, and, as such, WebFinger is not intended to be
used to return dynamic information like the temperature of a CPU or
the current toner level in a laser printer.
The WebFinger protocol is designed to be used across many
applications. Applications that wish to utilize WebFinger will need
to specify properties, titles, and link relation types that are
appropriate for the application. Further, applications will need to
define the appropriate URI scheme to utilize for the query target.
Use of WebFinger is illustrated in the examples in Section 3 and
described more formally in Section 4. Section 8 describes how
applications of WebFinger may be defined.
2. Terminology
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 RFC 2119 [1].
WebFinger makes heavy use of "link relations". A link relation is an
attribute-value pair in which the attribute identifies the type of
relationship between the linked entity or resource and the
information specified in the value. In Web Linking [4], the link
relation is represented using an HTTP entity-header of "Link", where
the "rel" attribute specifies the type of relationship and the "href"
attribute specifies the information that is linked to the entity or
resource. In WebFinger, the same concept is represented using a JSON
array of "links" objects, where each member named "rel" specifies the
type of relationship and each member named "href" specifies the
information that is linked to the entity or resource. Note that
WebFinger narrows the scope of a link relation beyond what is defined
for Web Linking by stipulating that the value of the "rel" member
needs to be either a single IANA-registered link relation type [8] or
a URI [6].
The use of URIs throughout this document refers to URIs following the
syntax specified in Section 3 of RFC 3986 [6]. Relative URIs, having
syntax following that of Section 4.2 of RFC 3986, are not used with
WebFinger.
3. Example Uses of WebFinger
This section shows a few sample uses of WebFinger. Any application
of WebFinger would be specified outside of this document, as
described in Section 8. The examples in this section should be
simple enough to understand without having seen the formal
specifications of the applications.
3.1. Identity Provider Discovery for OpenID Connect
Suppose Carol wishes to authenticate with a web site she visits using
OpenID Connect [15]. She would provide the web site with her OpenID
Connect identifier, say carol@example.com. The visited web site
would perform a WebFinger query looking for the OpenID Connect
provider. Since the site is interested in only one particular link
relation, the WebFinger resource might utilize the "rel" parameter as
described in Section 4.3:
GET /.well-known/webfinger?
resource=acct%3Acarol%40example.com&
rel=http%3A%2F%2Fopenid.net%2Fspecs%2Fconnect%2F1.0%2Fissuer
HTTP/1.1
Host: example.com
The server might respond like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/jrd+json
{
"subject" : "acct:carol@example.com",
"links" :
[
{
"rel" : "http://openid.net/specs/connect/1.0/issuer",
"href" : "https://openid.example.com"
}
]
}
Since the "rel" parameter only serves to filter the link relations
returned by the resource, other name/value pairs in the response,
including any aliases or properties, would be returned. Also, since
support for the "rel" parameter is not guaranteed, the client must
not assume the "links" array will contain only the requested link
relation.
3.2. Getting Author and Copyright Information for a Web Page
Suppose an application is defined to retrieve metadata information
about a web page URL, such as author and copyright information. To
retrieve that information, the client can utilize WebFinger to issue
a query for the specific URL. Suppose the URL of interest is
http://blog.example.com/article/id/314. The client would issue a
query similar to the following:
GET /.well-known/webfinger?
resource=http%3A%2F%2Fblog.example.com%2Farticle%2Fid%2F314
HTTP/1.1
Host: blog.example.com
The server might then reply in this way:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/jrd+json
{
"subject" : "http://blog.example.com/article/id/314",
"aliases" :
[
"http://blog.example.com/cool_new_thing",
"http://blog.example.com/steve/article/7"
],
"properties" :
{
"http://blgx.example.net/ns/version" : "1.3",
"http://blgx.example.net/ns/ext" : null
},
"links" :
[
{
"rel" : "copyright",
"href" : "http://www.example.com/copyright"
},
{
"rel" : "author",
"href" : "http://blog.example.com/author/steve",
"titles" :
{
"en-us" : "The Magical World of Steve",
"fr" : "Le Monde Magique de Steve"
},
"properties" :
{
"http://example.com/role" : "editor"
}
}
]
}
In the above example, we see that the server returned a list of
aliases, properties, and links related to the subject URL. The links
contain references to information for each link relation type. For
the author link, the server provided a reference to the author's
blog, along with a title for the blog in two languages. The server
also returned a single property related to the author, indicating the
author's role as editor of the blog.
It is worth noting that, while the server returned just two links in
the "links" array in this example, a server might return any number
of links when queried.
4. WebFinger Protocol
The WebFinger protocol is used to request information about an entity
identified by a query target (a URI). The client can optionally
specify one or more link relation types for which it would like to
receive information.
