OAuth 2.0 Device Authorization GrantGoogle1600 Amphitheatre PkwyMountain ViewCA94043USAwdenniss@google.comhttp://wdenniss.com/device-flowPing Identityve7jtb@ve7jtb.comhttp://www.thread-safe.com/Microsoftmbj@microsoft.comhttp://self-issued.info/ARM LimitedAustriaHannes.Tschofenig@gmx.nethttp://www.tschofenig.priv.at
Security Area
OAuthOAuthSecurityAuthorizationSmart ObjectsIoTInternet of ThingsInternet of Things SecurityOAuth for Constrained DevicesOAuth IoT Security
The OAuth 2.0 Device Authorization Grant is designed for internet-connected devices
that either lack a browser to perform a user-agent based authorization,
or are input-constrained to the extent that requiring the user to
input text in order to authenticate during the authorization flow
is impractical.
It enables OAuth clients on such devices (like smart TVs,
media consoles, digital picture frames, and
printers) to obtain user authorization to access protected
resources without using an on-device user-agent.
This OAuth 2.0 protocol extension,
sometimes referred to as "device flow", enables OAuth clients to request user
authorization from applications on devices that have limited
input capabilities or lack a suitable browser. Such devices include
those smart TVs, media console, picture frames and printers which
lack an easy input method or suitable browser required for
traditional OAuth interactions. The authorization flow defined by this specification instructs the
user to review the authorization request on a secondary device, such as
a smartphone which does have the requisite input and browser capabilities
to complete the user interaction.
The Device Authorization Grant is not intended to replace browser-based OAuth in native
apps on capable devices like smartphones. Those apps should follow
the practices specified in
OAuth 2.0 for Native Apps.
The operating requirements to be able to use this authorization
grant type are:
The device is already connected to the Internet.
The device is able to make outbound HTTPS requests.
The device is able to display or otherwise communicate a URI
and code sequence to the user.
The user has a secondary device (e.g., personal computer or smartphone)
from which they can process the request.
As the device authorization grant does not require two-way communication
between the OAuth client and the user-agent (unlike other
OAuth 2 grant types such as the Authorization Code and Implicit grant types),
it supports several use cases that cannot be
served by those other approaches.
Instead of interacting with the end user's user agent, the client
instructs the end user to use another computer or device and connect
to the authorization server to approve the access request. Since the
protocol supports clients that can't receive incoming requests,
clients poll the authorization
server repeatedly until the end user completes the approval process.
The device typically chooses the set of authorization servers to
support (i.e., its own authorization server, or those by providers
it has relationships with). It is not uncommon for the device application to support
only a single authorization server, such as with a TV application for
a specific media provider that supports only that media provider's
authorization server. The user may not have an established relationship yet
with that authorization provider, though one can potentially be
set up during the authorization flow.
The device authorization flow illustrated in includes the following steps:
(A) The client requests access from the authorization server and
includes its client identifier in the request.(B) The authorization server issues a device code, an end-user
code, and provides the end-user verification URI.(C) The client instructs the end user to use its user agent
(on another device) and visit the provided end-user verification URI.
The client provides the user with the end-user code to enter
in order to review the authorization request.(D) The authorization server authenticates the end user (via the
user agent) and prompts the user to grant the client's
access request. If the user agrees to the client's access
request, the user enters the user code provided by the
client. The authorization server validates the user code
provided by the user.(E) While the end user reviews the client's request
(step D), the client repeatedly polls the authorization server to
find out if the user completed the user authorization
step. The client includes the verification code and its client
identifier.(F) The authorization server
validates the verification code provided by the client and
responds back with the access token if the user granted access,
an error if they denied access, or indicates that the client should continue to poll.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
The authorization server's endpoint capable of issuing device
verification codes, user codes, and verification URLs.
A short-lived token representing an authorization session.
A short-lived token which the device displays to the end user,
is entered by the user on the authorization server, and is
thus used to bind the device to the user.
