This specification was obsoleted by RFC 5849: The OAuth 1.0 Protocol. Implementers should use RFC 5849 instead of this specification.



June 24, 2009


OAuth Core 1.0 Revision A

License

This specification was derived from the OAuth Core 1.0 specification which was made available under the OAuth Non-Assertion Covenant and Author's Contribution License For OAuth Specification 1.0 available at http://oauth.net/license/core/1.0. The changes between OAuth Core 1.0 and OAuth Core 1.0 Revision A have been licensed by Google and Yahoo! under the Open Web Foundation Agreement 0.9.

Changes from OAuth Core 1.0

OAuth Core 1.0 Revision A was created to address a session fixation attack identified in the OAuth Core 1.0 specification as detailed in http://oauth.net/advisories/2009-1. A full inventory of changes is available in the specification repository.

Abstract

The OAuth protocol enables websites or applications (Consumers) to access Protected Resources from a web service (Service Provider) via an API, without requiring Users to disclose their Service Provider credentials to the Consumers. More generally, OAuth creates a freely-implementable and generic methodology for API authentication.

An example use case is allowing printing service printer.example.com (the Consumer), to access private photos stored on photos.example.net (the Service Provider) without requiring Users to provide their photos.example.net credentials to printer.example.com.

OAuth does not require a specific user interface or interaction pattern, nor does it specify how Service Providers authenticate Users, making the protocol ideally suited for cases where authentication credentials are unavailable to the Consumer, such as with OpenID.

OAuth aims to unify the experience and implementation of delegated web service authentication into a single, community-driven protocol. OAuth builds on existing protocols and best practices that have been independently implemented by various websites. An open standard, supported by large and small providers alike, promotes a consistent and trusted experience for both application developers and the users of those applications.



Table of Contents

1.  Authors
2.  Notation and Conventions
3.  Definitions
4.  Documentation and Registration
    4.1.  Request URLs
    4.2.  Service Providers
    4.3.  Consumers
5.  Parameters
    5.1.  Parameter Encoding
    5.2.  Consumer Request Parameters
    5.3.  Service Provider Response Parameters
    5.4.  OAuth HTTP Authorization Scheme
6.  Authenticating with OAuth
    6.1.  Obtaining an Unauthorized Request Token
    6.2.  Obtaining User Authorization
    6.3.  Obtaining an Access Token
7.  Accessing Protected Resources
8.  Nonce and Timestamp
9.  Signing Requests
    9.1.  Signature Base String
    9.2.  HMAC-SHA1
    9.3.  RSA-SHA1
    9.4.  PLAINTEXT
10.  HTTP Response Codes
11.  Security Considerations
    11.1.  Credentials and Token Exchange
    11.2.  PLAINTEXT Signature Method
    11.3.  Confidentiality of Requests
    11.4.  Spoofing by Counterfeit Servers
    11.5.  Proxying and Caching of Authenticated Content
    11.6.  Plaintext Storage of Credentials
    11.7.  Secrecy of the Consumer Secret
    11.8.  Phishing Attacks
    11.9.  Scoping of Access Requests
    11.10.  Entropy of Secrets
    11.11.  Denial of Service / Resource Exhaustion Attacks
    11.12.  Cryptographic Attacks
    11.13.  Signature Base String Compatibility
    11.14.  Cross-Site Request Forgery (CSRF)
    11.15.  User Interface Redress
    11.16.  Automatic Processing of Repeat Authorizations
Appendix A.  Appendix A - Protocol Example
Appendix A.1.  Documentation and Registration
Appendix A.2.  Obtaining a Request Token
Appendix A.3.  Requesting User Authorization
Appendix A.4.  Obtaining an Access Token
Appendix A.5.  Accessing Protected Resources
12.  References
§  Author's Address




1.  Authors



2.  Notation and 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, B., “Key words for use in RFCs to Indicate Requirement Levels,” .). Domain name examples use [RFC2606] (Eastlake, D. and A. Panitz, “Reserved Top Level DNS Names,” .).



3.  Definitions

Service Provider:
A web application that allows access via OAuth.
User:
An individual who has an account with the Service Provider.
Consumer:
A website or application that uses OAuth to access the Service Provider on behalf of the User.
Protected Resource(s):
Data controlled by the Service Provider, which the Consumer can access through authentication.
Consumer Developer:
An individual or organization that implements a Consumer.
Consumer Key:
A value used by the Consumer to identify itself to the Service Provider.
Consumer Secret:
A secret used by the Consumer to establish ownership of the Consumer Key.
Request Token:
A value used by the Consumer to obtain authorization from the User, and exchanged for an Access Token.
Access Token:
A value used by the Consumer to gain access to the Protected Resources on behalf of the User, instead of using the User's Service Provider credentials.
Token Secret:
A secret used by the Consumer to establish ownership of a given Token.
OAuth Protocol Parameters:
Parameters with names beginning with oauth_.