A WebFinger request is an HTTPS request to a WebFinger resource. A
WebFinger resource is a well-known URI [3] using the HTTPS scheme
constructed along with the required query target and optional link
relation types. WebFinger resources MUST NOT be served with any
other URI scheme (such as HTTP).
A WebFinger resource is always given a query target, which is another
URI that identifies the entity whose information is sought. GET
requests to a WebFinger resource convey the query target in the
"resource" parameter of the WebFinger URI's query string; see Section
4.1 for details.
The host to which a WebFinger query is issued is significant. If the
query target contains a "host" portion (Section 3.2.2 of RFC 3986),
then the host to which the WebFinger query is issued SHOULD be the
same as the "host" portion of the query target, unless the client
receives instructions through some out-of-band mechanism to send the
query to another host. If the query target does not contain a "host"
portion, then the client chooses a host to which it directs the query
using additional information it has.
The path component of a WebFinger URI MUST be the well-known path
"/.well-known/webfinger". A WebFinger URI MUST contain a query
component that encodes the query target and optional link relation
types as specified in Section 4.1.
The WebFinger resource returns a JSON Resource Descriptor (JRD) as
the resource representation to convey information about an entity on
the Internet. Also, the Cross-Origin Resource Sharing (CORS) [7]
specification is utilized to facilitate queries made via a web
browser.
4.1. Constructing the Query Component of the Request URI
A WebFinger URI MUST contain a query component (see Section 3.4 of
RFC 3986). The query component MUST contain a "resource" parameter
and MAY contain one or more "rel" parameters. The "resource"
parameter MUST contain the query target (URI), and the "rel"
parameters MUST contain encoded link relation types according to the
encoding described in this section.
To construct the query component, the client performs the following
steps. First, each parameter value is percent-encoded, as per
Section 2.1 of RFC 3986. The encoding is done to conform to the
query production in Section 3.4 of that specification, with the
addition that any instances of the "=" and "&" characters within the
parameter values are also percent-encoded. Next, the client
constructs a string to be placed in the query component by
concatenating the name of the first parameter together with an equal
sign ("=") and the percent-encoded parameter value. For any
subsequent parameters, the client appends an ampersand ("&") to the
string, the name of the next parameter, an equal sign, and the
parameter value. The client MUST NOT insert any spaces while
constructing the string. The order in which the client places each
attribute-value pair within the query component does not matter in
the interpretation of the query component.
4.2. Performing a WebFinger Query
A WebFinger client issues a query using the GET method to the well-
known [3] resource identified by the URI whose path component is
"/.well-known/webfinger" and whose query component MUST include the
"resource" parameter exactly once and set to the value of the URI for
which information is being sought.
If the "resource" parameter is absent or malformed, the WebFinger
resource MUST indicate that the request is bad as per Section 10.4.1
of RFC 2616 [2].
If the "resource" parameter is a value for which the server has no
information, the server MUST indicate that it was unable to match the
request as per Section 10.4.5 of RFC 2616.
A client MUST query the WebFinger resource using HTTPS only. If the
client determines that the resource has an invalid certificate, the
resource returns a 4xx or 5xx status code, or if the HTTPS connection
cannot be established for any reason, then the client MUST accept
that the WebFinger query has failed and MUST NOT attempt to reissue
the WebFinger request using HTTP over a non-secure connection.
A WebFinger resource MUST return a JRD as the representation for the
resource if the client requests no other supported format explicitly
via the HTTP "Accept" header. The client MAY include the "Accept"
header to indicate a desired representation; representations other
than JRD might be defined in future specifications. The WebFinger
resource MUST silently ignore any requested representations that it
does not understand or support. The media type used for the JSON
Resource Descriptor (JRD) is "application/jrd+json" (see Section
10.2).
The properties, titles, and link relation types returned by the
server in a JRD might be varied and numerous. For example, the
server might return information about a person's blog, vCard [14],
avatar, OpenID Connect provider, RSS or ATOM feed, and so forth in a
reply. Likewise, if a server has no information to provide, it might
return a JRD with an empty "links" array or no "links" array.
A WebFinger resource MAY redirect the client; if it does, the
redirection MUST only be to an "https" URI and the client MUST
perform certificate validation again when redirected.
A WebFinger resource can include cache validators in a response to
enable conditional requests by the client and/or expiration times as
per Section 13 of RFC 2616.
4.3. The "rel" Parameter
When issuing a request to a WebFinger resource, the client MAY
utilize the "rel" parameter to request only a subset of the
information that would otherwise be returned without the "rel"
parameter. When the "rel" parameter is used and accepted, only the
link relation types that match the link relation type provided via
the "rel" parameter are included in the array of links returned in
the JRD. If there are no matching link relation types defined for
the resource, the "links" array in the JRD will be either absent or
empty. All other information present in a resource descriptor
remains present, even when "rel" is employed.