This specification defines a new OAuth endpoint, the device
authorization endpoint. This is separate from the OAuth authorization
endpoint defined in with which the user interacts
with via a user-agent (i.e., a browser). By comparison, when using the
device authorization endpoint, the OAuth client on the device interacts
with the authorization server directly without presenting the request
in a user-agent, and the end user authorizes the request on a separate
device. This interaction is defined as follows.The client initiates the authorization flow by requesting a set of verification
codes from the authorization server by making an HTTP "POST" request
to the device authorization endpoint.
The client constructs the request with the following parameters,
sent as the body of the request, encoded with the
application/x-www-form-urlencoded
encoding algorithm defined by Section 4.10.22.6 of :
REQUIRED, if the client is not authenticating with the
authorization server as described in Section 3.2.1. of
. The client identifier as
described in Section 2.2 of .
OPTIONAL. The scope of the access request as described by
Section 3.3 of .
For example, the client makes the following HTTPS request:
All requests from the device MUST use the Transport Layer Security (TLS)
protocol and
implement the best practices of BCP 195 .
Parameters sent without a value MUST be treated as if they were
omitted from the request. The authorization server MUST ignore
unrecognized request parameters. Request and response parameters
MUST NOT be included more than once.
The client authentication requirements of Section 3.2.1 of
apply to requests on this endpoint, which means that confidential
clients (those that have established client credentials) authenticate
in the same manner as when making requests to the token endpoint, and
public clients provide the "client_id" parameter to identify themselves.
Due to the polling nature of this protocol (as specified in ),
care is needed to avoid
overloading the capacity of the token endpoint. To avoid unneeded requests
on the token endpoint, the client SHOULD only commence a device
authorization request when prompted by the user, and not automatically,
such as when the app starts or when the previous authorization session
expires or fails.
In response, the authorization server generates a unique device verification code
and an end-user code that are valid for a limited time and includes them
in the HTTP response body using the "application/json" format
with a 200 (OK)
status code. The response contains the following parameters:
REQUIRED. The device verification code.
REQUIRED. The end-user verification code.
REQUIRED. The end-user verification URI on the authorization
server. The URI should be short and easy to remember as end users
will be asked to manually type it into their user-agent.
OPTIONAL. A verification URI that includes the
user_code (or other information with the same
function as the user_code), designed for
non-textual transmission.
REQUIRED. The lifetime in seconds of the
device_code and
user_code.
OPTIONAL. The minimum amount of time in seconds that the
client SHOULD wait between polling requests to the token
endpoint. If no value is provided, clients MUST use 5 as the default.
For example:
In the event of an error (such as an invalidly configured client),
the authorization server responds in the same way as the token endpoint
specified in Section 5.2 of .
After receiving a successful Authorization Response, the client displays or
otherwise communicates the user_code and the
verification_uri to the end user and instructs
them to visit the URI in a user agent on a secondary device
(for example, in a browser on their mobile phone), and enter the user code.
The authorizing user navigates to the verification_uri
and authenticates with the authorization server in a secure TLS-protected
() session.
The authorization server prompts the end user to identify the device authorization session by
entering the user_code provided by the client.
The authorization server should then inform the user about the action they
are undertaking and ask them to approve or deny the request. Once the user
interaction is complete, the server MAY inform the user to return to their
device.
During the user interaction, the device continuously polls the token
endpoint with the device_code, as detailed in
, until the user completes the interaction,
the code expires, or another error occurs. The
device_code is not intended for
the end user directly, and thus should not be displayed during
the interaction to avoid confusing the end user.
Authorization servers supporting this specification MUST implement a user
interaction sequence that starts with the user navigating to
verification_uri and continues with them
supplying the user_code at some stage during
the interaction. Other than that, the exact sequence and implementation of
the user interaction is up to the authorization server, for example,
the authorization server may enable new users to sign up for an account
during the authorization flow, or add additional security verification steps.
It is NOT RECOMMENDED for authorization servers to include the user code in
the verification URI (verification_uri),
as this increases the length and complexity of the URI that the user must type.
While the user must still type the same number of characters with
the user_code separated, once they successfully navigate to the
verification_uri, any errors in entering
the code can be highlighted by the authorization server to improve the
user experience.
The next section documents user interaction with verification_uri_complete,
which is designed to carry both pieces of information.