4.  Documentation and Registration

OAuth includes a Consumer Key and matching Consumer Secret that together authenticate the Consumer (as opposed to the User) to the Service Provider. Consumer-specific identification allows the Service Provider to vary access levels to Consumers (such as un-throttled access to resources).

Service Providers SHOULD NOT rely on the Consumer Secret as a method to verify the Consumer identity, unless the Consumer Secret is known to be inaccessible to anyone other than the Consumer and the Service Provider. The Consumer Secret MAY be an empty string (for example when no Consumer verification is needed, or when verification is achieved through other means such as RSA).



4.1.  Request URLs

OAuth defines three request URLs:

Request Token URL:
The URL used to obtain an unauthorized Request Token, described in Section 6.1 (Obtaining an Unauthorized Request Token).
User Authorization URL:
The URL used to obtain User authorization for Consumer access, described in Section 6.2 (Obtaining User Authorization).
Access Token URL:
The URL used to exchange the User-authorized Request Token for an Access Token, described in Section 6.3 (Obtaining an Access Token).

The three URLs MUST include scheme, authority, and path, and MAY include query and fragment as defined by [RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) section 3. The request URL query MUST NOT contain any OAuth Protocol Parameters. For example:

              http://sp.example.com/authorize



4.2.  Service Providers

The Service Provider's responsibility is to enable Consumer Developers to establish a Consumer Key and Consumer Secret. The process and requirements for provisioning these are entirely up to the Service Providers.

The Service Provider's documentation includes:

  1. The URLs (Request URLs) the Consumer will use when making OAuth requests, and the HTTP methods (i.e. GET, POST, etc.) used in the Request Token URL and Access Token URL.
  2. Signature methods supported by the Service Provider.
  3. Any additional request parameters that the Service Provider requires in order to obtain a Token. Service Provider specific parameters MUST NOT begin with oauth_.



4.3.  Consumers

The Consumer Developer MUST establish a Consumer Key and a Consumer Secret with the Service Provider. The Consumer Developer MAY also be required to provide additional information to the Service Provider upon registration.



5.  Parameters

OAuth Protocol Parameter names and values are case sensitive. Each OAuth Protocol Parameters MUST NOT appear more than once per request, and are REQUIRED unless otherwise noted.



5.1.  Parameter Encoding

All parameter names and values are escaped using the [RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) percent-encoding (%xx) mechanism. Characters not in the unreserved character set ([RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) section 2.3) MUST be encoded. Characters in the unreserved character set MUST NOT be encoded. Hexadecimal characters in encodings MUST be upper case. Text names and values MUST be encoded as UTF-8 octets before percent-encoding them per [RFC3629] (Yergeau, F., “UTF-8, a transformation format of Unicode and ISO 10646,” .).

            unreserved = ALPHA, DIGIT, '-', '.', '_', '~'


5.2.  Consumer Request Parameters

OAuth Protocol Parameters are sent from the Consumer to the Service Provider in one of three methods, in order of decreasing preference:

  1. In the HTTP Authorization header as defined in OAuth HTTP Authorization Scheme (OAuth HTTP Authorization Scheme).
  2. As the HTTP POST request body with a content-type of application/x-www-form-urlencoded.
  3. Added to the URLs in the query part (as defined by [RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) section 3).

In addition to these defined methods, future extensions may describe alternate methods for sending the OAuth Protocol Parameters. The methods for sending other request parameters are left undefined, but SHOULD NOT use the OAuth HTTP Authorization Scheme (OAuth HTTP Authorization Scheme) header.



5.3.  Service Provider Response Parameters

Response parameters are sent by the Service Provider to return Tokens and other information to the Consumer in the HTTP response body. The parameter names and values are first encoded as per Parameter Encoding (Parameter Encoding), and concatenated with the '&' character (ASCII code 38) as defined in [RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) Section 2.1. For example:

            oauth_token=ab3cd9j4ks73hf7g&oauth_token_secret=xyz4992k83j47x0b


5.4.  OAuth HTTP Authorization Scheme

This section defines an [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” .) extension to support OAuth. It uses the standard HTTP Authorization and WWW-Authenticate headers to pass OAuth Protocol Parameters.

It is RECOMMENDED that Service Providers accept the HTTP Authorization header. Consumers SHOULD be able to send OAuth Protocol Parameters in the OAuth Authorization header.

The extension auth-scheme (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,” .)) is OAuth and is case-insensitive.



5.4.1.  Authorization Header

The OAuth Protocol Parameters are sent in the Authorization header the following way:

  1. Parameter names and values are encoded per Parameter Encoding (Parameter Encoding).
  2. For each parameter, the 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 comma 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,” .).
  4. The OPTIONAL realm parameter is 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,” .), section 1.2.