The "rel" parameter MAY be included multiple times in order to
request multiple link relation types.
The purpose of the "rel" parameter is to return a subset of "link
relation objects" (see Section 4.4.4) that would otherwise be
returned in the resource descriptor. Use of the parameter might
reduce processing requirements on either the client or server, and it
might also reduce the bandwidth required to convey the partial
resource descriptor, especially if there are numerous link relation
values to convey for a given "resource" value. Note that if a client
requests a particular link relation type for which the server has no
information, the server MAY return a JRD with an empty "links" array
or no "links" array.
WebFinger resources SHOULD support the "rel" parameter. If the
resource does not support the "rel" parameter, it MUST ignore the
parameter and process the request as if no "rel" parameter values
were present.
The following example uses the "rel" parameter to request links for
two link relation types:
GET /.well-known/webfinger?
resource=acct%3Abob%40example.com&
rel=http%3A%2F%2Fwebfinger.example%2Frel%2Fprofile-page&
rel=http%3A%2F%2Fwebfinger.example%2Frel%2Fbusinesscard HTTP/1.1
Host: example.com
In this example, the client requests the link relations of type
"http://webfinger.example/rel/profile-page" and
"http://webfinger.example/rel/businesscard". The server then
responds with a message like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/jrd+json
{
"subject" : "acct:bob@example.com",
"aliases" :
[
"https://www.example.com/~bob/"
],
"properties" :
{
"http://example.com/ns/role" : "employee"
},
"links" :
[
{
"rel" : "http://webfinger.example/rel/profile-page",
"href" : "https://www.example.com/~bob/"
},
{
"rel" : "http://webfinger.example/rel/businesscard",
"href" : "https://www.example.com/~bob/bob.vcf"
}
]
}
As you can see in the response, the resource representation contains
only the links of the types requested by the client and for which the
server had information, but the other parts of the JRD are still
present. Note also in the above example that the links returned in
the "links" array all use HTTPS, which is important if the data
indirectly obtained via WebFinger needs to be returned securely.
4.4. The JSON Resource Descriptor (JRD)
The JSON Resource Descriptor (JRD), originally introduced in RFC 6415
[16] and based on the Extensible Resource Descriptor (XRD) format
[17], is a JSON object that comprises the following name/value pairs:
o subject
o aliases
o properties
o links
The member "subject" is a name/value pair whose value is a string,
"aliases" is an array of strings, "properties" is an object
comprising name/value pairs whose values are strings, and "links" is
an array of objects that contain link relation information.
When processing a JRD, the client MUST ignore any unknown member and
not treat the presence of an unknown member as an error.
Below, each of these members of the JRD is described in more detail.
4.4.1. subject
The value of the "subject" member is a URI that identifies the entity
that the JRD describes.
The "subject" value returned by a WebFinger resource MAY differ from
the value of the "resource" parameter used in the client's request.
This might happen, for example, when the subject's identity changes
(e.g., a user moves his or her account to another service) or when
the resource prefers to express URIs in canonical form.
The "subject" member SHOULD be present in the JRD.
4.4.2. aliases
The "aliases" array is an array of zero or more URI strings that
identify the same entity as the "subject" URI.
The "aliases" array is OPTIONAL in the JRD.
4.4.3. properties
The "properties" object comprises zero or more name/value pairs whose
names are URIs (referred to as "property identifiers") and whose
values are strings or null. Properties are used to convey additional
information about the subject of the JRD. As an example, consider
this use of "properties":
"properties" : { "http://webfinger.example/ns/name" : "Bob Smith" }
The "properties" member is OPTIONAL in the JRD.
4.4.4. links
The "links" array has any number of member objects, each of which
represents a link [4]. Each of these link objects can have the
following members:
o rel
o type
o href
o titles
o properties
The "rel" and "href" members are strings representing the link's
relation type and the target URI, respectively. The context of the
link is the "subject" (see Section 4.4.1).
The "type" member is a string indicating what the media type of the
result of dereferencing the link ought to be.
The order of elements in the "links" array MAY be interpreted as
indicating an order of preference. Thus, if there are two or more
link relations having the same "rel" value, the first link relation
would indicate the user's preferred link.
The "links" array is OPTIONAL in the JRD.
Below, each of the members of the objects found in the "links" array
is described in more detail. Each object in the "links" array,
referred to as a "link relation object", is completely independent
from any other object in the array; any requirement to include a
given member in the link relation object refers only to that
particular object.
