The OAuth Core 1.0 Protocol
draft-hammer-oauth-07

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

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Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Introduction
    1.1.  Terminology
    1.2.  Example
    1.3.  Notational Conventions
2.  Redirection-Based Authorization
    2.1.  Temporary Credentials
    2.2.  Resource Owner Authorization
    2.3.  Token Credentials
3.  Authenticated Requests
    3.1.  Making Requests
    3.2.  Verifying Requests
    3.3.  Nonce and Timestamp
    3.4.  Signature
        3.4.1.  Signature Base String
        3.4.2.  HMAC-SHA1
        3.4.3.  RSA-SHA1
        3.4.4.  PLAINTEXT
    3.5.  Parameter Transmission
        3.5.1.  Authorization Header
        3.5.2.  Form-Encoded Body
        3.5.3.  Request URI Query
    3.6.  Percent Encoding
4.  Security Considerations
    4.1.  Credentials Transmission
    4.2.  RSA-SHA1 Signature Method
    4.3.  PLAINTEXT Signature Method
    4.4.  Confidentiality of Requests
    4.5.  Spoofing by Counterfeit Servers
    4.6.  Proxying and Caching of Authenticated Content
    4.7.  Plaintext Storage of Credentials
    4.8.  Secrecy of the Client Credentials
    4.9.  Phishing Attacks
    4.10.  Scoping of Access Requests
    4.11.  Entropy of Secrets
    4.12.  Denial of Service / Resource Exhaustion Attacks
    4.13.  Cryptographic Attacks
    4.14.  Signature Base String Limitations
    4.15.  Cross-Site Request Forgery (CSRF)
    4.16.  User Interface Redress
    4.17.  Automatic Processing of Repeat Authorizations
5.  IANA Considerations
Appendix A.  Differences from the Community Edition
Appendix B.  Acknowledgments
Appendix C.  Document History
6.  References
    6.1.  Normative References
    6.2.  Informative References
§  Author's Address

1.  Introduction

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 substitute the need 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] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) 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 other transport protocol than [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) is undefined.

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 published at the oauth.net website.

This specification provides an informational documentation of OAuth Core 1.0 Revision A as finalized in June 2009, addressing several errata reported since that time, as well as numerous editorial clarifications. It 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.

1.1.  Terminology

client

An HTTP client (per [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) capable of making OAuth-authenticated requests (Authenticated Requests).

server

An HTTP server (per [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) capable of accepting OAuth-authenticated requests (Authenticated Requests).

protected resource

An access-restricted resource (per [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.)) which can be obtained from the server using an OAuth-authenticated request (Authenticated Requests).

resource owner

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

token

An 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 which 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] (Rescorla, E., “HTTP Over TLS,” May 2000.) request to the server:

  POST /initiate HTTP/1.1
  Host: photos.example.net
  Authorization: OAuth realm="http://photos.example.net/",
     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 TLS channel):

  POST /token HTTP/1.1
  Host: photos.example.net
  Authorization: OAuth realm="http://photos.example.net/",
     oauth_consumer_key="dpf43f3p2l4k3l03",
     oauth_token="hh5s93j4hdidpola",
     oauth_signature_method="HMAC-SHA1",
     oauth_timestamp="137131201",
     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
  Content-Type: application/x-www-form-urlencoded

  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
  Host: photos.example.net
  Authorization: OAuth realm="http://photos.example.net/",
     oauth_consumer_key="dpf43f3p2l4k3l03",
     oauth_token="nnch734d00sl2jdk",
     oauth_signature_method="HMAC-SHA1",
     oauth_timestamp="137131202",
     oauth_nonce="chapoH",
     oauth_signature="MdpQcU8iPSUjWoN%2FUDMsK2sui9I%3D"

The 'photos.example.net' sever 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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 (Temporary Credentials).

Resource Owner Authorization

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

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 (Token Credentials).

The three URIs advertised by the server MAY include a query component as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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 which 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 (Authenticated Requests) 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 parameter to the request (in addition to the other protocol parameters, using the same parameter transmission method):

oauth_callback:

An absolute URI to which the server will redirect the resource owner back when the Resource Owner Authorization step (Section 2.2 (Resource Owner Authorization)) 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 SHOULD require the use of a transport-layer mechanisms such as TLS or SSL. This is true regardless of the signature method used by the client.