For example:

                Authorization: OAuth realm="http://sp.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"



5.4.2.  WWW-Authenticate Header

Service Providers MAY indicate their support for the extension by returning the OAuth HTTP WWW-Authenticate header upon Consumer 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,” .) such a response MAY include additional HTTP WWW-Authenticate headers:

For example:

                WWW-Authenticate: OAuth realm="http://sp.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,” .), section 1.2.



6.  Authenticating with OAuth

OAuth authentication is the process in which Users grant access to their Protected Resources without sharing their credentials with the Consumer. OAuth uses Tokens generated by the Service Provider instead of the User's credentials in Protected Resources requests. The process uses two Token types:

Request Token:
Used by the Consumer to ask the User to authorize access to the Protected Resources. The User-authorized Request Token is exchanged for an Access Token, MUST only be used once, and MUST NOT be used for any other purpose. It is RECOMMENDED that Request Tokens have a limited lifetime.
Access Token:
Used by the Consumer to access the Protected Resources on behalf of the User. Access Tokens MAY limit access to certain Protected Resources, and MAY have a limited lifetime. Service Providers SHOULD allow Users to revoke Access Tokens. Only the Access Token SHALL be used to access the Protect Resources.

OAuth Authentication is done in three steps:

  1. The Consumer obtains an unauthorized Request Token.
  2. The User authorizes the Request Token.
  3. The Consumer exchanges the Request Token for an Access Token.



6.1.  Obtaining an Unauthorized Request Token

The Consumer obtains an unauthorized Request Token by asking the Service Provider to issue a Token. The Request Token's sole purpose is to receive User approval and can only be used to obtain an Access Token. The Request Token process goes as follows:



6.1.1.  Consumer Obtains a Request Token

To obtain a Request Token, the Consumer sends an HTTP request to the Service Provider's Request Token URL. The Service Provider documentation specifies the HTTP method for this request, and HTTP POST is RECOMMENDED. The request MUST be signed and contains the following parameters:

oauth_consumer_key:
The Consumer Key.
oauth_signature_method:
The signature method the Consumer used to sign the request.
oauth_signature:
The signature as defined in Signing Requests (Signing Requests).
oauth_timestamp:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_nonce:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_version:
OPTIONAL. If present, value MUST be 1.0 . Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.
oauth_callback:
An absolute URL to which the Service Provider will redirect the User back when the Obtaining User Authorization (Obtaining User Authorization) step is completed. If the Consumer is unable to receive callbacks or a callback URL has been established via other means, the parameter value MUST be set to oob (case sensitive), to indicate an out-of-band configuration.
Additional parameters:
Any additional parameters, as defined by the Service Provider.



6.1.2.  Service Provider Issues an Unauthorized Request Token

The Service Provider verifies the signature and Consumer Key. If successful, it generates a Request Token and Token Secret and returns them to the Consumer in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters). The Service Provider MUST ensure the Request Token cannot be exchanged for an Access Token until the User successfully grants access in Obtaining User Authorization (Obtaining User Authorization).

The response contains the following parameters:

oauth_token:
The Request Token.
oauth_token_secret:
The Token Secret.
oauth_callback_confirmed:
MUST be present and set to true. The Consumer MAY use this to confirm that the Service Provider received the callback value.
Additional parameters:
Any additional parameters, as defined by the Service Provider.

If the request fails verification or is rejected for other reasons, the Service Provider SHOULD respond with the appropriate response code as defined in HTTP Response Codes (HTTP Response Codes). The Service Provider MAY include some further details about why the request was rejected in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters).



6.2.  Obtaining User Authorization

The Consumer cannot use the Request Token until it has been authorized by the User. Obtaining User authorization includes the following steps:



6.2.1.  Consumer Directs the User to the Service Provider

In order for the Consumer to be able to exchange the Request Token for an Access Token, the Consumer MUST obtain approval from the User by directing the User to the Service Provider. The Consumer constructs an HTTP GET request to the Service Provider's User Authorization URL with the following parameter:

oauth_token:
OPTIONAL. The Request Token obtained in the previous step. The Service Provider MAY declare this parameter as REQUIRED, or accept requests to the User Authorization URL without it, in which case it will prompt the User to enter it manually.
Additional parameters:
Any additional parameters, as defined by the Service Provider.

Once the request URL has been constructed the Consumer redirects the User to the URL via the User's web browser. If the Consumer is incapable of automatic HTTP redirection, the Consumer SHALL notify the User how to manually go to the constructed request URL.

Note: If a Service Provider knows a Consumer to be running on a mobile device or set-top box, the Service Provider SHOULD ensure that the User Authorization URL and Request Token are suitable for manual entry.



6.2.2.  Service Provider Authenticates the User and Obtains Consent

The Service Provider verifies the User's identity and asks for consent as detailed. OAuth does not specify how the Service Provider authenticates the User. However, it does define a set of REQUIRED steps:

When displaying any identifying information about the Consumer to the User based on the Consumer Key, the Service Provider MUST inform the User if it is unable to assure the Consumer's true identity. The method in which the Service Provider informs the User and the quality of the identity assurance is beyond the scope of this specification.