4.4.4.1. rel
The value of the "rel" member is a string that is either a URI or a
registered relation type [8] (see RFC 5988 [4]). The value of the
"rel" member MUST contain exactly one URI or registered relation
type. The URI or registered relation type identifies the type of the
link relation.
The other members of the object have meaning only once the type of
link relation is understood. In some instances, the link relation
will have associated semantics enabling the client to query for other
resources on the Internet. In other instances, the link relation
will have associated semantics enabling the client to utilize the
other members of the link relation object without fetching additional
external resources.
URI link relation type values are compared using the "Simple String
Comparison" algorithm of Section 6.2.1 of RFC 3986.
The "rel" member MUST be present in the link relation object.
4.4.4.2. type
The value of the "type" member is a string that indicates the media
type [9] of the target resource (see RFC 6838 [10]).
The "type" member is OPTIONAL in the link relation object.
4.4.4.3. href
The value of the "href" member is a string that contains a URI
pointing to the target resource.
The "href" member is OPTIONAL in the link relation object.
4.4.4.4. titles
The "titles" object comprises zero or more name/value pairs whose
names are a language tag [11] or the string "und". The string is
human-readable and describes the link relation. More than one title
for the link relation MAY be provided for the benefit of users who
utilize the link relation, and, if used, a language identifier SHOULD
be duly used as the name. If the language is unknown or unspecified,
then the name is "und".
A JRD SHOULD NOT include more than one title identified with the same
language tag (or "und") within the link relation object. Meaning is
undefined if a link relation object includes more than one title
named with the same language tag (or "und"), though this MUST NOT be
treated as an error. A client MAY select whichever title or titles
it wishes to utilize.
Here is an example of the "titles" object:
"titles" :
{
"en-us" : "The Magical World of Steve",
"fr" : "Le Monde Magique de Steve"
}
The "titles" member is OPTIONAL in the link relation object.
4.4.4.5. properties
The "properties" object within the link relation object comprises
zero or more name/value pairs whose names are URIs (referred to as
"property identifiers") and whose values are strings or null.
Properties are used to convey additional information about the link
relation. As an example, consider this use of "properties":
"properties" : { "http://webfinger.example/mail/port" : "993" }
The "properties" member is OPTIONAL in the link relation object.
4.5. WebFinger and URIs
WebFinger requests include a "resource" parameter (see Section 4.1)
specifying the query target (URI) for which the client requests
information. WebFinger is neutral regarding the scheme of such a
URI: it could be an "acct" URI [18], an "http" or "https" URI, a
"mailto" URI [19], or some other scheme.
5. Cross-Origin Resource Sharing (CORS)
WebFinger resources might not be accessible from a web browser due to
"Same-Origin" policies. The current best practice is to make
resources available to browsers through Cross-Origin Resource Sharing
(CORS) [7], and servers MUST include the Access-Control-Allow-Origin
HTTP header in responses. Servers SHOULD support the least
restrictive setting by allowing any domain access to the WebFinger
resource:
Access-Control-Allow-Origin: *
There are cases where defaulting to the least restrictive setting is
not appropriate. For example, a server on an intranet that provides
sensitive company information SHOULD NOT allow CORS requests from any
domain, as that could allow leaking of that sensitive information. A
server that wishes to restrict access to information from external
entities SHOULD use a more restrictive Access-Control-Allow-Origin
header.
6. Access Control
As with all web resources, access to the WebFinger resource could
require authentication. Further, failure to provide required
credentials might result in the server forbidding access or providing
a different response than had the client authenticated with the
server.
Likewise, a WebFinger resource MAY provide different responses to
different clients based on other factors, such as whether the client
is inside or outside a corporate network. As a concrete example, a
query performed on the internal corporate network might return link
relations to employee pictures, whereas link relations for employee
pictures might not be provided to external entities.
Further, link relations provided in a WebFinger resource
representation might point to web resources that impose access
restrictions. For example, the aforementioned corporate server may
provide both internal and external entities with URIs to employee
pictures, but further authentication might be required in order for
the client to access the picture resources if the request comes from
outside the corporate network.
The decisions made with respect to what set of link relations a
WebFinger resource provides to one client versus another and what
resources require further authentication, as well as the specific
authentication mechanisms employed, are outside the scope of this
document.
7. Hosted WebFinger Services
As with most services provided on the Internet, it is possible for a
domain owner to utilize "hosted" WebFinger services. By way of
example, a domain owner might control most aspects of their domain
but use a third-party hosting service for email. In the case of
email, mail exchange (MX) records identify mail servers for a domain.
An MX record points to the mail server to which mail for the domain
should be delivered. To the sending server, it does not matter
whether those MX records point to a server in the destination domain
or a different domain.
