OAuth 2.0 Device Flow for Browserless and Input Constrained DevicesGoogle1600 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
This OAuth 2.0 authorization flow for browserless and input constrained
devices, often referred to as the device flow, enables OAuth clients to
request user authorization from devices that have an Internet
connection, but don't have an easy input method (such as a smart TV,
media console, picture frame, or printer), or lack a suitable browser
for a more traditional OAuth flow. This authorization flow instructs the
user to perform the authorization request on a secondary device, such as
a smartphone. There is no requirement for communication between the
constrained device and the user's secondary device.
This OAuth 2.0 protocol flow for browserless and input constrained
devices, often referred to as the device flow, enables OAuth clients to
request user authorization from devices that have an internet
connection, but don't have an easy input method (such as a smart TV,
media console, picture frame, or printer), or lack a suitable browser
for a more traditional OAuth flow. This authorization flow instructs the
user to perform the authorization request on a secondary device, such as
a smartphone.
The device flow 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
OAuth 2.0 for Native Apps.
The only requirements to use this flow are that the device is connected
to the Internet, and able to make outbound HTTPS requests, be able to
display or otherwise communicate a URI and code sequence to the user,
and that the user has a secondary device (e.g., personal computer or
smartphone) from which to process the request. There is no requirement
for two-way communication between the OAuth client and the user-agent,
enabling a broad range of use-cases.
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
client cannot receive incoming requests, it polls the authorization
server repeatedly until the end-user completes the approval process.
The device 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 verification code, an end-user
code, and provides the end-user verification URI.(C) The client instructs the end-user to use its user-agent
(elsewhere) and visit the provided end-user verification URI.
The client provides the end-user with the end-user code to enter
in order to grant access.(D) The authorization server authenticates the end-user (via the
user-agent) and prompts the end-user to grant the client's
access request. If the end-user agrees to the client's access
request, the end-user enters the end-user code provided by the
client. The authorization server validates the end-user code
provided by the end-user.(E) While the end-user authorizes (or denies) the client's request
(step D), the client repeatedly polls the authorization server to
find out if the end-user completed the end-user authorization
step. The client includes the verification code and its client
identifier.(F) Assuming the end-user granted access, the authorization server
validates the verification code provided by the client and
responds back with the access token.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 .
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 end-user on the authorization server, and is
thus used to bind the device to the end-user.
The client initiates the 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, encoded with the
application/x-www-form-urlencoded content type:
REQUIRED. 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 (line
breaks are for display purposes only):
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.
In response, the authorization server generates a 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.
OPTIONAL. 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.
For example:
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 informs 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
and MUST NOT be displayed or communicated.
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 and is out of scope
of this specification.
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.
The next section documents user interaction with verification_uri_complete,
which is designed to carry this 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 codes or NFC, 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 may still require the user to confirm it to disambiguate devices,
or as a remote phishing mitigation (See ).
After displaying instructions to the user, the client makes an Access Token
Request to the token endpoint 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
) 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
.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 by the
device flow for use 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.
The end-user denied the authorization request.
The client is polling too quickly and should back off at a
reasonable rate.
The device_code has expired. The client will need to make a new
Device Authorization Request.The error codes authorization_pending and
slow_down are considered soft errors. The client
should continue to poll the token endpoint by repeating the Device
Token Request () when receiving soft errors,
increasing the time between polls if a slow_down error
is received. Other error codes are considered hard errors; the client should
stop polling and react accordingly, for example, by displaying an error to
the user.
If the verification codes have expired, the server SHOULD respond with the
error code expired_token. Clients
MAY then choose to start a new device authorization session.
The interval at which the client polls MUST NOT be more frequent than the
interval parameter returned in the Device Authorization
Response (see ).
If no interval was provided, the client MUST use a reasonable default
polling interval.
The assumption of this specification is that the secondary device the
user is authorizing the request on does not have a way to communicate back
to the OAuth client. Only a one-way channel is required to make this flow
useful in many scenarios. For example, an HTML application on a TV that can
only make 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.
Such behavior is, however, outside the scope of this specification.
Support for the device flow MAY be declared in the OAuth 2.0
Authorization Server Metadata
with the following metadata:
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.
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. In some applications this attack may not
make much economic sense, for example for a video app, the owner of
the device may then be able to purchase movies with the attacker's
account, however there are still privacy considerations in that case
as well as other uses of the device flow whereby the granting
account may be able to perform sensitive actions such as controlling
the victim's device.
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.
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 authorization being requested 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, the 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 spy on the device user interface 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.
Most device clients are incapable of being confidential clients, as secrets
that are statically included as part of an app distributed to
multiple users cannot be considered confidential.
For such clients, the recommendations of
Section 5.3.1 of and
Section 8.5 of apply.
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, or within range of a short-range
wireless signal.
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, with an 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, with an entropy of
10^9: 019-450-730.
The server should ignore any characters like punctuation that are
not in the user-code character set. Provided that the character set doesn't include
characters of different case, the comparison should be case insensitive.
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 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 ParametersIANA
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:
Brian Campbell,
Roshni Chandrashekhar,
Eric Fazendin,
Torsten Lodderstedt,
James Manger,
Breno de Medeiros,
Simon Moffatt,
Stein Myrseth,
Justin Richer,
Nat Sakimura,
Andrew Sciberras,
Marius Scurtescu,
Ken Wang, and
Steven E. Wright.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-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.