6.2.3.  Service Provider Directs the User Back to the Consumer

After the User authenticates with the Service Provider and grants permission for Consumer access, the Consumer MUST be notified that the Request Token has been authorized and ready to be exchanged for an Access Token. If the User denies access, the Consumer MAY be notified that the Request Token has been revoked.

To make sure that the User granting access is the same User returning back to the Consumer to complete the process, the Service Provider MUST generate a verification code: an unguessable value passed to the Consumer via the User and REQUIRED to complete the process.

If the Consumer provided a callback URL (using the oauth_callback parameter in Section 6.1.1 (Consumer Obtains a Request Token) or by other means), the Service Provider uses it to constructs an HTTP request, and directs the User's web browser to that URL with the following parameters added:

oauth_token:
The Request Token the User authorized or denied.
oauth_verifier:
The verification code.

The callback URL MAY include Consumer provided query parameters. The Service Provider MUST retain them unmodified and append the OAuth parameters to the existing query.

If the Consumer did not provide a callback URL, the Service Provider SHOULD display the value of the verification code, and instruct the User to manually inform the Consumer that authorization is completed. If the Service Provider knows a Consumer to be running on a mobile device or set-top box, the Service Provider SHOULD ensure that the verifier value is suitable for manual entry.



6.3.  Obtaining an Access Token

The Consumer exchanges the Request Token for an Access Token capable of accessing the Protected Resources. Obtaining an Access Token includes the following steps:



6.3.1.  Consumer Requests an Access Token

The Request Token and Token Secret MUST be exchanged for an Access Token and Token Secret.

To request an Access Token, the Consumer makes an HTTP request to the Service Provider's Access Token URL. The Service Provider documentation specifies the HTTP method for this request, and HTTP POST is RECOMMENDED. The request MUST be signed per Signing Requests (Signing Requests), and contains the following parameters:

oauth_consumer_key:
The Consumer Key.
oauth_token:
The Request Token obtained previously.
oauth_signature_method:
The signature method the Consumer used to sign the request.
oauth_signature:
The signature as defined in Signing Requests (Signing Requests).
oauth_timestamp:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_nonce:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_version:
OPTIONAL. If present, value MUST be 1.0 . Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.
oauth_verifier:
The verification code received from the Service Provider in the Service Provider Directs the User Back to the Consumer (Service Provider Directs the User Back to the Consumer) step.

No additional Service Provider specific parameters are allowed when requesting an Access Token to ensure all Token related information is present prior to seeking User approval.



6.3.2.  Service Provider Grants an Access Token

The Service Provider MUST ensure that:

If successful, the Service Provider generates an Access Token and Token Secret and returns them in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters). The Access Token and Token Secret are stored by the Consumer and used when signing Protected Resources requests. The response contains the following parameters:

oauth_token:
The Access Token.
oauth_token_secret:
The Token Secret.
Additional parameters:
Any additional parameters, as defined by the Service Provider.

If the request fails verification or is rejected for other reasons, the Service Provider SHOULD respond with the appropriate response code as defined in HTTP Response Codes (HTTP Response Codes). The Service Provider MAY include some further details about why the request was rejected in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters).



7.  Accessing Protected Resources

After successfully receiving the Access Token and Token Secret, the Consumer is able to access the Protected Resources on behalf of the User. The request MUST be signed per Signing Requests (Signing Requests), and contains the following parameters:

oauth_consumer_key:
The Consumer Key.
oauth_token:
The Access Token.
oauth_signature_method:
The signature method the Consumer used to sign the request.
oauth_signature:
The signature as defined in Signing Requests (Signing Requests).
oauth_timestamp:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_nonce:
As defined in Nonce and Timestamp (Nonce and Timestamp).
oauth_version:
OPTIONAL. If present, value MUST be 1.0. Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.
Additional parameters:
Any additional parameters, as defined by the Service Provider.



8.  Nonce and Timestamp

Unless otherwise specified by the Service Provider, the timestamp is expressed in the number of seconds since January 1, 1970 00:00:00 GMT. The timestamp value MUST be a positive integer and MUST be equal or greater than the timestamp used in previous requests.

The Consumer SHALL then generate a Nonce value that is unique for all requests with that timestamp. A nonce is a random string, uniquely generated for each request. The nonce allows the Service Provider to verify that a request has never been made before and helps prevent replay attacks when requests are made over a non-secure channel (such as HTTP).



9.  Signing Requests

All Token requests and Protected Resources requests MUST be signed by the Consumer and verified by the Service Provider. The purpose of signing requests is to prevent unauthorized parties from using the Consumer Key and Tokens when making Token requests or Protected Resources requests. The signature process encodes the Consumer Secret and Token Secret into a verifiable value which is included with the request.