Likewise, a domain owner might utilize the services of a third party
to provide WebFinger services on behalf of its users. Just as a
domain owner is required to insert MX records into DNS to allow for
hosted email services, the domain owner is required to redirect HTTP
queries to its domain to allow for hosted WebFinger services.
When a query is issued to the WebFinger resource, the web server MUST
return a response with a redirection status code that includes a
Location header pointing to the location of the hosted WebFinger
service URI. This WebFinger service URI does not need to point to
the well-known WebFinger location on the hosting service provider
server.
As an example, assume that example.com's WebFinger services are
hosted by wf.example.net. Suppose a client issues a query for
acct:alice@example.com like this:
GET /.well-known/webfinger?
resource=acct%3Aalice%40example.com HTTP/1.1
Host: example.com
The server might respond with this:
HTTP/1.1 307 Temporary Redirect
Access-Control-Allow-Origin: *
Location: https://wf.example.net/example.com/webfinger?
resource=acct%3Aalice%40example.com
The client can then follow the redirection, reissuing the request to
the URI provided in the Location header. Note that the server will
include any required URI parameters in the Location header value,
which could be different than the URI parameters the client
originally used.
8. Definition of WebFinger Applications
This specification details the protocol syntax used to query a domain
for information about a URI, the syntax of the JSON Resource
Descriptor (JRD) that is returned in response to that query, security
requirements and considerations, hosted WebFinger services, various
expected HTTP status codes, and so forth. However, this
specification does not enumerate the various possible properties or
link relation types that might be used in conjunction with WebFinger
for a particular application, nor does it define what properties or
link relation types one might expect to see in response to querying
for a particular URI or URI scheme. Nonetheless, all of these
unspecified elements are important in order to implement an
interoperable application that utilizes the WebFinger protocol and
MUST be specified in the relevant document(s) defining the particular
application making use of the WebFinger protocol according to the
procedures described in this section.
8.1. Specification of the URI Scheme and URI
Any application that uses WebFinger MUST specify the URI scheme(s),
and to the extent appropriate, what forms the URI(s) might take. For
example, when querying for information about a user's account at some
domain, it might make sense to specify the use of the "acct" URI
scheme [18]. When trying to obtain the copyright information for a
web page, it makes sense to specify the use of the web page URI
(either http or https).
The examples in Sections 3.1 and 3.2 illustrate the use of different
URI schemes with WebFinger applications. In the example in Section
3.1, WebFinger is used to retrieve information pertinent to OpenID
Connect. In the example in Section 3.2, WebFinger is used to
discover metadata information about a web page, including author and
copyright information. Each of these WebFinger applications needs to
be fully specified to ensure interoperability.
8.2. Host Resolution
As explained in Section 4, the host to which a WebFinger query is
issued is significant. In general, WebFinger applications would
adhere to the procedures described in Section 4 in order to properly
direct a WebFinger query.
However, some URI schemes do not have host portions and there might
be some applications of WebFinger for which the host portion of a URI
cannot or should not be utilized. In such instances, the application
specification MUST clearly define the host resolution procedures,
which might include provisioning a "default" host within the client
to which queries are directed.
8.3. Specification of Properties
WebFinger defines both subject-specific properties (i.e., properties
described in Section 4.4.3 that relate to the URI for which
information is queried) and link-specific properties (see Section
4.4.4.5). This section refers to subject-specific properties.
Applications that utilize subject-specific properties MUST define the
URIs used in identifying those properties, along with valid property
values.
Consider this portion of the JRD found in the example in Section 3.2.
"properties" :
{
"http://blgx.example.net/ns/version" : "1.3",
"http://blgx.example.net/ns/ext" : null
}
Here, two properties are returned in the WebFinger response. Each of
these would be defined in a WebFinger application specification.
These two properties might be defined in the same WebFinger
application specification or separately in different specifications.
Since the latter is possible, it is important that WebFinger clients
not assume that one property has any specific relationship with
another property, unless some relationship is explicitly defined in
the particular WebFinger application specification.
8.4. Specification of Links
The links returned in a WebFinger response each comprise several
pieces of information, some of which are optional (refer to Section
4.4.4). The WebFinger application specification MUST define each
link and any values associated with a link, including the link
relation type ("rel"), the expected media type ("type"), properties,
and titles.
The target URI to which the link refers (i.e., the "href"), if
present, would not normally be specified in an application
specification. However, the URI scheme or any special
characteristics of the URI would usually be specified. If a
particular link does not require an external reference, then all of
the semantics related to the use of that link MUST be defined within
the application specification. Such links might rely only on
properties or titles in the link to convey meaning.