When verification_uri_complete is included
in the Authorization Response (),
clients MAY present this URI in a non-textual manner using
any method that results in the browser being opened with the URI, such as with
QR (Quick Response) codes or NFC (Near Field Communication), to save the user typing the URI.
For usability reasons,
it is RECOMMENDED for clients to still display the
textual verification URI (verification_uri) for
users not able to use such a shortcut. Clients MUST
still display the user_code, as the authorization
server will require the user to confirm it to disambiguate devices,
or as a remote phishing mitigation (See ).
If the user starts the user interaction by browsing to verification_uri_complete,
then the user interaction described in
is still followed, but with the optimization that the user does not need to type the user_code.
The server SHOULD display the user_code to the user
and ask them to verify that it matches the user_code being
displayed on the device, to confirm they are authorizing the correct device.
As before, in addition to taking steps to confirm the identity of the device,
the user should also be afforded the choice to approve or deny the authorization request.
After displaying instructions to the user, the client makes an Access Token
Request to the token endpoint (as defined by Section 3.2 of )
with a grant_type
of urn:ietf:params:oauth:grant-type:device_code.
This is an extension grant type (as defined by Section 4.5 of
) created by this specification, with the following parameters:
REQUIRED. Value MUST be set to
urn:ietf:params:oauth:grant-type:device_code.
REQUIRED. The device verification code, device_code from the
Device Authorization Response, defined in .
REQUIRED, if the client is not authenticating with the
authorization server as described in Section 3.2.1. of
. The client identifier as
described in Section 2.2 of .For example, the client makes the following HTTPS request (line
breaks are for display purposes only):
If the client was issued client
credentials (or assigned other authentication requirements), the
client MUST authenticate with the authorization server as described
in Section 3.2.1 of . Note that there are security
implications of statically distributed client credentials, see
.
The response to this request is defined in .
Unlike other OAuth grant types, it is expected for the client to try the
Access Token Request repeatedly in a polling fashion, based on the error
code in the response.
If the user has approved the grant, the token endpoint responds with
a success response defined in Section 5.1 of ;
otherwise it responds with an error, as defined in Section 5.2 of
.
In addition to the error codes defined in Section 5.2 of
, the following error codes are specified
for use with the device authorization grant in token endpoint responses:
The authorization request is still pending as the end user hasn't
yet completed the user interaction steps ().
The client SHOULD repeat the Access Token Request to the token
endpoint (a process known as polling). Before each new request the
client MUST wait at least the number of seconds specified by the
interval parameter of the Device
Authorization Response (see ),
or 5 seconds if none was provided, and respect any increase in the
polling interval required by the
slow_down error.
A variant of authorization_pending,
the authorization request is still pending and polling should
continue, but the interval MUST be increased by 5 seconds
for this and all subsequent requests.
The end user denied the authorization request.
The device_code has expired and
the device authorization session has concluded.
The client MAY commence a new Device Authorization Request
but SHOULD wait for user interaction before restarting
to avoid unnecessary polling.
The authorization_pending and
slow_down error codes define particularly unique
behavior, as they indicate that the OAuth client should
continue to poll the token endpoint by repeating the token request
(implementing the precise behavior defined above).
If the client receives an error response with any other error code,
it MUST stop polling and SHOULD react accordingly,
for example, by displaying an error to the user.
On encountering a connection timeout, clients MUST unilaterally reduce
their polling frequency before retrying. The use of an exponential
backoff algorithm to achieve this, such as by doubling the polling
interval on each such connection timeout, is RECOMMENDED.
The assumption of this specification is that the separate device the
user is authorizing the request on does not have a way to communicate back
to device with the OAuth client. This protocol only requires a one-way
channel in order to maximise the viability of the protocol in restricted
environments, like an application running on a TV that is only capable of
outbound requests. If a return channel were to exist for the chosen user
interaction interface, then the device MAY wait until notified on that channel
that the user has completed the action before initiating the token request
(as an alternative to polling).
Such behavior is, however, outside the scope of this specification.
Support for this specification MAY be declared in the OAuth 2.0
Authorization Server Metadata by
including the value urn:ietf:params:oauth:grant-type:device_code in the
grant_types_supported parameter, and by adding the following
new parameter:
OPTIONAL.