OAuth does not mandate a particular signature method, as each implementation can have its own unique requirements. The protocol defines three signature methods: HMAC-SHA1, RSA-SHA1, and PLAINTEXT, but Service Providers are free to implement and document their own methods. Recommending any particular method is beyond the scope of this specification.

The Consumer declares a signature method in the oauth_signature_method parameter, generates a signature, and stores it in the oauth_signature parameter. The Service Provider verifies the signature as specified in each method. When verifying a Consumer signature, the Service Provider SHOULD check the request nonce to ensure it has not been used in a previous Consumer request.

The signature process MUST NOT change the request parameter names or values, with the exception of the oauth_signature parameter.



9.1.  Signature Base String

The Signature Base String is a consistent reproducible concatenation of the request elements into a single string. The string is used as an input in hashing or signing algorithms. The HMAC-SHA1 signature method provides both a standard and an example of using the Signature Base String with a signing algorithm to generate signatures. All the request parameters MUST be encoded as described in Parameter Encoding (Parameter Encoding) prior to constructing the Signature Base String.



9.1.1.  Normalize Request Parameters

The request parameters are collected, sorted and concatenated into a normalized string:

The oauth_signature parameter MUST be excluded.

The parameters are normalized into a single string as follows:

  1. Parameters are sorted by name, using lexicographical byte value ordering. If two or more parameters share the same name, they are sorted by their value. For example:
                        a=1, c=hi%20there, f=25, f=50, f=a, z=p, z=t
    
  2. Parameters are concatenated in their sorted order into a single string. For each parameter, the name is separated from the corresponding value by an '=' character (ASCII code 61), even if the value is empty. Each name-value pair is separated by an '&' character (ASCII code 38). For example:
                        a=1&c=hi%20there&f=25&f=50&f=a&z=p&z=t
    



9.1.2.  Construct Request URL

The Signature Base String includes the request absolute URL, tying the signature to a specific endpoint. The URL used in the Signature Base String MUST include the scheme, authority, and path, and MUST exclude the query and fragment as defined by [RFC3986] (Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” .) section 3.

If the absolute request URL is not available to the Service Provider (it is always available to the Consumer), it can be constructed by combining the scheme being used, the HTTP Host header, and the relative HTTP request URL. If the Host header is not available, the Service Provider SHOULD use the host name communicated to the Consumer in the documentation or other means.

The Service Provider SHOULD document the form of URL used in the Signature Base String to avoid ambiguity due to URL normalization. Unless specified, URL scheme and authority MUST be lowercase and include the port number; http default port 80 and https default port 443 MUST be excluded.

For example, the request:

                HTTP://Example.com:80/resource?id=123

Is included in the Signature Base String as:

                http://example.com/resource



9.1.3.  Concatenate Request Elements

The following items MUST be concatenated in order into a single string. Each item is encoded (Parameter Encoding) and separated by an '&' character (ASCII code 38), even if empty.

  1. The HTTP request method used to send the request. Value MUST be uppercase, for example: HEAD, GET , POST, etc.
  2. The request URL from Section 9.1.2 (Construct Request URL).
  3. The normalized request parameters string from Section 9.1.1 (Normalize Request Parameters).

See Signature Base String example in Appendix A.5.1 (Generating Signature Base String).



9.2.  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,” .) where the Signature Base String is the text and the key is the concatenated values (each first encoded per Parameter Encoding (Parameter Encoding)) of the Consumer Secret and Token Secret, separated by an '&' character (ASCII code 38) even if empty.



9.2.1.  Generating Signature

oauth_signature is set to the calculated digest octet string, first base64-encoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” .) section 6.8, then URL-encoded per Parameter Encoding (Parameter Encoding).



9.2.2.  Verifying Signature

The Service Provider verifies the request by generating a new request signature octet string, and comparing it to the signature provided by the Consumer, first URL-decoded per Parameter Encoding (Parameter Encoding), then base64-decoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” .) section 6.8. The signature is generated using the request parameters as provided by the Consumer, and the Consumer Secret and Token Secret as stored by the Service Provider.



9.3.  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,” .) section 8.2 (more simply known as PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5. It is assumed that the Consumer has provided its RSA public key in a verified way to the Service Provider, in a manner which is beyond the scope of this specification.



9.3.1.  Generating Signature

The Signature Base String is signed using the Consumer's RSA private key per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography; Specifications Version 2.1,” .) section 8.2.1, where K is the Consumer's RSA private key, M the Signature Base String, and S is the result signature octet string:

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

oauth_signature is set to S, first base64-encoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” .) section 6.8, then URL-encoded per Parameter Encoding (Parameter Encoding).



9.3.2.  Verifying Signature

The Service Provider verifies the signature per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography; Specifications Version 2.1,” .) section 8.2.2, where (n, e) is the Consumer's RSA public key, M is the Signature Base String, and S is the octet string representation of the oauth_signature value:

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



9.4.  PLAINTEXT

The PLAINTEXT method does not provide any security protection and SHOULD only be used over a secure channel such as HTTPS. It does not use the Signature Base String.



