Twitter Authentication

Internet Engineering Task Force (IETF) E. Hammer-Lahav, Ed.
Request for Comments: 5849 April 2010
Category: Informational
ISSN: 2070-1721

The OAuth 1.0 Protocol

Abstract

OAuth provides a method for clients to access server resources on
behalf of a resource owner (such as a different client or an end-
user). It also provides a process for end-users to authorize third-
party access to their server resources without sharing their
credentials (typically, a username and password pair), using user-
agent redirections.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc5849.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust’s Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Hammer-Lahav Informational [Page 1]

RFC 5849 OAuth 1.0 April 2010

Table of Contents

1. Introduction ....................................................3
1.1. Terminology ................................................4
1.2. Example ....................................................5
1.3. Notational Conventions .....................................7
2. Redirection-Based Authorization .................................8
2.1. Temporary Credentials ......................................9
2.2. Resource Owner Authorization ..............................10
2.3. Token Credentials .........................................12
3. Authenticated Requests .........................................14
3.1. Making Requests ...........................................14
3.2. Verifying Requests ........................................16
3.3. Nonce and Timestamp .......................................17
3.4. Signature .................................................18
3.4.1. Signature Base String ..............................18
3.4.2. HMAC-SHA1 ..........................................25
3.4.3. RSA-SHA1 ...........................................25
3.4.4. PLAINTEXT ..........................................26
3.5. Parameter Transmission ....................................26
3.5.1. Authorization Header ...............................27
3.5.2. Form-Encoded Body ..................................28
3.5.3. Request URI Query ..................................28
3.6. Percent Encoding ..........................................29
4. Security Considerations ........................................29
4.1. RSA-SHA1 Signature Method .................................29
4.2. Confidentiality of Requests ...............................30
4.3. Spoofing by Counterfeit Servers ...........................30
4.4. Proxying and Caching of Authenticated Content .............30
4.5. Plaintext Storage of Credentials ..........................30
4.6. Secrecy of the Client Credentials .........................31
4.7. Phishing Attacks ..........................................31
4.8. Scoping of Access Requests ................................31
4.9. Entropy of Secrets ........................................32
4.10. Denial-of-Service / Resource-Exhaustion Attacks ..........32
4.11. SHA-1 Cryptographic Attacks ..............................33
4.12. Signature Base String Limitations ........................33
4.13. Cross-Site Request Forgery (CSRF) ........................33
4.14. User Interface Redress ...................................34
4.15. Automatic Processing of Repeat Authorizations ............34
5. Acknowledgments ................................................35
Appendix A. Differences from the Community Edition ...............36
6. References .....................................................37
6.1. Normative References ......................................37
6.2. Informative References ....................................38



1. Introduction

The OAuth protocol was originally created by a small community of web
developers from a variety of websites and other Internet services who
wanted to solve the common problem of enabling delegated access to
protected resources. The resulting OAuth protocol was stabilized at
version 1.0 in October 2007, and revised in June 2009 (Revision A) as
published at <http://oauth.net/core/1.0a>.

This specification provides an informational documentation of OAuth
Core 1.0 Revision A, addresses several errata reported since that
time, and makes numerous editorial clarifications. While this
specification is not an item of the IETF’s OAuth Working Group, which
at the time of writing is working on an OAuth version that can be
appropriate for publication on the standards track, it has been
transferred to the IETF for change control by authors of the original
work.

In the traditional client-server authentication model, the client
uses its credentials to access its resources hosted by the server.
With the increasing use of distributed web services and cloud
computing, third-party applications require access to these server-
hosted resources.

OAuth introduces a third role to the traditional client-server
authentication model: the resource owner. In the OAuth model, the
client (which is not the resource owner, but is acting on its behalf)
requests access to resources controlled by the resource owner, but
hosted by the server. In addition, OAuth allows the server to verify
not only the resource owner authorization, but also the identity of
the client making the request.

OAuth provides a method for clients to access server resources on
behalf of a resource owner (such as a different client or an end-
user). It also provides a process for end-users to authorize third-
party access to their server resources without sharing their
credentials (typically, a username and password pair), using user-
agent redirections.

For example, a web user (resource owner) can grant a printing service
(client) access to her private photos stored at a photo sharing
service (server), without sharing her username and password with the
printing service. Instead, she authenticates directly with the photo
sharing service which issues the printing service delegation-specific
credentials.



In order for the client to access resources, it first has to obtain
permission from the resource owner. This permission is expressed in
the form of a token and matching shared-secret. The purpose of the
token is to make it unnecessary for the resource owner to share its
credentials with the client. Unlike the resource owner credentials,
tokens can be issued with a restricted scope and limited lifetime,
and revoked independently.

This specification consists of two parts. The first part defines a
redirection-based user-agent process for end-users to authorize
client access to their resources, by authenticating directly with the
server and provisioning tokens to the client for use with the
authentication method. The second part defines a method for making
authenticated HTTP [RFC2616] requests using two sets of credentials,
one identifying the client making the request, and a second
identifying the resource owner on whose behalf the request is being
made.

The use of OAuth with any transport protocol other than [RFC2616] is
undefined.

1.1. Terminology

client
An HTTP client (per [RFC2616]) capable of making OAuth-
authenticated requests (Section 3).

server
An HTTP server (per [RFC2616]) capable of accepting OAuth-
authenticated requests (Section 3).

protected resource
An access-restricted resource that can be obtained from the
server using an OAuth-authenticated request (Section 3).

resource owner
An entity capable of accessing and controlling protected
resources by using credentials to authenticate with the server.

credentials
Credentials are a pair of a unique identifier and a matching
shared secret. OAuth defines three classes of credentials:
client, temporary, and token, used to identify and authenticate
the client making the request, the authorization request, and
the access grant, respectively.



token
A unique identifier issued by the server and used by the client
to associate authenticated requests with the resource owner
whose authorization is requested or has been obtained by the
client. Tokens have a matching shared-secret that is used by
the client to establish its ownership of the token, and its
authority to represent the resource owner.

The original community specification used a somewhat different
terminology that maps to this specifications as follows (original
community terms provided on left):

Consumer: client

Service Provider: server

User: resource owner

Consumer Key and Secret: client credentials

Request Token and Secret: temporary credentials

Access Token and Secret: token credentials

1.2. Example

Jane (resource owner) has recently uploaded some private vacation
photos (protected resources) to her photo sharing site
’photos.example.net’ (server). She would like to use the
’printer.example.com’ website (client) to print one of these photos.
Typically, Jane signs into ’photos.example.net’ using her username
and password.