For example, the client makes the following HTTPS request:

  POST /request_temp_credentials HTTP/1.1
  Host: server.example.com
  Authorization: OAuth realm="http://server.example.com/",
     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 (Verifying Requests) 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).

The response contains the following 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
  Content-Type: application/x-www-form-urlencoded

  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 query parameter to the Resource Owner Authorization endpoint URI:

oauth_token

REQUIRED. The temporary credentials identifier obtained in Section 2.1 (Temporary Credentials) 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
  Host: server.example.com

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 parameter 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 (Authenticated Requests) 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 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 SHOULD require the use of a transport-layer mechanisms such as TLS or SSL. This is true regardless of the signature method used by the client.

For example, the client makes the following HTTPS request:

  POST /request_token HTTP/1.1
  Host: server.example.com
  Authorization: OAuth realm="http://server.example.com/",
     oauth_consumer_key="jd83jd92dhsh93js",
     oauth_signature_method="PLAINTEXT",
     oauth_verifier="473f82d3",
     oauth_signature="ja893SD9%26xyz4992k83j47x0b"

The server MUST verify (Verifying Requests) 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).

The response contains the following parameters:

oauth_token

The token identifier.

oauth_token_secret

The token shared-secret.

For example:

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

  oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk

The token credentials issued by the server MUST reflect the exact scope, duration, and other attributes approved by the resource owner.

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 request (Authenticated Requests) using the client credentials together with the token credentials received.

3.  Authenticated Requests

The HTTP authentication methods defined by [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), 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 allows 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 (Redirection-Based Authorization) 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 4.8 (Secrecy of the Client Credentials).

Making authenticated requests requires prior knowledge of the server's configuration. OAuth includes multiple methods for transmitting protocol parameters with requests (Section 3.5 (Parameter Transmission)), as well as multiple methods for the client to prove its rightful ownership of the credentials used (Section 3.4 (Signature)). 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 (Signature).

   oauth_timestamp

   The timestamp value as defined in Section 3.3 (Nonce and Timestamp). The parameter MAY be omitted when using the PLAINTEXT signature method.

   oauth_nonce

   The nonce value as defined in Section 3.3 (Nonce and Timestamp). 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 (Parameter Transmission). 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 (Signature) and adds the parameter to the request using the same method used in the previous step.
4. The client sends the authenticated HTTP request to the server.

For example, to make the following HTTP request authenticated:

  GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1
  Host: example.com
  Content-Type: application/x-www-form-urlencoded

  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 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 parameter to the request using the OAuth HTTP Authorization header:

  Authorization: OAuth realm="http://example.com/",
                 oauth_consumer_key="9djdj82h48djs9d2",
                 oauth_token="kkk9d7dh3k39sjv7",
                 oauth_signature_method="HMAC-SHA1",
                 oauth_timestamp="137131201",
                 oauth_nonce="7d8f3e4a"

Then calculates the value of the oauth_signature parameter, adds it to the request, and sends the HTTP request to the server:

  GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1
  Host: example.com
  Content-Type: application/x-www-form-urlencoded
  Authorization: OAuth realm="http://example.com/",
                 oauth_consumer_key="9djdj82h48djs9d2",
                 oauth_token="kkk9d7dh3k39sjv7",
                 oauth_signature_method="HMAC-SHA1",
                 oauth_timestamp="137131201",
                 oauth_nonce="7d8f3e4a",
                 oauth_signature="djosJKDKJSD8743243%2Fjdk33klY%3D"

  c2&a3=2+q

3.2.  Verifying Requests

Servers receiving an authenticated request MUST validate it by:

• Recalculate the request signature independently as described in Section 3.4 (Signature) and compare it to the value received from the client via the oauth_signature parameter.
• If using the HMAC-SHA1 or RSA-SHA1 signature methods, ensure 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 (Nonce and Timestamp)).
• If a token is present, verify 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).
• If the oauth_version parameter is present, ensure 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, 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, invalid or expired token, invalid signature, or 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. Servers applying such a restriction SHOULD provide a way for the client to sync its clock with the server's clock, which is 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 (Security Considerations) 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:

• The HTTP request method (e.g. GET, POST, etc.).
• The authority as declared by the HTTP Host request header.
• The path and query components of the request resource URI.
• The protocol parameters excluding the oauth_signature.
• Parameters included in the request entity-body if they comply with the strict restrictions defined in Section 3.4.1.3 (Request Parameters).