9.4.1.  Generating Signature

oauth_signature is set to the concatenated encoded values of the Consumer Secret and Token Secret, separated by a '&' character (ASCII code 38), even if either secret is empty. The result MUST be encoded again.

These examples show the value of oauth_signature for Consumer Secret djr9rjt0jd78jf88 and 3 different Token Secrets:

jjd999tj88uiths3:
oauth_signature=djr9rjt0jd78jf88%26jjd999tj88uiths3
jjd99$tj88uiths3:
oauth_signature=djr9rjt0jd78jf88%26jjd99%2524tj88uiths3
Empty:
oauth_signature=djr9rjt0jd78jf88%26



9.4.2.  Verifying Signature

The Service Provider verifies the request by breaking the signature value into the Consumer Secret and Token Secret, and ensures they match the secrets stored locally.



10.  HTTP Response Codes

This section applies only to the Request Token and Access Token requests. In general, the Service Provider SHOULD use the response codes defined in [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” .) Section 10. When the Service Provider rejects a Consumer request, it SHOULD respond with HTTP 400 Bad Request or HTTP 401 Unauthorized.



11.  Security Considerations



11.1.  Credentials and Token Exchange

The OAuth specification does not describe any mechanism for protecting Tokens and secrets from eavesdroppers when they are transmitted from the Service Provider to the Consumer in Section 6.1.2 (Service Provider Issues an Unauthorized Request Token) and Section 6.3.2 (Service Provider Grants an Access Token). Service Providers should ensure that these transmissions are protected using transport-layer mechanisms such as TLS or SSL.



11.2.  PLAINTEXT Signature Method

When used with PLAINTEXT signatures, the OAuth protocol makes no attempts to protect User credentials from eavesdroppers or man-in-the-middle attacks. The PLAINTEXT signature algorithm is only intended to be used in conjunction with a transport-layer security mechanism such as TLS or SSL which does provide such protection. If transport-layer protection is unavailable, the PLAINTEXT signature method should not be used.



11.3.  Confidentiality of Requests

While OAuth 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. Service Providers 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.



11.4.  Spoofing by Counterfeit Servers

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



11.5.  Proxying and Caching of Authenticated Content

The HTTP Authorization scheme (OAuth HTTP Authorization Scheme) 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,” .) 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. Service Providers not using the HTTP Authorization scheme (OAuth HTTP Authorization Scheme) should take care to use other mechanisms, such as the Cache-Control header, to ensure that authenticated content is protected.



11.6.  Plaintext Storage of Credentials

The Consumer Secret and Token Secret function the same way passwords do in traditional authentication systems. In order to compute the signatures used in the non-PLAINTEXT methods, the Service Provider 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 Service Provider's database of all such secrets - he or she would be able to perform any action on behalf of any User. Accordingly, it is critical that Service Providers protect these secrets from unauthorized access.



11.7.  Secrecy of the Consumer Secret

In many applications, the Consumer application will be under the control of potentially untrusted parties. For example, if the Consumer is a freely available desktop application, an attacker may be able to download a copy for analysis. In such cases, attackers will be able to recover the Consumer Secret used to authenticate the Consumer to the Service Provider.

Accordingly, Service Providers should not use the Consumer Secret alone to verify the identity of the Consumer. Where possible, other factors such as IP address should be used as well.



11.8.  Phishing Attacks

Wide deployment of OAuth and similar protocols may cause Users to become inured to the practice of being redirected to websites where they are asked to enter their passwords. If Users 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 Users' passwords.

Service Providers should attempt to educate Users about the risks phishing attacks pose, and should provide mechanisms that make it easy for Users to confirm the authenticity of their sites.



11.9.  Scoping of Access Requests

By itself, OAuth does not provide any method for scoping the access rights granted to a Consumer. A Consumer either has access to Protected Resources or it doesn't. Many applications will, however, require greater granularity of access rights. For example, Service Providers 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 OAuth, Service Providers should consider the types of access Users may wish to grant Consumers, and should provide mechanisms to do so. Service Providers should also take care to ensure that Users understand the access they are granting, as well as any risks that may be involved.



11.10.  Entropy of Secrets

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

For example, if Token Secrets are valid for two weeks, Service Providers should ensure that it is not possible to mount a brute force attack that recovers the Token Secret in less than two weeks. Of course, Service Providers 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. Implementors should be careful to use cryptographically secure PRNGs to avoid these problems.



11.11.  Denial of Service / Resource Exhaustion Attacks

The OAuth protocol has a number of features which may make resource exhaustion attacks against Service Providers possible. For example, if a Service Provider includes a nontrivial amount of entropy in Token Secrets as recommended above, then an attacker may be able to exhaust the Service Provider's entropy pool very quickly by repeatedly obtaining Request Tokens from the Service Provider.