However, Jane does not wish to share her username and password with
the ’printer.example.com’ website, which needs to access the photo in
order to print it. In order to provide its users with better
service, ’printer.example.com’ has signed up for a set of
’photos.example.net’ client credentials ahead of time:

Client Identifier
dpf43f3p2l4k3l03

Client Shared-Secret:
kd94hf93k423kf44

The ’printer.example.com’ website has also configured its application
to use the protocol endpoints listed in the ’photos.example.net’ API
documentation, which use the "HMAC-SHA1" signature method:



Temporary Credential Request
https://photos.example.net/initiate

Resource Owner Authorization URI:
https://photos.example.net/authorize

Token Request URI:
https://photos.example.net/token

Before ’printer.example.com’ can ask Jane to grant it access to the
photos, it must first establish a set of temporary credentials with
’photos.example.net’ to identify the delegation request. To do so,
the client sends the following HTTPS [RFC2818] request to the server:

POST /initiate HTTP/1.1
Host: photos.example.net
Authorization: OAuth realm="Photos",
oauth_consumer_key="dpf43f3p2l4k3l03",
oauth_signature_method="HMAC-SHA1",
oauth_timestamp="137131200",
oauth_nonce="wIjqoS",
oauth_callback="http%3A%2F%2Fprinter.example.com%2Fready",
oauth_signature="74KNZJeDHnMBp0EMJ9ZHt%2FXKycU%3D"

The server validates the request and replies with a set of temporary
credentials in the body of the HTTP response (line breaks are for
display purposes only):

HTTP/1.1 200 OK
Content-Type: application/x-www-form-urlencoded

oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03&
oauth_callback_confirmed=true

The client redirects Jane’s user-agent to the server’s Resource Owner
Authorization endpoint to obtain Jane’s approval for accessing her
private photos:

https://photos.example.net/authorize?oauth_token=hh5s93j4hdidpola

The server requests Jane to sign in using her username and password
and if successful, asks her to approve granting ’printer.example.com’
access to her private photos. Jane approves the request and her
user-agent is redirected to the callback URI provided by the client
in the previous request (line breaks are for display purposes only):

http://printer.example.com/ready?
oauth_token=hh5s93j4hdidpola&oauth_verifier=hfdp7dh39dks9884



The callback request informs the client that Jane completed the
authorization process. The client then requests a set of token
credentials using its temporary credentials (over a secure Transport
Layer Security (TLS) channel):

POST /token HTTP/1.1
oauth_token="hh5s93j4hdidpola",
oauth_nonce="walatlh",
oauth_verifier="hfdp7dh39dks9884",
oauth_signature="gKgrFCywp7rO0OXSjdot%2FIHF7IU%3D"

The server validates the request and replies with a set of token
credentials in the body of the HTTP response:

HTTP/1.1 200 OK

oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00

With a set of token credentials, the client is now ready to request
the private photo:

GET /photos?file=vacation.jpg&size=original HTTP/1.1
oauth_token="nnch734d00sl2jdk",
oauth_nonce="chapoH",
oauth_signature="MdpQcU8iPSUjWoN%2FUDMsK2sui9I%3D"

The ’photos.example.net’ server validates the request and responds
with the requested photo. ’printer.example.com’ is able to continue
accessing Jane’s private photos using the same set of token
credentials for the duration of Jane’s authorization, or until Jane
revokes access.

1.3. Notational Conventions

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 [RFC2119].



2. Redirection-Based Authorization

OAuth uses tokens to represent the authorization granted to the
client by the resource owner. Typically, token credentials are
issued by the server at the resource owner’s request, after
authenticating the resource owner’s identity (usually using a
username and password).

There are many ways in which a server can facilitate the provisioning
of token credentials. This section defines one such way, using HTTP
redirections and the resource owner’s user-agent. This redirection-
based authorization method includes three steps:

1. The client obtains a set of temporary credentials from the server
(in the form of an identifier and shared-secret). The temporary
credentials are used to identify the access request throughout
the authorization process.

2. The resource owner authorizes the server to grant the client’s
access request (identified by the temporary credentials).

3. The client uses the temporary credentials to request a set of
token credentials from the server, which will enable it to access
the resource owner’s protected resources.

The server MUST revoke the temporary credentials after being used
once to obtain the token credentials. It is RECOMMENDED that the
temporary credentials have a limited lifetime. Servers SHOULD enable
resource owners to revoke token credentials after they have been
issued to clients.

In order for the client to perform these steps, the server needs to
advertise the URIs of the following three endpoints:

Temporary Credential Request
The endpoint used by the client to obtain a set of temporary
credentials as described in Section 2.1.

Resource Owner Authorization
The endpoint to which the resource owner is redirected to grant
authorization as described in Section 2.2.

Token Request
The endpoint used by the client to request a set of token
credentials using the set of temporary credentials as described
in Section 2.3.



The three URIs advertised by the server MAY include a query component
as defined by [RFC3986], Section 3, but if present, the query MUST
NOT contain any parameters beginning with the "oauth_" prefix, to
avoid conflicts with the protocol parameters added to the URIs when
used.

The methods in which the server advertises and documents its three
endpoints are beyond the scope of this specification. Clients should
avoid making assumptions about the size of tokens and other server-
generated values, which are left undefined by this specification. In
addition, protocol parameters MAY include values that require
encoding when transmitted. Clients and servers should not make
assumptions about the possible range of their values.

2.1. Temporary Credentials

The client obtains a set of temporary credentials from the server by
making an authenticated (Section 3) HTTP "POST" request to the
Temporary Credential Request endpoint (unless the server advertises
another HTTP request method for the client to use). The client
constructs a request URI by adding the following REQUIRED parameter
to the request (in addition to the other protocol parameters, using
the same parameter transmission method):

oauth_callback: An absolute URI back to which the server will
redirect the resource owner when the Resource Owner
Authorization step (Section 2.2) is completed. If
the client is unable to receive callbacks or a
callback URI has been established via other means,
the parameter value MUST be set to "oob" (case
sensitive), to indicate an out-of-band
configuration.

Servers MAY specify additional parameters.

When making the request, the client authenticates using only the
client credentials. The client MAY omit the empty "oauth_token"
protocol parameter from the request and MUST use the empty string as
the token secret value.

Since the request results in the transmission of plain text
credentials in the HTTP response, the server MUST require the use of
a transport-layer mechanisms such as TLS or Secure Socket Layer (SSL)
(or a secure channel with equivalent protections).