URL of the authorization server's device authorization endpoint
defined in .
Since the user code is typed by the user, shorter codes are more
desirable for usability reasons. This means the entropy is typically
less than would be used for the device code or other OAuth bearer
token types where the code length does not impact usability.
It is therefore recommended that the server rate-limit
user code attempts.
The user code SHOULD have enough entropy that when
combined with rate limiting and other mitigations makes
a brute-force attack infeasible. For example, it's generally held that
128-bit symmetric keys for encryption are seen as good enough today
because an attacker has to put in 2^96 work to have a 2^-32 chance
of guessing correctly via brute force. The rate limiting and finite
lifetime on the user code places an artificial limit on the amount
of work an attacker can "do", so if, for instance, one uses a 8-character base-20
user code (with roughly 34.5 bits of entropy), the rate-limiting
interval and validity period would need to only allow 5
attempts in order to get the same 2^-32 probability of success
by random guessing.
A successful brute forcing of the user code would enable the attacker
to authenticate with their own credentials and make an authorization
grant to the device. This is the opposite scenario to an OAuth bearer
token being brute forced, whereby the attacker gains control of the
victim's authorization grant. Such attacks may not always make
economic sense, for example for a video app the device owner may
then be able to purchase movies using the attacker's account, though
a privacy risk would still remain and thus is important to protect
against. Furthermore, some uses of the device flow give
the granting account the ability to perform actions such as controlling
the device, which needs to be protected.
The precise length of the user code and the entropy contained within
is at the discretion of the authorization server, which needs to
consider the sensitivity of their specific protected resources,
the practicality of the code length from a usability standpoint, and
any mitigations that are in place such as rate-limiting, when
determining the user code format.
An attacker who guesses the device code would be able to potentially
obtain the authorization code once the user completes the flow.
As the device code is not displayed to the user and thus there
are no usability considerations on the length, a very high entropy
code SHOULD be used.
Unlike other native application OAuth 2.0 flows, the device requesting the
authorization is not the same as the device that the user grants access
from. Thus, signals from the approving user's session and device are
not relevant to the trustworthiness of the client device.
Note that if an authorization server used with this flow is malicious,
then it could man-in-the-middle the backchannel flow to another authorization server.
In this scenario, the man-in-the-middle is not completely hidden from sight,
as the end user would end up on the authorization page of the wrong service,
giving them an opportunity to notice that the URL in the browser's address bar is wrong.
For this to be possible, the device manufacturer must either directly be the attacker,
shipping a device intended to perform the man-in-the-middle attack,
or be using an authorization server that is controlled by an attacker,
possibly because the attacker compromised the authorization server used by the device.
In part,
the person purchasing the device is counting on it and its business partners to be trustworthy.
It is possible for the device flow to be initiated on a device in
an attacker's possession. For example, an attacker might send an email
instructing the target user to visit the verification URL and
enter the user code. To mitigate such an attack, it is RECOMMENDED to
inform the user that they are authorizing a device during the user
interaction step (see ), and to
confirm that the device is in their possession.
The authorization server SHOULD display information about the device
so that the person can notice if a software client
was attempting to impersonating a hardware device.
For authorization servers that support the option specified in
for the client to
append the user code to the authorization URI, it is particularly
important to confirm that the device is in the user's possession,
as the user no longer has to type the code manually.
One possibility is to display the code during the authorization flow
and asking the user to verify that the same code is being displayed on
the device they are setting up.
The user code needs to have a long enough lifetime to be useable (allowing
the user to retrieve their secondary device, navigate to the verification URI, login, etc.),
but should be sufficiently short to limit the
usability of a code obtained for phishing. This doesn't prevent a
phisher presenting a fresh token, particularly in the case they
are interacting with the user in real time,
but it does limit the viability of codes sent over email or SMS.
While the device is pending authorization, it may be possible for a
malicious user to physically spy on the device user interface
(by viewing the screen on which it's displayed, for example) and hijack the
session by completing the authorization faster than the user that
initiated it. Devices SHOULD take into account the operating
environment when considering how to communicate the code to the user
to reduce the chances it will be observed by a malicious user.