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 (Percent Encoding).
2. An & character (ASCII code 38).
3. The base string URI from Section 3.4.1.2 (Base String URI), after being encoded (Percent Encoding).
4. An & character (ASCII code 38).
5. The request parameters as normalized in Section 3.4.1.3.2 (Parameters Normalization), after being encoded (Percent Encoding).

For example, the HTTP request:

  GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1
  Host: example.com
  Content-Type: application/x-www-form-urlencoded
  Authorization: OAuth realm="http://example.com/",
                 oauth_consumer_key="9djdj82h48djs9d2",
                 oauth_token="kkk9d7dh3k39sjv7",
                 oauth_signature_method="HMAC-SHA1",
                 oauth_timestamp="137131201",
                 oauth_nonce="7d8f3e4a",
                 oauth_signature="djosJKDKJSD8743243%2Fjdk33klY%3D"

  c2&a3=2+q

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

  GET&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_k
  ey%3D9djdj82h48djs9d2%26oauth_nonce%3D7d8f3e4a%26oauth_signature_me
  thod%3DHMAC-SHA1%26oauth_timestamp%3D137131201%26oauth_token%3Dkkk9
  d7dh3k39sjv7

3.4.1.2.  Base String URI

The scheme, authority, and path of the request resource URI [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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.
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] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) to port 80 or when making an HTTPS request [RFC2818] (Rescorla, E., “HTTP Over TLS,” May 2000.) to port 443. All other non-default port numbers MUST be included.

For example, the HTTP request:

  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 which 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:

• The query component of the HTTP request URI as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.) section 17.13.4.
• The OAuth HTTP Authorization header (Section 3.5.1 (Authorization Header)) 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 (Authorization Header).
• 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).
  • The HTTP request entity-header includes the Content-Type header 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.) 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).

For example, the HTTP request:

    GET /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1
    Host: example.com
    Content-Type: application/x-www-form-urlencoded
    Authorization: OAuth realm="http://example.com/",
                   oauth_consumer_key="9djdj82h48djs9d2",
                   oauth_token="kkk9d7dh3k39sjv7",
                   oauth_signature_method="HMAC-SHA1",
                   oauth_timestamp="137131201",
                   oauth_nonce="7d8f3e4a",
                   oauth_signature="djosJKDKJSD8743243%2Fjdk33klY%3D"

    c2&a3=2+q

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

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 (Request Parameters) are normalized into a single string as follows:

1. First, the name and value of each parameter are encoded (Percent Encoding).
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 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:

Sorted:

Concatenated Pairs:

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] (Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” February 1997.):

  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 (String Construction).

key

is set to the concatenated values of:

1. The client shared-secret, after being encoded (Percent Encoding).
2. An & character (ASCII code 38), which MUST be included even when either secret is empty.
3. The token shared-secret, after being encoded (Percent Encoding).

digest

is used to set the value of the oauth_signature protocol parameter, after the result octet string is base64-encoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) 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] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) 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 which included its RSA public key (in a manner which is beyond the scope of this specification).

The signature base string is signed using the client's RSA private key per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) 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 (String Construction), 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] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) section 6.8.

The server verifies the signature per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) 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 (String Construction), 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 and does not provide any security on its own as it transmits secrets in the clear. It SHOULD only be used with a transport-layer mechanism such as TLS or SSL as explained in Section 4.3 (PLAINTEXT Signature Method). It does not utilize the signature base string nor 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 (Percent Encoding).
2. An & character (ASCII code 38), which MUST be included even when either secret is empty.
3. The token shared-secret, after being encoded (Percent Encoding).