For example, the client makes the following HTTPS request:

POST /request_temp_credentials HTTP/1.1
Host: server.example.com
Authorization: OAuth realm="Example",
oauth_consumer_key="jd83jd92dhsh93js",
oauth_signature_method="PLAINTEXT",
oauth_callback="http%3A%2F%2Fclient.example.net%2Fcb%3Fx%3D1",
oauth_signature="ja893SD9%26"

The server MUST verify (Section 3.2) the request and if valid,
respond back to the client with a set of temporary credentials (in
the form of an identifier and shared-secret). The temporary
credentials are included in the HTTP response body using the
"application/x-www-form-urlencoded" content type as defined by
[W3C.REC-html40-19980424] with a 200 status code (OK).

The response contains the following REQUIRED parameters:

oauth_token
The temporary credentials identifier.

oauth_token_secret
The temporary credentials shared-secret.

oauth_callback_confirmed
MUST be present and set to "true". The parameter is used to
differentiate from previous versions of the protocol.

Note that even though the parameter names include the term ’token’,
these credentials are not token credentials, but are used in the next
two steps in a similar manner to token credentials.

For example (line breaks are for display purposes only):

HTTP/1.1 200 OK

oauth_token=hdk48Djdsa&oauth_token_secret=xyz4992k83j47x0b&
oauth_callback_confirmed=true

2.2. Resource Owner Authorization

Before the client requests a set of token credentials from the
server, it MUST send the user to the server to authorize the request.
The client constructs a request URI by adding the following REQUIRED
query parameter to the Resource Owner Authorization endpoint URI:



oauth_token
The temporary credentials identifier obtained in Section 2.1 in
the "oauth_token" parameter. Servers MAY declare this
parameter as OPTIONAL, in which case they MUST provide a way
for the resource owner to indicate the identifier through other
means.

Servers MAY specify additional parameters.

The client directs the resource owner to the constructed URI using an
HTTP redirection response, or by other means available to it via the
resource owner’s user-agent. The request MUST use the HTTP "GET"
method.

For example, the client redirects the resource owner’s user-agent to
make the following HTTPS request:

GET /authorize_access?oauth_token=hdk48Djdsa HTTP/1.1

The way in which the server handles the authorization request,
including whether it uses a secure channel such as TLS/SSL is beyond
the scope of this specification. However, the server MUST first
verify the identity of the resource owner.

When asking the resource owner to authorize the requested access, the
server SHOULD present to the resource owner information about the
client requesting access based on the association of the temporary
credentials with the client identity. When displaying any such
information, the server SHOULD indicate if the information has been
verified.

After receiving an authorization decision from the resource owner,
the server redirects the resource owner to the callback URI if one
was provided in the "oauth_callback" parameter or by other means.

To make sure that the resource owner granting access is the same
resource owner returning back to the client to complete the process,
the server MUST generate a verification code: an unguessable value
passed to the client via the resource owner and REQUIRED to complete
the process. The server constructs the request URI by adding the
following REQUIRED parameters to the callback URI query component:

oauth_token
The temporary credentials identifier received from the client.



oauth_verifier
The verification code.

If the callback URI already includes a query component, the server
MUST append the OAuth parameters to the end of the existing query.

For example, the server redirects the resource owner’s user-agent to
make the following HTTP request:

GET /cb?x=1&oauth_token=hdk48Djdsa&oauth_verifier=473f82d3 HTTP/1.1
Host: client.example.net

If the client did not provide a callback URI, the server SHOULD
display the value of the verification code, and instruct the resource
owner to manually inform the client that authorization is completed.
If the server knows a client to be running on a limited device, it
SHOULD ensure that the verifier value is suitable for manual entry.

2.3. Token Credentials

The client obtains a set of token credentials from the server by
making an authenticated (Section 3) HTTP "POST" request to the Token
Request endpoint (unless the server advertises another HTTP request
method for the client to use). The client constructs a request URI
by adding the following REQUIRED parameter to the request (in
addition to the other protocol parameters, using the same parameter
transmission method):

oauth_verifier
The verification code received from the server in the previous
step.

When making the request, the client authenticates using the client
credentials as well as the temporary credentials. The temporary
credentials are used as a substitute for token credentials in the
authenticated request and transmitted using the "oauth_token"
parameter.

Since the request results in the transmission of plain text
credentials in the HTTP response, the server MUST require the use of
a transport-layer mechanism such as TLS or SSL (or a secure channel
with equivalent protections).



For example, the client makes the following HTTPS request:

POST /request_token HTTP/1.1
oauth_consumer_key="jd83jd92dhsh93js",
oauth_token="hdk48Djdsa",
oauth_signature_method="PLAINTEXT",
oauth_verifier="473f82d3",
oauth_signature="ja893SD9%26xyz4992k83j47x0b"

The server MUST verify (Section 3.2) the validity of the request,
ensure that the resource owner has authorized the provisioning of
token credentials to the client, and ensure that the temporary
credentials have not expired or been used before. The server MUST
also verify the verification code received from the client. If the
request is valid and authorized, the token credentials are included
in the HTTP response body using the
"application/x-www-form-urlencoded" content type as defined by
[W3C.REC-html40-19980424] with a 200 status code (OK).

The response contains the following REQUIRED parameters:

oauth_token
The token identifier.

oauth_token_secret
The token shared-secret.

For example:

HTTP/1.1 200 OK

oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk

The server must retain the scope, duration, and other attributes
approved by the resource owner, and enforce these restrictions when
receiving a client request made with the token credentials issued.

Once the client receives and stores the token credentials, it can
proceed to access protected resources on behalf of the resource owner
by making authenticated requests (Section 3) using the client
credentials together with the token credentials received.



3. Authenticated Requests

The HTTP authentication methods defined by [RFC2617] enable clients
to make authenticated HTTP requests. Clients using these methods
gain access to protected resources by using their credentials
(typically, a username and password pair), which allow the server to
verify their authenticity. Using these methods for delegation
requires the client to assume the role of the resource owner.

OAuth provides a method designed to include two sets of credentials
with each request, one to identify the client, and another to
identify the resource owner. Before a client can make authenticated
requests on behalf of the resource owner, it must obtain a token
authorized by the resource owner. Section 2 provides one such method
through which the client can obtain a token authorized by the
resource owner.

The client credentials take the form of a unique identifier and an
associated shared-secret or RSA key pair. Prior to making
authenticated requests, the client establishes a set of credentials
with the server. The process and requirements for provisioning these
are outside the scope of this specification. Implementers are urged
to consider the security ramifications of using client credentials,
some of which are described in Section 4.6.

Making authenticated requests requires prior knowledge of the
server’s configuration. OAuth includes multiple methods for
transmitting protocol parameters with requests (Section 3.5), as well
as multiple methods for the client to prove its rightful ownership of
the credentials used (Section 3.4). The way in which clients
discover the required configuration is outside the scope of this
specification.