8.5. One URI, Multiple Applications
It is important to be mindful of the fact that different WebFinger
applications might specify the use of the same URI scheme, and in
effect, the same URI for different purposes. That should not be a
problem, since each of property identifier (see Sections 4.4.3 and
4.4.4.5) and link relation type would be uniquely defined for a
specific application.
It should be noted that when a client requests information about a
particular URI and receives a response with a number of different
property identifiers or link relation types that the response is
providing information about the URI without any particular semantics.
How the client interprets the information SHOULD be in accordance
with the particular application specification or set of
specifications the client implements.
Any syntactically valid properties or links the client receives and
that are not fully understood SHOULD be ignored and SHOULD NOT cause
the client to report an error.
8.6. Registration of Link Relation Types and Properties
Application specifications MAY define a simple token as a link
relation type for a link. In that case, the link relation type MUST
be registered with IANA as specified in Sections 10.3.
Further, any defined properties MUST be registered with IANA as
described in Section 10.4.
9. Security Considerations
9.1. Transport-Related Issues
Since this specification utilizes Cross-Origin Resource Sharing
(CORS) [7], all of the security considerations applicable to CORS are
also applicable to this specification.
The use of HTTPS is REQUIRED to ensure that information is not
modified during transit. It should be acknowledged that in
environments where a web server is normally available, there exists
the possibility that a compromised network might have its WebFinger
resource operating on HTTPS replaced with one operating only over
HTTP. As such, clients MUST NOT issue queries over a non-secure
connection.
Clients MUST verify that the certificate used on an HTTPS connection
is valid (as defined in [12]) and accept a response only if the
certificate is valid.
9.2. User Privacy Considerations
Service providers and users should be aware that placing information
on the Internet means that any user can access that information, and
WebFinger can be used to make it even easier to discover that
information. While WebFinger can be an extremely useful tool for
discovering one's avatar, blog, or other personal data, users should
also understand the risks.
Systems or services that expose personal data via WebFinger MUST
provide an interface by which users can select which data elements
are exposed through the WebFinger interface. For example, social
networking sites might allow users to mark certain data as "public"
and then utilize that marking as a means of determining what
information to expose via WebFinger. The information published via
WebFinger would thus comprise only the information marked as public
by the user. Further, the user has the ability to remove information
from publication via WebFinger by removing this marking.
WebFinger MUST NOT be used to provide any personal data unless
publishing that data via WebFinger by the relevant service was
explicitly authorized by the person whose information is being
shared. Publishing one's personal data within an access-controlled
or otherwise limited environment on the Internet does not equate to
providing implicit authorization of further publication of that data
via WebFinger.
The privacy and security concerns with publishing personal data via
WebFinger are worth emphasizing again with respect to personal data
that might reveal a user's current context (e.g., the user's
location). The power of WebFinger comes from providing a single
place where others can find pointers to information about a person,
but service providers and users should be mindful of the nature of
that information shared and the fact that it might be available for
the entire world to see. Sharing location information, for example,
would potentially put a person in danger from any individual who
might seek to inflict harm on that person.
Users should be aware of how easily personal data that one might
publish can be used in unintended ways. In one study relevant to
WebFinger-like services, Balduzzi et al. [20] took a large set of
leaked email addresses and demonstrated a number of potential privacy
concerns, including the ability to cross-correlate the same user's
accounts over multiple social networks. The authors also describe
potential mitigation strategies.
The easy access to user information via WebFinger was a design goal
of the protocol, not a limitation. If one wishes to limit access to
information available via WebFinger, such as WebFinger resources for
use inside a corporate network, the network administrator needs to
take necessary measures to limit access from outside the network.
Using standard methods for securing web resources, network
administrators do have the ability to control access to resources
that might return sensitive information. Further, a server can be
employed in such a way as to require authentication and prevent
disclosure of information to unauthorized entities.
9.3. Abuse Potential
Service providers should be mindful of the potential for abuse using
WebFinger.
As one example, one might query a WebFinger server only to discover
whether or not a given URI is valid. With such a query, the person
may deduce that an email identifier is valid, for example. Such an
approach could help spammers maintain a current list of known email
addresses and to discover new ones.
WebFinger could be used to associate a name or other personal data
with an email address, allowing spammers to craft more convincing
email messages. This might be of particular value in phishing
attempts.
It is RECOMMENDED that implementers of WebFinger server software take
steps to mitigate abuse, including malicious over-use of the server
and harvesting of user information. Although there is no mechanism
that can guarantee that publicly accessible WebFinger databases won't
be harvested, rate-limiting by IP address will prevent or at least
dramatically slow harvest by private individuals without access to
botnets or other distributed systems. The reason these mitigation
strategies are not mandatory is that the correct choice of mitigation
strategy (if any) depends greatly on the context. Implementers
should not construe this as meaning that they do not need to consider
whether to use a mitigation strategy, and if so, what strategy to
use.