Device clients are generally incapable of maintaining the confidentiality
of their credentials, as users in possession of the device can reverse
engineer it and extract the credentials. Therefore, unless additional
measures are taken, they should be treated as public clients
(as defined by Section 2.1 of OAuth 2.0)
susceptible to impersonation. The security considerations of
Section 5.3.1 of and
Sections 8.5 and 8.6 of apply to such clients.
The user may also be able to obtain the device_code and/or other OAuth
bearer tokens issued to their client, which would allow them to use their
own authorization grant directly by impersonating the client. Given
that the user in possession of the client credentials can already
impersonate the client and create a new authorization grant (with
a new device_code), this doesn't represent a separate impersonation
vector.
There is no requirement that the user code be displayed by the
device visually. Other methods of one-way communication can potentially be
used, such as text-to-speech audio, or Bluetooth Low Energy. To mitigate an attack
in which a malicious user can bootstrap their credentials on a device
not in their control, it is RECOMMENDED that any chosen communication
channel only be accessible by people in close proximity. E.g., users
who can see, or hear the device.
This section is a non-normative discussion of usability considerations.
For many users, their nearest Internet-connected device will be their
mobile phone, and typically these devices offer input methods that are
more time consuming than a computer keyboard to change the case or
input numbers. To improve usability (improving entry speed, and
reducing retries), these limitations should be taken into account
when selecting the user-code character set.
One way to improve input speed is to restrict the character set to
case-insensitive A-Z characters, with no digits. These characters can
typically be entered on a mobile keyboard without using modifier keys.
Further removing vowels to avoid randomly creating words
results in the base-20 character set:
BCDFGHJKLMNPQRSTVWXZ. Dashes or other
punctuation may be included for readability.
An example user code following this guideline containing 8 significant
characters and dashes added for end-user readability, with a
resulting entropy of 20^8:
WDJB-MJHT.
Pure numeric codes are also a good choice for usability, especially
for clients targeting locales where A-Z character keyboards are not
used, though their length needs to be longer to maintain a high
entropy.
An example numeric user code
containing 9 significant digits and dashes added for end-user readability,
with an entropy of
10^9: 019-450-730.
When processing the inputted user code, the server should strip dashes and
other punctuation it added for readability (making the inclusion
of that punctuation by the user optional).
For codes using only characters in the A-Z range as
with the base-20 charset defined above, the user's input
should be upper-cased before comparison to account for the fact
that the user may input the equivalent lower-case characters.
Further stripping of all characters outside the user_code charset
is recommended to reduce instances where an errantly typed
character (like a space character) invalidates otherwise valid
input.
It is RECOMMENDED to avoid character sets that contain two
or more characters that can easily be confused with each other
like "0" and "O", or "1", "l" and "I".
Furthermore, the extent practical, where a character set contains one
character that may be confused with characters outside the character set
the character outside the set MAY be substituted with the one
in the character set that it is commonly confused with
(for example, "O" for "0" when using a numerical 0-9 character set).
Devices and authorization servers MAY negotiate an alternative code
transmission and user interaction method in addition to the one described
in . Such an alternative user interaction flow could obviate the need for a
browser and manual input of the code, for example, by using Bluetooth to
transmit the code to the authorization server's companion app. Such
interaction methods can utilize this protocol, as ultimately, the user just
needs to identify the authorization session to the authorization server;
however, user interaction other than via the
verification URI is outside the scope of this
specification.
This specification registers the following values in the
IANA "OAuth Parameters" registry
established by .
Parameter name: device_codeParameter usage location: token requestChange controller: IESGSpecification Document: of [[ this specification ]]
This specification registers the following values in the
IANA "OAuth URI" registry
established by .
URN: urn:ietf:params:oauth:grant-type:device_codeCommon Name: Device flow grant type for OAuth 2.0Change controller: IESGSpecification Document: of [[ this specification ]]
This specification registers the following values in the
IANA "OAuth Extensions Error Registry" registry
established by .