3.1. Making Requests

An authenticated request includes several protocol parameters. Each
parameter name begins with the "oauth_" prefix, and the parameter
names and values are case sensitive. Clients make authenticated
requests by calculating the values of a set of protocol parameters
and adding them to the HTTP request as follows:

1. The client assigns value to each of these REQUIRED (unless
specified otherwise) protocol parameters:



oauth_consumer_key
The identifier portion of the client credentials (equivalent to
a username). The parameter name reflects a deprecated term
(Consumer Key) used in previous revisions of the specification,
and has been retained to maintain backward compatibility.

oauth_token
The token value used to associate the request with the resource
owner. If the request is not associated with a resource owner
(no token available), clients MAY omit the parameter.

oauth_signature_method
The name of the signature method used by the client to sign the
request, as defined in Section 3.4.

oauth_timestamp
The timestamp value as defined in Section 3.3. The parameter
MAY be omitted when using the "PLAINTEXT" signature method.

oauth_nonce
The nonce value as defined in Section 3.3. The parameter MAY
be omitted when using the "PLAINTEXT" signature method.

oauth_version
OPTIONAL. If present, MUST be set to "1.0". Provides the
version of the authentication process as defined in this
specification.

2. The protocol parameters are added to the request using one of the
transmission methods listed in Section 3.5. Each parameter MUST
NOT appear more than once per request.

3. The client calculates and assigns the value of the
"oauth_signature" parameter as described in Section 3.4 and adds
the parameter to the request using the same method as in the
previous step.

4. The client sends the authenticated HTTP request to the server.

For example, to make the following HTTP request authenticated (the
"c2&a3=2+q" string in the following examples is used to illustrate
the impact of a form-encoded entity-body):

POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1
Host: example.com

c2&a3=2+q



The client assigns values to the following protocol parameters using
its client credentials, token credentials, the current timestamp, a
uniquely generated nonce, and indicates that it will use the
"HMAC-SHA1" signature method:

oauth_consumer_key: 9djdj82h48djs9d2
oauth_token: kkk9d7dh3k39sjv7
oauth_signature_method: HMAC-SHA1
oauth_timestamp: 137131201
oauth_nonce: 7d8f3e4a

The client adds the protocol parameters to the request using the
OAuth HTTP "Authorization" header field:

oauth_consumer_key="9djdj82h48djs9d2",
oauth_token="kkk9d7dh3k39sjv7",
oauth_nonce="7d8f3e4a"

Then, it calculates the value of the "oauth_signature" parameter
(using client secret "j49sk3j29djd" and token secret "dh893hdasih9"),
adds it to the request, and sends the HTTP request to the server:

Host: example.com
oauth_nonce="7d8f3e4a",
oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D"

c2&a3=2+q

3.2. Verifying Requests

Servers receiving an authenticated request MUST validate it by:

o Recalculating the request signature independently as described in
Section 3.4 and comparing it to the value received from the client
via the "oauth_signature" parameter.



o If using the "HMAC-SHA1" or "RSA-SHA1" signature methods, ensuring
that the combination of nonce/timestamp/token (if present)
received from the client has not been used before in a previous
request (the server MAY reject requests with stale timestamps as
described in Section 3.3).

o If a token is present, verifying the scope and status of the
client authorization as represented by the token (the server MAY
choose to restrict token usage to the client to which it was
issued).

o If the "oauth_version" parameter is present, ensuring its value is
"1.0".

If the request fails verification, the server SHOULD respond with the
appropriate HTTP response status code. The server MAY include
further details about why the request was rejected in the response
body.

The server SHOULD return a 400 (Bad Request) status code when
receiving a request with unsupported parameters, an unsupported
signature method, missing parameters, or duplicated protocol
parameters. The server SHOULD return a 401 (Unauthorized) status
code when receiving a request with invalid client credentials, an
invalid or expired token, an invalid signature, or an invalid or used
nonce.

3.3. Nonce and Timestamp

The timestamp value MUST be a positive integer. Unless otherwise
specified by the server’s documentation, the timestamp is expressed
in the number of seconds since January 1, 1970 00:00:00 GMT.

A nonce is a random string, uniquely generated by the client to allow
the server to verify that a request has never been made before and
helps prevent replay attacks when requests are made over a non-secure
channel. The nonce value MUST be unique across all requests with the
same timestamp, client credentials, and token combinations.

To avoid the need to retain an infinite number of nonce values for
future checks, servers MAY choose to restrict the time period after
which a request with an old timestamp is rejected. Note that this
restriction implies a level of synchronization between the client’s
and server’s clocks. Servers applying such a restriction MAY provide
a way for the client to sync with the server’s clock; alternatively,
both systems could synchronize with a trusted time service. Details
of clock synchronization strategies are beyond the scope of this
specification.



3.4. Signature

OAuth-authenticated requests can have two sets of credentials: those
passed via the "oauth_consumer_key" parameter and those in the
"oauth_token" parameter. In order for the server to verify the
authenticity of the request and prevent unauthorized access, the
client needs to prove that it is the rightful owner of the
credentials. This is accomplished using the shared-secret (or RSA
key) part of each set of credentials.

OAuth provides three methods for the client to prove its rightful
ownership of the credentials: "HMAC-SHA1", "RSA-SHA1", and
"PLAINTEXT". These methods are generally referred to as signature
methods, even though "PLAINTEXT" does not involve a signature. In
addition, "RSA-SHA1" utilizes an RSA key instead of the shared-
secrets associated with the client credentials.

OAuth does not mandate a particular signature method, as each
implementation can have its own unique requirements. Servers are
free to implement and document their own custom methods.
Recommending any particular method is beyond the scope of this
specification. Implementers should review the Security
Considerations section (Section 4) before deciding on which method to
support.

The client declares which signature method is used via the
"oauth_signature_method" parameter. It then generates a signature
(or a string of an equivalent value) and includes it in the
"oauth_signature" parameter. The server verifies the signature as
specified for each method.

The signature process does not change the request or its parameters,
with the exception of the "oauth_signature" parameter.

3.4.1. Signature Base String

The signature base string is a consistent, reproducible concatenation
of several of the HTTP request elements into a single string. The
string is used as an input to the "HMAC-SHA1" and "RSA-SHA1"
signature methods.

The signature base string includes the following components of the
HTTP request:

o The HTTP request method (e.g., "GET", "POST", etc.).

o The authority as declared by the HTTP "Host" request header field.



o The path and query components of the request resource URI.

o The protocol parameters excluding the "oauth_signature".

o Parameters included in the request entity-body if they comply with
the strict restrictions defined in Section 3.4.1.3.