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 as described in Section 3.5.1 (Authorization Header).
2. The HTTP request entity-body as described in Section 3.5.2 (Form-Encoded Body).
3. The HTTP request URI query as described in Section 3.5.3 (Request URI Query).

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 as defined by [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) with the auth-scheme name set to OAuth (case-insensitive).

For example:

  Authorization: OAuth realm="http://server.example.com/",
     oauth_consumer_key="0685bd9184jfhq22",
     oauth_token="ad180jjd733klru7",
     oauth_signature_method="HMAC-SHA1",
     oauth_signature="wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%3D",
     oauth_timestamp="137131200",
     oauth_nonce="4572616e48616d6d65724c61686176",
     oauth_version="1.0"

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

1. Parameter names and values are encoded per Parameter Encoding (Percent Encoding).
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] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.).
4. The OPTIONAL realm parameter MAY be added and interpreted per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) section 1.2.

Servers MAY indicate their support for the OAuth auth-scheme by returning the HTTP WWW-Authenticate response header upon client requests for protected resources. As per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) such a response MAY include additional HTTP WWW-Authenticate headers:

For example:

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

The realm parameter defines a protection realm per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) 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:

• 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).
• The HTTP request entity-header includes the Content-Type header set to application/x-www-form-urlencoded.

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

  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

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3.

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

  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

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) and [W3C.REC‑html40‑19980424] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.). It is used only in the construction of the signature base string and the authorization header (Authorization Header).

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

1. Text values are first encoded as UTF-8 octets per [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) if they are not already. This does not include binary values which are not intended for human consumption.
2. The values are then escaped using the [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) percent-encoding (%XX) mechanism as follows:
   • Characters in the unreserved character set as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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 upper case.

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 lower case hexadecimal characters).

4.  Security Considerations

As stated in [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), 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.  Credentials Transmission

This specification does not describe any mechanism for protecting tokens and shared-secrets from eavesdroppers when they are transmitted from the server to the client during the authorization phase. Servers should ensure that these transmissions are protected using transport-layer mechanisms such as TLS or SSL.

4.2.  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.

4.3.  PLAINTEXT Signature Method

When used with the PLAINTEXT method, the protocol makes no attempt to protect credentials from eavesdroppers or man-in-the-middle attacks. The PLAINTEXT method is only intended to be used in conjunction with a transport-layer security mechanism such as TLS or SSL which does provide such protection.

4.4.  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.

4.5.  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.

4.6.  Proxying and Caching of Authenticated Content

The HTTP Authorization scheme (Authorization Header) is optional. However, [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) relies on the Authorization and WWW-Authenticate headers to distinguish authenticated content so that it can be protected. Proxies and caches, in particular, may fail to adequately protect requests not using these headers.

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

4.7.  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.

4.8.  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.

4.9.  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.

4.10.  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.

4.11.  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 which 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 which nevertheless exhibit patterns or other weaknesses which make cryptanalysis or brute force attacks easier. Implementers should be careful to use cryptographically secure PRNGs to avoid these problems.

4.12.  Denial of Service / Resource Exhaustion Attacks

This specification includes a number of features which 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.

4.13.  Cryptographic Attacks

SHA-1, the hash algorithm used in HMAC-SHA1 signatures, has been shown (De Canniere, C. and C. Rechberger, “Finding SHA-1 Characteristics: General Results and Applications,” .) [SHA1‑CHARACTERISTICS] to have a number of cryptographic weaknesses that significantly reduce its resistance to collision attacks. 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 NIST has announced (National Institute of Standards and Technology, NIST., “NIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1, August, 2004.,” .) [SHA‑COMMENTS] that it will phase out use of SHA-1 by 2010. Servers should take this into account when considering whether SHA-1 provides an adequate level of security for their applications.

4.14.  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.

4.15.  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.

4.16.  User Interface Redress

Servers should protect the authorization process against 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 through the following techniques:

• Javascript frame busting.
• Javascript frame busting, and requiring that browsers have javascript enabled on the authorization page.
• Browser-specific anti-framing techniques.
• Requiring password reentry before issuing OAuth tokens.