Similarly, OAuth requires Service Providers 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, OAuth can require Service Providers 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 Service Provider.

Resource Exhaustion attacks are by no means specific to OAuth. However, OAuth implementors should be careful to consider the additional avenues of attack that OAuth 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 OAuth, Service Providers should consider which of these presents a more serious risk for their application and design accordingly.



11.12.  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] 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 OAuth. They may, however, make more efficient attacks possible, and NIST has announced (National Institute of Standards and Technolog, NIST., “NIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1,” .) [NIST] that it will phase out use of SHA-1 by 2010. Service Providers should take this into account when considering whether SHA-1 provides an adequate level of security for their applications.



11.13.  Signature Base String Compatibility

The Signature Base String has been designed to support the signature methods defined in this specification. When designing additional signature methods, the Signature Base String should be evaluated to ensure compatibility with the algorithms used.

The Signature Base String cannot guarantee the order in which parameters are sent. If parameter ordering is important and affects the result of a request, the Signature Base String will not protect against request manipulation.



11.14.  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 OAuth approvals can allow an attacker to obtain authorization to OAuth Protected Resources without the consent of the User. Service Providers SHOULD strongly consider best practices in CSRF prevention at all OAuth endpoints.

CSRF attacks on OAuth callback URLs hosted by Consumers are also possible. Consumers should prevent CSRF attacks on OAuth callback URLs by verifying that the User at the Consumer site intended to complete the OAuth negotiation with the Service Provider.



11.15.  User Interface Redress

Service Providers 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. Service Providers can mitigate the risk of UI redress attacks through the following techniques:



11.16.  Automatic Processing of Repeat Authorizations

Service Providers may wish to automatically process authorization requests (Section 6.2 (Obtaining User Authorization)) from Consumers which have been previously authorized by the user. When the User is redirected to the Service Provider to grant access, the Service Provider detects that the User has already granted access to that particular Consumer. Instead of prompting the User for approval, the Service Provider automatically redirects the User back to the Provider.

If the Consumer Secret is compromised, automatic processing creates additional security risks. An attacker can use the stolen Consumer Key and Secret to redirect the User to the Service Provider with an authorization request. The Service Provider will then grant access to the User's data without the User's explicit approval, or even awareness of an attack. If no automatic approval is implemented, an attacker must use social engineering to convince the User to approve access.

Service Providers can mitigate the risks associated with automatic processing by limiting the scope of Access Tokens obtained through automated approvals. Access Tokens obtained through explicit User consent can remain unaffected. Consumers can mitigate the risks associated with automatic processing by protecting their Consumer Secret.



Appendix A.  Appendix A - Protocol Example

In this example, the Service Provider photos.example.net is a photo sharing website, and the Consumer printer.example.com is a photo printing website. Jane, the User, would like printer.example.com to print the private photo vacation.jpg stored at photos.example.net.

When Jane signs-into photos.example.net using her username and password, she can access the photo by going to the URL http://photos.example.net/photo?file=vacation.jpg. Other Users cannot access that photo, and Jane does not want to share her username and password with printer.example.com.

The requests in this example use the URL query method when sending parameters. This is done to simplify the example and should not be taken as an endorsement of one method over the others.



Appendix A.1.  Documentation and Registration

The Service Provider documentation explains how to register for a Consumer Key and Consumer Secret, and declares the following URLs:

Request Token URL:
https://photos.example.net/request_token, using HTTP POST
User Authorization URL:
http://photos.example.net/authorize, using HTTP GET
Access Token URL:
https://photos.example.net/access_token, using HTTP POST
Photo (Protected Resource) URL:
http://photos.example.net/photo with required parameter file and optional parameter size

The Service Provider declares support for the HMAC-SHA1 signature method for all requests, and PLAINTEXT only for secure (HTTPS) requests.

The Consumer printer.example.com already established a Consumer Key and Consumer Secret with photos.example.net and advertizes its printing services for photos stored on photos.example.net. The Consumer registration is:

Consumer Key:
dpf43f3p2l4k3l03
Consumer Secret:
kd94hf93k423kf44



Appendix A.2.  Obtaining a Request Token

After Jane informs printer.example.com that she would like to print her vacation photo stored at photos.example.net, the printer website tries to access the photo and receives HTTP 401 Unauthorized indicating it is private. The Service Provider includes the following header with the response:

              WWW-Authenticate: OAuth realm="http://photos.example.net/"

The Consumer sends the following HTTP POST request to the Service Provider:

              https://photos.example.net/request_token?oauth_consumer_key=dpf43f3p2l4k3l03&oauth_signature_method=PLAINTEXT&oauth_signature=kd94hf93k423kf44%26&oauth_timestamp=1191242090&oauth_nonce=hsu94j3884jdopsl&oauth_version=1.0&oauth_callback=http%3A%2F%2Fprinter.example.com%2Frequest_token_ready

The Service Provider checks the signature and replies with an unauthorized Request Token in the body of the HTTP response:

              oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03&oauth_callback_confirmed=true



Appendix A.3.  Requesting User Authorization

The Consumer redirects Jane's browser to the Service Provider User Authorization URL to obtain Jane's approval for accessing her private photos.