The signature base string does not cover the entire HTTP request.
Most notably, it does not include the entity-body in most requests,
nor does it include most HTTP entity-headers. It is important to
note that the server cannot verify the authenticity of the excluded
request components without using additional protections such as SSL/
TLS or other methods.

3.4.1.1. String Construction

The signature base string is constructed by concatenating together,
in order, the following HTTP request elements:

1. The HTTP request method in uppercase. For example: "HEAD",
"GET", "POST", etc. If the request uses a custom HTTP method, it
MUST be encoded (Section 3.6).

2. An "&" character (ASCII code 38).

3. The base string URI from Section 3.4.1.2, after being encoded
(Section 3.6).

4. An "&" character (ASCII code 38).

5. The request parameters as normalized in Section 3.4.1.3.2, after
being encoded (Section 3.6).

For example, the HTTP request:

Host: example.com
oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D"

c2&a3=2+q



is represented by the following signature base string (line breaks
are for display purposes only):

POST&http%3A%2F%2Fexample.com%2Frequest&a2%3Dr%2520b%26a3%3D2%2520q
%26a3%3Da%26b5%3D%253D%25253D%26c%2540%3D%26c2%3D%26oauth_consumer_
key%3D9djdj82h48djs9d2%26oauth_nonce%3D7d8f3e4a%26oauth_signature_m
ethod%3DHMAC-SHA1%26oauth_timestamp%3D137131201%26oauth_token%3Dkkk
9d7dh3k39sjv7

3.4.1.2. Base String URI

The scheme, authority, and path of the request resource URI [RFC3986]
are included by constructing an "http" or "https" URI representing
the request resource (without the query or fragment) as follows:

1. The scheme and host MUST be in lowercase.

2. The host and port values MUST match the content of the HTTP
request "Host" header field.

3. The port MUST be included if it is not the default port for the
scheme, and MUST be excluded if it is the default. Specifically,
the port MUST be excluded when making an HTTP request [RFC2616]
to port 80 or when making an HTTPS request [RFC2818] to port 443.
All other non-default port numbers MUST be included.


GET /r%20v/X?id=123 HTTP/1.1
Host: EXAMPLE.COM:80

is represented by the base string URI: "http://example.com/r%20v/X".

In another example, the HTTPS request:

GET /?q=1 HTTP/1.1
Host: www.example.net:8080

is represented by the base string URI:
"https://www.example.net:8080/".

3.4.1.3. Request Parameters

In order to guarantee a consistent and reproducible representation of
the request parameters, the parameters are collected and decoded to
their original decoded form. They are then sorted and encoded in a
particular manner that is often different from their original
encoding scheme, and concatenated into a single string.



3.4.1.3.1. Parameter Sources

The parameters from the following sources are collected into a single
list of name/value pairs:

o The query component of the HTTP request URI as defined by
[RFC3986], Section 3.4. The query component is parsed into a list
of name/value pairs by treating it as an
"application/x-www-form-urlencoded" string, separating the names
and values and decoding them as defined by
[W3C.REC-html40-19980424], Section 17.13.4.

o The OAuth HTTP "Authorization" header field (Section 3.5.1) if
present. The header’s content is parsed into a list of name/value
pairs excluding the "realm" parameter if present. The parameter
values are decoded as defined by Section 3.5.1.

o The HTTP request entity-body, but only if all of the following
conditions are met:

* The entity-body is single-part.

* The entity-body follows the encoding requirements of the
"application/x-www-form-urlencoded" content-type as defined by
[W3C.REC-html40-19980424].

* The HTTP request entity-header includes the "Content-Type"
header field set to "application/x-www-form-urlencoded".

The entity-body is parsed into a list of decoded name/value pairs
as described in [W3C.REC-html40-19980424], Section 17.13.4.

The "oauth_signature" parameter MUST be excluded from the signature
base string if present. Parameters not explicitly included in the
request MUST be excluded from the signature base string (e.g., the
"oauth_version" parameter when omitted).




Host: example.com
oauth_signature="djosJKDKJSD8743243%2Fjdk33klY%3D"

c2&a3=2+q

contains the following (fully decoded) parameters used in the
signature base sting:

+------------------------+------------------+
| Name | Value |
+------------------------+------------------+
| b5 | =%3D |
| a3 | a |
| c@ | |
| a2 | r b |
| oauth_consumer_key | 9djdj82h48djs9d2 |
| oauth_token | kkk9d7dh3k39sjv7 |
| oauth_signature_method | HMAC-SHA1 |
| oauth_timestamp | 137131201 |
| oauth_nonce | 7d8f3e4a |
| c2 | |
| a3 | 2 q |
+------------------------+------------------+

Note that the value of "b5" is "=%3D" and not "==". Both "c@" and
"c2" have empty values. While the encoding rules specified in this
specification for the purpose of constructing the signature base
string exclude the use of a "+" character (ASCII code 43) to
represent an encoded space character (ASCII code 32), this practice
is widely used in "application/x-www-form-urlencoded" encoded values,
and MUST be properly decoded, as demonstrated by one of the "a3"
parameter instances (the "a3" parameter is used twice in this
request).



3.4.1.3.2. Parameters Normalization

The parameters collected in Section 3.4.1.3 are normalized into a
single string as follows:

1. First, the name and value of each parameter are encoded
(Section 3.6).

2. The parameters are sorted by name, using ascending byte value
ordering. If two or more parameters share the same name, they
are sorted by their value.

3. The name of each parameter is concatenated to its corresponding
value using an "=" character (ASCII code 61) as a separator, even
if the value is empty.

4. The sorted name/value pairs are concatenated together into a
single string by using an "&" character (ASCII code 38) as
separator.