4.17.  Automatic Processing of Repeat Authorizations

Servers may wish to automatically process authorization requests (Section 2.2 (Resource Owner Authorization)) from clients which 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.  IANA Considerations

This memo includes no request to IANA.

Appendix A.  Differences from the Community Edition

This specification includes the following changes made to the original community document in order to correct mistakes and omissions identified since the document has been published:

• Adjusted nonce language to indicate it is unique per token/timestamp/client combination.
• Removed the requirement for timestamps to be equal to or greater than the timestamp used in the previous request.
• Changed the nonce and timestamp parameters to OPTIONAL when using the PLAINTEXT signature method.
• Extended signature base string coverage which 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.
• Incorporated corrections to the instructions in each signature method to encode the signature value before inserting it into the oauth_signature parameter, removing errors which would have caused double-encoded values.
• Allowed omitting the oauth_token parameter when empty.
• Permitted sending requests for temporary credentials with an empty oauth_token parameter.
• Removed the restrictions from defining additional oauth_ parameters.

Appendix B.  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, Peter Saint-Andre, Joseph Smarr, and Paul Walker.

Appendix C.  Document History

[[ To be removed by the RFC editor before publication as an RFC. ]]

-07

• Corrected typos.
• Changed examples to use HTTPS in authorization step.
• Clarified the use of 'oauth_' tokens in the three static endpoint URIs.
• Edited abstract and introduction.

-06

• Moved parameter section into client instructions.
• Added more examples in the delegation section, including more complete HTTP responses.
• Removed language about restricting the use of the 'oauth_' prefix.
• Changed the nonce and timestamp parameters to optional when using PLAINTEXT.
• Allowed empty token in temporary credentials requests.

-05

• Replaced the word 'lexicographical' with 'ascending' in sorting instructions.
• Added note that the 'a3' parameter appears twice in the example.
• Made the note about the encoding scheme being different from most common implementations more explicit.
• Moved and cleaned up the example from appendix to introduction. Removed crypto details from the example.

-04

• Corrected typo and other minor editorial changes.
• Added warning about token sizes.
• Clarified that all 'oauth_' parameters must be transmitted using the same method.
• Added explicit requirement to exclude parameters not transmitted in a request in the signature base string (for example oauth_version when omitted).
• Explicitly set OAuth to use HTTP 1.1.
• Rearranged the signature base string section to provide a better narrative of how the HTTP request is normalized. Added the protocol parameters to the examples to better demonstrate how they are incorporated in practice.
• Flipped the document order between authentication and authorization.
• Removed the requirement for timestamps to be equal or greater than previous timestamps.

-03

• Updated draft with changes from OAuth Core 1.0 Revision A to fix a session fixation exploit.
• Small editorial corrections.
• Removed confusing language from 'Denial of Service / Resource Exhaustion Attacks'.
• Added new 'Differences from the Community Edition' appendix.
• Updated acknowledgements section.

-02

• Corrected mistake in parameter sorting order (c%40 comes before c2).
• Added requirement to normalize empty paths as '/'.

-01

• Complete rewrite of the entire specification from scratch. Separated the spec structure into two parts and flipped their order.
• Corrected errors in instructions to encode the signature base string by some methods. The signature value is encoded using the transport rules, not the spec method for encoding.
• Replaced the entire terminology.

-00

• Initial draft based on the OAuth Core 1.0 community specification with the following changes.
• Various changes required to accommodate the strict format requirements of the IETF, such as moving sections around (Security, Contributors, Introduction, etc.), cleaning references, adding IETF specific text, etc.
• Moved the Parameter Encoding sub-section from section 5 (Parameters) to section 9.1 (Signature Base String) to make it clear it only applies to the signature base string.
• Nonce language adjusted to indicate it is unique per token/timestamp/consumer combination.
• Added security language regarding lack of token secrets in RSA-SHA1.
• Fixed the bug in the Normalize Request Parameters section. Removed the GET limitation from the third bullet (query parameters).
• Removed restriction of only signing application/x-www-form-urlencoded in POST requests, allowing the entity-body to be used with all HTTP request methods.

6.  References

6.1. Normative References

6.2. Informative References

Author's Address