WebFinger client developers should also be aware of potential abuse
by spammers or those phishing for information about users. As an
example, suppose a mail client was configured to automatically
perform a WebFinger query on the sender of each received mail
message. If a spammer sent an email using a unique identifier in the
'From' header, then when the WebFinger query was performed, the
spammer would be able to associate the request with a particular
user's email address. This would provide information to the spammer,
including the user's IP address, the fact the user just checked
email, what kind of WebFinger client the user utilized, and so on.
For this reason, it is strongly advised that clients not perform
WebFinger queries unless authorized by the user to do so.
9.4. Information Reliability
A WebFinger resource has no means of ensuring that information
provided by a user is accurate. Likewise, neither the resource nor
the client can be absolutely guaranteed that information has not been
manipulated either at the server or along the communication path
between the client and server. Use of HTTPS helps to address some
concerns with manipulation of information along the communication
path, but it clearly cannot address issues where the resource
provided incorrect information, either due to being provided false
information or due to malicious behavior on the part of the server
administrator. As with any information service available on the
Internet, users should be wary of information received from untrusted
sources.
10. IANA Considerations
10.1. Well-Known URI
This specification registers the "webfinger" well-known URI in the
"Well-Known URIs" registry as defined by RFC 5785 [3].
URI suffix: webfinger
Change controller: IETF
Specification document(s): RFC 7033
Related information: The query to the WebFinger resource will
include one or more parameters in the query string; see Section 4.1
of RFC 7033. Resources at this location are able to return a JSON
Resource Descriptor (JRD) as described in Section 4.4 of RFC 7033.
10.2. JSON Resource Descriptor (JRD) Media Type
This specification registers the media type application/jrd+json for
use with WebFinger in accordance with media type registration
procedures defined in RFC 6838 [10].
Type name: application
Subtype name: jrd+json
Required parameters: N/A
Optional parameters: N/A
In particular, because RFC 4627 already defines the character
encoding for JSON, no "charset" parameter is used.
Encoding considerations: See RFC 6839, Section 3.1.
Security considerations:
The JSON Resource Descriptor (JRD) is a JavaScript Object Notation
(JSON) object. It is a text format that must be parsed by entities
that wish to utilize the format. Depending on the language and
mechanism used to parse a JSON object, it is possible for an
attacker to inject behavior into a running program. Therefore,
care must be taken to properly parse a received JRD to ensure that
only a valid JSON object is present and that no JavaScript or other
code is injected or executed unexpectedly.
Interoperability considerations:
This media type is a JavaScript Object Notation (JSON) object and
can be consumed by any software application that can consume JSON
objects.
Published specification: RFC 7033
Applications that use this media type:
The JSON Resource Descriptor (JRD) is used by the WebFinger
protocol (RFC 7033) to enable the exchange of information between a
client and a WebFinger resource over HTTPS.
Fragment identifier considerations:
The syntax and semantics of fragment identifiers SHOULD be as
specified for "application/json". (At publication of this
document, there is no fragment identification syntax defined for
"application/json".)
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): jrd
Macintosh file type code(s): N/A
Person & email address to contact for further information:
Paul E. Jones <paulej@packetizer.com>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Paul E. Jones <paulej@packetizer.com>
Change controller:
IESG has change control over this registration.
Provisional registration? (standards tree only): N/A
10.3. Registering Link Relation Types
RFC 5988 established a "Link Relation Types" registry that is reused
by WebFinger applications.
Link relation types used by WebFinger applications are registered in
the "Link Relation Types" registry as per the procedures of Section
6.2.1 of RFC 5988. The "Notes" entry for the registration SHOULD
indicate if property values associated with the link relation type
are registered in the "WebFinger Properties" registry with a link to
the registry.
10.4. Establishment of the "WebFinger Properties" Registry
WebFinger utilizes URIs to identify properties of a subject or link
and the associated values (see Sections 8.3 and 8.6). This
specification establishes a new "WebFinger Properties" registry to
record property identifiers.
10.4.1. The Registration Template
The registration template for WebFinger properties is:
o Property Identifier:
o Link Type:
o Description:
o Reference:
o Notes: [optional]
The "Property Identifier" must be a URI that identifies the property
being registered.
The "Link Type" contains the name of a link relation type with which
this property identifier is used. If the property is a subject-
specific property, then this field is specified as "N/A".
The "Description" is intended to explain the purpose of the property.
The "Reference" field points to the specification that defines the
registered property.
The optional "Notes" field is for conveying any useful information
about the property that might be of value to implementers.