For example, the list of parameters from the previous section would
be normalized as follows:

Encoded:

+------------------------+------------------+
| Name | Value |
+------------------------+------------------+
| b5 | %3D%253D |
| a3 | a |
| c%40 | |
| a2 | r%20b |
| c2 | |
| a3 | 2%20q |
+------------------------+------------------+



Sorted:

+------------------------+------------------+
| Name | Value |
+------------------------+------------------+
| a2 | r%20b |
| a3 | 2%20q |
| a3 | a |
| b5 | %3D%253D |
| c%40 | |
| c2 | |
+------------------------+------------------+

Concatenated Pairs:

+-------------------------------------+
| Name=Value |
+-------------------------------------+
| a2=r%20b |
| a3=2%20q |
| a3=a |
| b5=%3D%253D |
| c%40= |
| c2= |
| oauth_consumer_key=9djdj82h48djs9d2 |
| oauth_nonce=7d8f3e4a |
| oauth_signature_method=HMAC-SHA1 |
| oauth_timestamp=137131201 |
| oauth_token=kkk9d7dh3k39sjv7 |
+-------------------------------------+

and concatenated together into a single string (line breaks are for
display purposes only):

a2=r%20b&a3=2%20q&a3=a&b5=%3D%253D&c%40=&c2=&oauth_consumer_key=9dj
dj82h48djs9d2&oauth_nonce=7d8f3e4a&oauth_signature_method=HMAC-SHA1
&oauth_timestamp=137131201&oauth_token=kkk9d7dh3k39sjv7



3.4.2. HMAC-SHA1

The "HMAC-SHA1" signature method uses the HMAC-SHA1 signature
algorithm as defined in [RFC2104]:

digest = HMAC-SHA1 (key, text)

The HMAC-SHA1 function variables are used in following way:

text is set to the value of the signature base string from
Section 3.4.1.1.

key is set to the concatenated values of:

1. The client shared-secret, after being encoded
(Section 3.6).

2. An "&" character (ASCII code 38), which MUST be included
even when either secret is empty.

3. The token shared-secret, after being encoded
(Section 3.6).

digest is used to set the value of the "oauth_signature" protocol
parameter, after the result octet string is base64-encoded
per [RFC2045], Section 6.8.

3.4.3. RSA-SHA1

The "RSA-SHA1" signature method uses the RSASSA-PKCS1-v1_5 signature
algorithm as defined in [RFC3447], Section 8.2 (also known as
PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5. To
use this method, the client MUST have established client credentials
with the server that included its RSA public key (in a manner that is
beyond the scope of this specification).

The signature base string is signed using the client’s RSA private
key per [RFC3447], Section 8.2.1:

S = RSASSA-PKCS1-V1_5-SIGN (K, M)

Where:

K is set to the client’s RSA private key,

M is set to the value of the signature base string from
Section 3.4.1.1, and



S is the result signature used to set the value of the
"oauth_signature" protocol parameter, after the result octet
string is base64-encoded per [RFC2045] section 6.8.

The server verifies the signature per [RFC3447] section 8.2.2:

RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S)

Where:

(n, e) is set to the client’s RSA public key,

M is set to the value of the signature base string from
Section 3.4.1.1, and

S is set to the octet string value of the "oauth_signature"
protocol parameter received from the client.

3.4.4. PLAINTEXT

The "PLAINTEXT" method does not employ a signature algorithm. It
MUST be used with a transport-layer mechanism such as TLS or SSL (or
sent over a secure channel with equivalent protections). It does not
utilize the signature base string or the "oauth_timestamp" and
"oauth_nonce" parameters.

The "oauth_signature" protocol parameter is set to the concatenated
value of:

1. The client shared-secret, after being encoded (Section 3.6).

2. An "&" character (ASCII code 38), which MUST be included even
when either secret is empty.

3. The token shared-secret, after being encoded (Section 3.6).

3.5. Parameter Transmission

When making an OAuth-authenticated request, protocol parameters as
well as any other parameter using the "oauth_" prefix SHALL be
included in the request using one and only one of the following
locations, listed in order of decreasing preference:

1. The HTTP "Authorization" header field as described in
Section 3.5.1.

2. The HTTP request entity-body as described in Section 3.5.2.



3. The HTTP request URI query as described in Section 3.5.3.

In addition to these three methods, future extensions MAY define
other methods for including protocol parameters in the request.

3.5.1. Authorization Header

Protocol parameters can be transmitted using the HTTP "Authorization"
header field as defined by [RFC2617] with the auth-scheme name set to
"OAuth" (case insensitive).

For example:

oauth_consumer_key="0685bd9184jfhq22",
oauth_token="ad180jjd733klru7",
oauth_signature="wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%3D",
oauth_nonce="4572616e48616d6d65724c61686176",
oauth_version="1.0"

Protocol parameters SHALL be included in the "Authorization" header
field as follows:

1. Parameter names and values are encoded per Parameter Encoding
(Section 3.6).

2. Each parameter’s name is immediately followed by an "=" character
(ASCII code 61), a """ character (ASCII code 34), the parameter
value (MAY be empty), and another """ character (ASCII code 34).

3. Parameters are separated by a "," character (ASCII code 44) and
OPTIONAL linear whitespace per [RFC2617].

4. The OPTIONAL "realm" parameter MAY be added and interpreted per
[RFC2617] section 1.2.

Servers MAY indicate their support for the "OAuth" auth-scheme by
returning the HTTP "WWW-Authenticate" response header field upon
client requests for protected resources. As per [RFC2617], such a
response MAY include additional HTTP "WWW-Authenticate" header
fields:

For example:

WWW-Authenticate: OAuth realm="http://server.example.com/"



The realm parameter defines a protection realm per [RFC2617], Section
1.2.

3.5.2. Form-Encoded Body

Protocol parameters can be transmitted in the HTTP request entity-
body, but only if the following REQUIRED conditions are met:

o The entity-body is single-part.

o The entity-body follows the encoding requirements of the
"application/x-www-form-urlencoded" content-type as defined by
[W3C.REC-html40-19980424].

o The HTTP request entity-header includes the "Content-Type" header
field set to "application/x-www-form-urlencoded".


oauth_consumer_key=0685bd9184jfhq22&oauth_token=ad180jjd733klr
u7&oauth_signature_method=HMAC-SHA1&oauth_signature=wOJIO9A2W5
mFwDgiDvZbTSMK%2FPY%3D&oauth_timestamp=137131200&oauth_nonce=4
572616e48616d6d65724c61686176&oauth_version=1.0

The entity-body MAY include other request-specific parameters, in
which case, the protocol parameters SHOULD be appended following the
request-specific parameters, properly separated by an "&" character
(ASCII code 38).

3.5.3. Request URI Query

Protocol parameters can be transmitted by being added to the HTTP
request URI as a query parameter as defined by [RFC3986], Section 3.


GET /example/path?oauth_consumer_key=0685bd9184jfhq22&
oauth_token=ad180jjd733klru7&oauth_signature_method=HM
AC-SHA1&oauth_signature=wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%
3D&oauth_timestamp=137131200&oauth_nonce=4572616e48616
d6d65724c61686176&oauth_version=1.0 HTTP/1.1

The request URI MAY include other request-specific query parameters,
in which case, the protocol parameters SHOULD be appended following
the request-specific parameters, properly separated by an "&"
character (ASCII code 38).