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

The Service Provider asks Jane to sign-in using her username and password and, if successful, asks her if she approves granting printer.example.com access to her private photos. If Jane approves the request, the Service Provider generates a verification code and redirects her back to the Consumer's callback URL:

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



Appendix A.4.  Obtaining an Access Token

Now that the Consumer knows Jane approved the Request Token, it asks the Service Provider to exchange it for an Access Token:

              https://photos.example.net/access_token?oauth_consumer_key=dpf43f3p2l4k3l03&oauth_token=hh5s93j4hdidpola&oauth_signature_method=PLAINTEXT&oauth_signature=kd94hf93k423kf44%26hdhd0244k9j7ao03&oauth_timestamp=1191242092&oauth_nonce=dji430splmx33448&oauth_version=1.0&oauth_verifier=hfdp7dh39dks9884

The Service Provider checks the signature and the verification code and replies with an Access Token in the body of the HTTP response:

              oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00



Appendix A.5.  Accessing Protected Resources

The Consumer is now ready to request the private photo. Since the photo URL is not secure (HTTP), it must use HMAC-SHA1.



Appendix A.5.1.  Generating Signature Base String

To generate the signature, it first needs to generate the Signature Base String. The request contains the following parameters (oauth_signature excluded) which are ordered and concatenated into a normalized string:

oauth_consumer_key:
dpf43f3p2l4k3l03
oauth_token:
nnch734d00sl2jdk
oauth_signature_method:
HMAC-SHA1
oauth_timestamp:
1191242096
oauth_nonce:
kllo9940pd9333jh
oauth_version:
1.0
file:
vacation.jpg
size:
original

The following inputs are used to generate the Signature Base String:

  1. GET
  2. http://photos.example.net/photos
  3. file=vacation.jpg&oauth_consumer_key=dpf43f3p2l4k3l03&oauth_nonce=kllo9940pd9333jh&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1191242096&oauth_token=nnch734d00sl2jdk&oauth_version=1.0&size=original

The Signature Base String is:

                GET&http%3A%2F%2Fphotos.example.net%2Fphotos&file%3Dvacation.jpg%26oauth_consumer_key%3Ddpf43f3p2l4k3l03%26oauth_nonce%3Dkllo9940pd9333jh%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1191242096%26oauth_token%3Dnnch734d00sl2jdk%26oauth_version%3D1.0%26size%3Doriginal



Appendix A.5.2.  Calculating Signature Value

HMAC-SHA1 produces the following digest value as a base64-encoded string (using the Signature Base String as text and kd94hf93k423kf44&pfkkdhi9sl3r4s00 as key):

                tR3+Ty81lMeYAr/Fid0kMTYa/WM=



Appendix A.5.3.  Requesting Protected Resource

All together, the Consumer request for the photo is:

                http://photos.example.net/photos?file=vacation.jpg&size=original

                Authorization: OAuth realm="http://photos.example.net/",
                oauth_consumer_key="dpf43f3p2l4k3l03",
                oauth_token="nnch734d00sl2jdk",
                oauth_signature_method="HMAC-SHA1",
                oauth_signature="tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D",
                oauth_timestamp="1191242096",
                oauth_nonce="kllo9940pd9333jh",
                oauth_version="1.0"

And if using query parameters:

                http://photos.example.net/photos?file=vacation.jpg&size=original&oauth_consumer_key=dpf43f3p2l4k3l03&oauth_token=nnch734d00sl2jdk&oauth_signature_method=HMAC-SHA1&oauth_signature=tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D&oauth_timestamp=1191242096&oauth_nonce=kllo9940pd9333jh&oauth_version=1.0

photos.example.net checks the signature and responds with the requested photo.



12. References

[NIST] National Institute of Standards and Technolog, NIST., “NIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1.”
[RFC2045] Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” RFC 2045.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” RFC 2104.
[RFC2119] Bradner, B., “Key words for use in RFCs to Indicate Requirement Levels,” RFC 2119.
[RFC2606] Eastlake, D. and A. Panitz, “Reserved Top Level DNS Names,” RFC 2606.
[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.
[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.
[RFC3447] Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography; Specifications Version 2.1,” RFC 3447.
[RFC3629] Yergeau, F., “UTF-8, a transformation format of Unicode and ISO 10646,” RFC 3629.
[RFC3986] Berners-Lee, T., “Uniform Resource Identifiers (URI): Generic Syntax,” RFC 3986.
[SHA1] De Canniere, C. and C. Rechberger, “Finding SHA-1 Characteristics: General Results and Applications.”


Author's Address

  OAuth Core Workgroup
Email:  spec@oauth.net