Error name: authorization_pendingError usage location: Token endpoint responseRelated protocol extension: [[ this specification ]]Change controller: IETFSpecification Document: of [[ this specification ]]Error name: access_deniedError usage location: Token endpoint responseRelated protocol extension: [[ this specification ]]Change controller: IETFSpecification Document: of [[ this specification ]]Error name: slow_downError usage location: Token endpoint response Related protocol extension: [[ this specification ]]Change controller: IETFSpecification Document: of [[ this specification ]]Error name: expired_tokenError usage location: Token endpoint response Related protocol extension: [[ this specification ]]Change controller: IETFSpecification Document: of [[ this specification ]]
This specification registers the following values in the
IANA "OAuth 2.0 Authorization Server Metadata" registry
established by .
Metadata name: device_authorization_endpointMetadata Description: The Device Authorization Endpoint.Change controller: IESGSpecification Document: of [[ this specification ]]OAuth ParametersIANAHTML5IANA
The starting point for this document was the Internet-Draft
draft-recordon-oauth-v2-device, authored by David Recordon and Brent Goldman,
which itself was based on content in draft versions of the OAuth 2.0 protocol specification
removed prior to publication due to a then lack of sufficient deployment expertise.
Thank you to the OAuth working group members who contributed to those earlier drafts.
This document was produced in the OAuth working group under the
chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
Benjamin Kaduk, Kathleen Moriarty, and Eric Rescorla serving as Security
Area Directors.
The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:
Alissa Cooper,
Ben Campbell,
Brian Campbell,
Roshni Chandrashekhar,
Eric Fazendin,
Benjamin Kaduk,
Jamshid Khosravian,
Torsten Lodderstedt,
James Manger,
Dan McNulty,
Breno de Medeiros,
Simon Moffatt,
Stein Myrseth,
Emond Papegaaij,
Justin Richer,
Adam Roach,
Nat Sakimura,
Andrew Sciberras,
Marius Scurtescu,
Filip Skokan,
Ken Wang, and
Steven E. Wright.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-15
Renamed and dropped most usage of the term "flow"
Documented error responses on the authorization endpoint
Documented client authentication for the authorization endpoint
-14
Added more normative text on polling behavior.
Added discussion on risk of user retrieving their own device_code.
Editorial improvements.
-13
Added a longer discussion about entropy, proposed by Benjamin Kaduk.
Added device_code to OAuth IANA registry.
Expanded explanation of "case insensitive".
Added security section on Device Code Brute Forcing.
application/x-www-form-urlencoded normativly referenced.
Editorial improvements.
-12
Set a default polling interval to 5s explicitly.
Defined the slow_down behavior that it should
increase the current interval by 5s.
expires_in now REQUIRED
Other changes in response to review feedback.
-11
Updated reference to OAuth 2.0 Authorization Server Metadata.
-10
Added a missing definition of access_denied for use on the token endpoint.
Corrected text documenting which error code should be returned for expired tokens
(it's "expired_token", not "invalid_grant").
Corrected section reference to RFC 8252 (the section numbers had changed after the initial reference was made).
Fixed line length of one diagram (was causing xml2rfc warnings).
Added line breaks so the URN grant_type is presented on an unbroken line.
Typos fixed and other stylistic improvements.
-09
Addressed review comments by Security Area Director Eric Rescorla
about the potential of a confused deputy attack.
-08
Expanded the User Code Brute Forcing section to include more detail on this attack.
-07
Replaced the "user_code" URI parameter optimization with
verification_uri_complete following
the IETF99 working group discussion.
Added security consideration about spying.
Required that device_code not be shown.
Added text regarding a minimum polling interval.
-06
Clarified usage of the "user_code" URI parameter optimization following
the IETF98 working group discussion.
-05
response_type parameter removed from authorization request.
Added option for clients to include the user_code on the
verification URI.
Clarified token expiry, and other nits.
-04
Security & Usability sections. OAuth Discovery Metadata.
-03
device_code is now a URN.
Added IANA Considerations
-02
Added token request & response specification.
-01
Applied spelling and grammar corrections and added the Document History appendix.
-00
Initial working group draft based on draft-recordon-oauth-v2-device.