3.6. Percent Encoding

Existing percent-encoding methods do not guarantee a consistent
construction of the signature base string. The following percent-
encoding method is not defined to replace the existing encoding
methods defined by [RFC3986] and [W3C.REC-html40-19980424]. It is
used only in the construction of the signature base string and the
"Authorization" header field.

This specification defines the following method for percent-encoding
strings:

1. Text values are first encoded as UTF-8 octets per [RFC3629] if
they are not already. This does not include binary values that
are not intended for human consumption.

2. The values are then escaped using the [RFC3986] percent-encoding
(%XX) mechanism as follows:

* Characters in the unreserved character set as defined by
[RFC3986], Section 2.3 (ALPHA, DIGIT, "-", ".", "_", "˜") MUST
NOT be encoded.

* All other characters MUST be encoded.

* The two hexadecimal characters used to represent encoded
characters MUST be uppercase.

This method is different from the encoding scheme used by the
"application/x-www-form-urlencoded" content-type (for example, it
encodes space characters as "%20" and not using the "+" character).
It MAY be different from the percent-encoding functions provided by
web-development frameworks (e.g., encode different characters, use
lowercase hexadecimal characters).

4. Security Considerations

As stated in [RFC2617], the greatest sources of risks are usually
found not in the core protocol itself but in policies and procedures
surrounding its use. Implementers are strongly encouraged to assess
how this protocol addresses their security requirements.

4.1. RSA-SHA1 Signature Method

Authenticated requests made with "RSA-SHA1" signatures do not use the
token shared-secret, or any provisioned client shared-secret. This
means the request relies completely on the secrecy of the private key
used by the client to sign requests.



4.2. Confidentiality of Requests

While this protocol provides a mechanism for verifying the integrity
of requests, it provides no guarantee of request confidentiality.
Unless further precautions are taken, eavesdroppers will have full
access to request content. Servers should carefully consider the
kinds of data likely to be sent as part of such requests, and should
employ transport-layer security mechanisms to protect sensitive
resources.

4.3. Spoofing by Counterfeit Servers

This protocol makes no attempt to verify the authenticity of the
server. A hostile party could take advantage of this by intercepting
the client’s requests and returning misleading or otherwise incorrect
responses. Service providers should consider such attacks when
developing services using this protocol, and should require
transport-layer security for any requests where the authenticity of
the server or of request responses is an issue.

4.4. Proxying and Caching of Authenticated Content

The HTTP Authorization scheme (Section 3.5.1) is optional. However,
[RFC2616] relies on the "Authorization" and "WWW-Authenticate" header
fields to distinguish authenticated content so that it can be
protected. Proxies and caches, in particular, may fail to adequately
protect requests not using these header fields.

For example, private authenticated content may be stored in (and thus
retrievable from) publicly accessible caches. Servers not using the
HTTP "Authorization" header field should take care to use other
mechanisms, such as the "Cache-Control" header field, to ensure that
authenticated content is protected.

4.5. Plaintext Storage of Credentials

The client shared-secret and token shared-secret function the same
way passwords do in traditional authentication systems. In order to
compute the signatures used in methods other than "RSA-SHA1", the
server must have access to these secrets in plaintext form. This is
in contrast, for example, to modern operating systems, which store
only a one-way hash of user credentials.

If an attacker were to gain access to these secrets -- or worse, to
the server’s database of all such secrets -- he or she would be able
to perform any action on behalf of any resource owner. Accordingly,
it is critical that servers protect these secrets from unauthorized
access.



4.6. Secrecy of the Client Credentials

In many cases, the client application will be under the control of
potentially untrusted parties. For example, if the client is a
desktop application with freely available source code or an
executable binary, an attacker may be able to download a copy for
analysis. In such cases, attackers will be able to recover the
client credentials.

Accordingly, servers should not use the client credentials alone to
verify the identity of the client. Where possible, other factors
such as IP address should be used as well.

4.7. Phishing Attacks

Wide deployment of this and similar protocols may cause resource
owners to become inured to the practice of being redirected to
websites where they are asked to enter their passwords. If resource
owners are not careful to verify the authenticity of these websites
before entering their credentials, it will be possible for attackers
to exploit this practice to steal resource owners’ passwords.

Servers should attempt to educate resource owners about the risks
phishing attacks pose, and should provide mechanisms that make it
easy for resource owners to confirm the authenticity of their sites.
Client developers should consider the security implications of how
they interact with a user-agent (e.g., separate window, embedded),
and the ability of the end-user to verify the authenticity of the
server website.

4.8. Scoping of Access Requests

By itself, this protocol does not provide any method for scoping the
access rights granted to a client. However, most applications do
require greater granularity of access rights. For example, servers
may wish to make it possible to grant access to some protected
resources but not others, or to grant only limited access (such as
read-only access) to those protected resources.

When implementing this protocol, servers should consider the types of
access resource owners may wish to grant clients, and should provide
mechanisms to do so. Servers should also take care to ensure that
resource owners understand the access they are granting, as well as
any risks that may be involved.



4.9. Entropy of Secrets

Unless a transport-layer security protocol is used, eavesdroppers
will have full access to authenticated requests and signatures, and
will thus be able to mount offline brute-force attacks to recover the
credentials used. Servers should be careful to assign shared-secrets
that are long enough, and random enough, to resist such attacks for
at least the length of time that the shared-secrets are valid.

For example, if shared-secrets are valid for two weeks, servers
should ensure that it is not possible to mount a brute force attack
that recovers the shared-secret in less than two weeks. Of course,
servers are urged to err on the side of caution, and use the longest
secrets reasonable.

It is equally important that the pseudo-random number generator
(PRNG) used to generate these secrets be of sufficiently high
quality. Many PRNG implementations generate number sequences that
may appear to be random, but that nevertheless exhibit patterns or
other weaknesses that make cryptanalysis or brute force attacks
easier. Implementers should be careful to use cryptographically
secure PRNGs to avoid these problems.

4.10. Denial-of-Service / Resource-Exhaustion Attacks

This specification includes a number of features that may make
resource exhaustion attacks against servers possible. For example,
this protocol requires servers to track used nonces. If an attacker
is able to use many nonces quickly, the resources required to track
them may exhaust available capacity. And again, this protocol can
require servers to perform potentially expensive computations in
order to verify the signature on incoming requests. An attacker may
exploit this to perform a denial-of-service attack by sending a large
number of invalid requests to the server.