10.4.2. The Registration Procedures
The IETF has created a mailing list, webfinger@ietf.org, which can be
used for public discussion of the WebFinger protocol and any
applications that use it. Prior to registration of a WebFinger
property, discussion on the mailing list is strongly encouraged. The
IESG has appointed Designated Experts [13] who will monitor the
webfinger@ietf.org mailing list and review registrations.
A WebFinger property is registered with a Specification Required (see
RFC 5226 [13]) after a review by the Designated Experts. The review
is normally expected to take on the order of two to four weeks.
However, the Designated Experts may approve a registration prior to
publication of a specification once the Designated Experts are
satisfied that such a specification will be published. In evaluating
registration requests, the Designated Experts should make an effort
to avoid registering two different properties that have the same
meaning. Where a proposed property is similar to an already-defined
property, the Designated Experts should insist that enough text be
included in the description or notes section of the template to
sufficiently differentiate the new property from an existing one.
The registration procedure begins with a completed registration
template (as defined above) sent to webfinger@ietf.org. Once
consensus is reached on the mailing list, the registration template
is sent to iana@iana.org. IANA will then contact the Designated
Experts and communicate the results to the registrant. The WebFinger
mailing list provides an opportunity for community discussion and
input, and the Designated Experts may use that input to inform their
review. Denials should include an explanation and, if applicable,
suggestions as to how to make the request successful if resubmitted.
The specification registering the WebFinger property MUST include the
completed registration template shown above. Once the registration
procedure concludes successfully, IANA creates or modifies the
corresponding record in the "WebFinger Properties" registry.
11. Acknowledgments
This document has benefited from extensive discussion and review by
many of the members of the APPSAWG working group. The authors would
like to especially acknowledge the invaluable input of Eran Hammer-
Lahav, Blaine Cook, Brad Fitzpatrick, Laurent-Walter Goix, Joe
Clarke, Peter Saint-Andre, Dick Hardt, Tim Bray, James Snell, Melvin
Carvalho, Evan Prodromou, Mark Nottingham, Elf Pavlik, Bjoern
Hoehrmann, Subramanian Moonesamy, Joe Gregorio, John Bradley, and
others that we have undoubtedly, but inadvertently, missed.
The authors would also like to express their gratitude to the chairs
of the APPSAWG working group, especially Salvatore Loreto for his
assistance in shepherding this document. We also want to thank Barry
Leiba and Pete Resnick, the Applications Area Directors, for their
support and exhaustive reviews.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol
-- HTTP/1.1", RFC 2616, June 1999.
[3] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April 2010.
[4] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[5] Crockford, D., "The application/json Media Type for JavaScript
Object Notation (JSON)", RFC 4627, July 2006.
[6] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[7] Van Kesteren, A., "Cross-Origin Resource Sharing", W3C CORS,
July 2010, <http://www.w3.org/TR/cors/>.
[8] IANA, "Link Relations",
<http://www.iana.org/assignments/link-relations/>.
[9] IANA, "MIME Media Types",
<http://www.iana.org/assignments/media-types>.
[10] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC 6838,
January 2013.
[11] Phillips, A., Ed., and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[12] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
12.2. Informative References
[14] Perreault, S., "vCard Format Specification", RFC 6350, August
2011.
[15] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B.,
Mortimore, C., and E. Jay, "OpenID Connect Messages 1.0",
July 2013,
<http://openid.net/specs/openid-connect-messages-1_0.html>.
[16] Hammer-Lahav, E., Ed., and B. Cook, "Web Host Metadata", RFC
6415, October 2011.
[17] Hammer-Lahav, E. and W. Norris, "Extensible Resource Descriptor
(XRD) Version 1.0",
<http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html>.
[18] Saint-Andre, P., "The 'acct' URI Scheme", Work in Progress,
July 2013.
[19] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' URI
Scheme", RFC 6068, October 2010.
[20] Balduzzi, M., Platzer, C., Thorsten, H., Kirda, E., Balzarotti,
D., and C. Kruegel "Abusing Social Networks for Automated User
Profiling", Recent Advances in Intrusion Detection, Springer
Berlin Heidelberg, March 2010,
<https://www.eurecom.fr/en/publication/3042/download/
rs-publi-3042_1.pdf>.
Authors' Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 476 2048
EMail: paulej@packetizer.com
IM: xmpp:paulej@packetizer.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 392 3266
EMail: gsalguei@cisco.com
IM: xmpp:gsalguei@cisco.com
Michael B. Jones
Microsoft
EMail: mbj@microsoft.com
URI: http://self-issued.info/
Joseph Smarr
Google
EMail: jsmarr@google.com
URI: http://josephsmarr.com/
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