Resource Exhaustion attacks are by no means specific to this
specification. However, implementers should be careful to consider
the additional avenues of attack that this protocol exposes, and
design their implementations accordingly. For example, entropy
starvation typically results in either a complete denial of service
while the system waits for new entropy or else in weak (easily
guessable) secrets. When implementing this protocol, servers should
consider which of these presents a more serious risk for their
application and design accordingly.



4.11. SHA-1 Cryptographic Attacks

SHA-1, the hash algorithm used in "HMAC-SHA1" and "RSA-SHA1"
signature methods, has been shown to have a number of cryptographic
weaknesses that significantly reduce its resistance to collision
attacks. While these weaknesses do not seem to affect the use of
SHA-1 with the Hash-based Message Authentication Code (HMAC) and
should not affect the "HMAC-SHA1" signature method, it may affect the
use of the "RSA-SHA1" signature method. NIST has announced that it
will phase out use of SHA-1 in digital signatures by 2010
[NIST_SHA-1Comments].

Practically speaking, these weaknesses are difficult to exploit, and
by themselves do not pose a significant risk to users of this
protocol. They may, however, make more efficient attacks possible,
and servers should take this into account when considering whether
SHA-1 provides an adequate level of security for their applications.

4.12. Signature Base String Limitations

The signature base string has been designed to support the signature
methods defined in this specification. Those designing additional
signature methods, should evaluated the compatibility of the
signature base string with their security requirements.

Since the signature base string does not cover the entire HTTP
request, such as most request entity-body, most entity-headers, and
the order in which parameters are sent, servers should employ
additional mechanisms to protect such elements.

4.13. Cross-Site Request Forgery (CSRF)

Cross-Site Request Forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from a user that the website trusts or has
authenticated. CSRF attacks on authorization approvals can allow an
attacker to obtain authorization to protected resources without the
consent of the User. Servers SHOULD strongly consider best practices
in CSRF prevention at all the protocol authorization endpoints.

CSRF attacks on OAuth callback URIs hosted by clients are also
possible. Clients should prevent CSRF attacks on OAuth callback URIs
by verifying that the resource owner at the client site intended to
complete the OAuth negotiation with the server. The methods for
preventing such CSRF attacks are beyond the scope of this
specification.



4.14. User Interface Redress

Servers should protect the authorization process against user
interface (UI) redress attacks (also known as "clickjacking"). As of
the time of this writing, no complete defenses against UI redress are
available. Servers can mitigate the risk of UI redress attacks using
the following techniques:

o JavaScript frame busting.

o JavaScript frame busting, and requiring that browsers have
JavaScript enabled on the authorization page.

o Browser-specific anti-framing techniques.

o Requiring password reentry before issuing OAuth tokens.

4.15. Automatic Processing of Repeat Authorizations

Servers may wish to automatically process authorization requests
(Section 2.2) from clients that have been previously authorized by
the resource owner. When the resource owner is redirected to the
server to grant access, the server detects that the resource owner
has already granted access to that particular client. Instead of
prompting the resource owner for approval, the server automatically
redirects the resource owner back to the client.

If the client credentials are compromised, automatic processing
creates additional security risks. An attacker can use the stolen
client credentials to redirect the resource owner to the server with
an authorization request. The server will then grant access to the
resource owner’s data without the resource owner’s explicit approval,
or even awareness of an attack. If no automatic approval is
implemented, an attacker must use social engineering to convince the
resource owner to approve access.

Servers can mitigate the risks associated with automatic processing
by limiting the scope of token credentials obtained through automated
approvals. Tokens credentials obtained through explicit resource
owner consent can remain unaffected. Clients can mitigate the risks
associated with automatic processing by protecting their client
credentials.



5. Acknowledgments

This specification is directly based on the OAuth Core 1.0 Revision A
community specification, which in turn was modeled after existing
proprietary protocols and best practices that have been independently
implemented by various companies.

The community specification was edited by Eran Hammer-Lahav and
authored by: Mark Atwood, Dirk Balfanz, Darren Bounds, Richard M.
Conlan, Blaine Cook, Leah Culver, Breno de Medeiros, Brian Eaton,
Kellan Elliott-McCrea, Larry Halff, Eran Hammer-Lahav, Ben Laurie,
Chris Messina, John Panzer, Sam Quigley, David Recordon, Eran
Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy
Smith.

The editor would like to thank the following individuals for their
invaluable contribution to the publication of this edition of the
protocol: Lisa Dusseault, Justin Hart, Avshalom Houri, Chris Messina,
Mark Nottingham, Tim Polk, Peter Saint-Andre, Joseph Smarr, and Paul
Walker.



Appendix A. Differences from the Community Edition

This specification includes the following changes made to the
original community document [OAuthCore1.0_RevisionA] in order to
correct mistakes and omissions identified since the document was
originally published at <http://oauth.net>.

o Changed using TLS/SSL when sending or requesting plain text
credentials from SHOULD to MUST. This change affects any use of
the "PLAINTEXT" signature method, as well as requesting temporary
credentials (Section 2.1) and obtaining token credentials
(Section 2.3).

o Adjusted nonce language to indicate it is unique per token/
timestamp/client combination.

o Removed the requirement for timestamps to be equal to or greater
than the timestamp used in the previous request.

o Changed the nonce and timestamp parameters to OPTIONAL when using
the "PLAINTEXT" signature method.

o Extended signature base string coverage that includes
"application/x-www-form-urlencoded" entity-body parameters when
the HTTP method used is other than "POST" and URI query parameters
when the HTTP method used is other than "GET".

o Incorporated corrections to the instructions in each signature
method to encode the signature value before inserting it into the
"oauth_signature" parameter, removing errors that would have
caused double-encoded values.

o Allowed omitting the "oauth_token" parameter when empty.

o Permitted sending requests for temporary credentials with an empty
"oauth_token" parameter.

o Removed the restrictions from defining additional "oauth_"
parameters.



6. References

6.1. Normative References

[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2616] 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.

[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[W3C.REC-html40-19980424]
Hors, A., Raggett, D., and I. Jacobs, "HTML 4.0
Specification", World Wide Web Consortium
Recommendation REC-html40-19980424, April 1998,
<http://www.w3.org/TR/1998/REC-html40-19980424>.



6.2. Informative References

[NIST_SHA-1Comments]
Burr, W., "NIST Comments on Cryptanalytic Attacks on
SHA-1",
<http://csrc.nist.gov/groups/ST/hash/statement.html>.

[OAuthCore1.0_RevisionA]
OAuth Community, "OAuth Core 1.0 Revision A",
<http://oauth.net/core/1.0a>.

Author’s Address

Eran Hammer-Lahav (editor)

EMail: eran@hueniverse.com
URI: http://hueniverse.com


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