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Internet Engineering Task Force (IETF) E. Hammer-Lahav, Ed. Request for Comments: 5849 April 2010 Category: Informational ISSN: 2070-1721 The OAuth 1.0 Protocol Abstract OAuth provides a method for clients to access server resources on behalf of a resource owner (such as a different client or an end- user). It also provides a process for end-users to authorize third- party access to their server resources without sharing their credentials (typically, a username and password pair), using user- agent redirections. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5849. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Hammer-Lahav Informational [Page 1] RFC 5849 OAuth 1.0 April 2010 Table of Contents 1. Introduction ....................................................3 1.1. Terminology ................................................4 1.2. Example ....................................................5 1.3. Notational Conventions .....................................7 2. Redirection-Based Authorization .................................8 2.1. Temporary Credentials ......................................9 2.2. Resource Owner Authorization ..............................10 2.3. Token Credentials .........................................12 3. Authenticated Requests .........................................14 3.1. Making Requests ...........................................14 3.2. Verifying Requests ........................................16 3.3. Nonce and Timestamp .......................................17 3.4. Signature .................................................18 3.4.1. Signature Base String ..............................18 3.4.2. HMAC-SHA1 ..........................................25 3.4.3. RSA-SHA1 ...........................................25 3.4.4. PLAINTEXT ..........................................26 3.5. Parameter Transmission ....................................26 3.5.1. Authorization Header ...............................27 3.5.2. Form-Encoded Body ..................................28 3.5.3. Request URI Query ..................................28 3.6. Percent Encoding ..........................................29 4. Security Considerations ........................................29 4.1. RSA-SHA1 Signature Method .................................29 4.2. Confidentiality of Requests ...............................30 4.3. Spoofing by Counterfeit Servers ...........................30 4.4. Proxying and Caching of Authenticated Content .............30 4.5. Plaintext Storage of Credentials ..........................30 4.6. Secrecy of the Client Credentials .........................31 4.7. Phishing Attacks ..........................................31 4.8. Scoping of Access Requests ................................31 4.9. Entropy of Secrets ........................................32 4.10. Denial-of-Service / Resource-Exhaustion Attacks ..........32 4.11. SHA-1 Cryptographic Attacks ..............................33 4.12. Signature Base String Limitations ........................33 4.13. Cross-Site Request Forgery (CSRF) ........................33 4.14. User Interface Redress ...................................34 4.15. Automatic Processing of Repeat Authorizations ............34 5. Acknowledgments ................................................35 Appendix A. Differences from the Community Edition ...............36 6. References .....................................................37 6.1. Normative References ......................................37 6.2. Informative References ....................................38 Hammer-Lahav Informational [Page 2] RFC 5849 OAuth 1.0 April 2010 1. Introduction The OAuth protocol was originally created by a small community of web developers from a variety of websites and other Internet services who wanted to solve the common problem of enabling delegated access to protected resources. The resulting OAuth protocol was stabilized at version 1.0 in October 2007, and revised in June 2009 (Revision A) as published at <http://oauth.net/core/1.0a>. This specification provides an informational documentation of OAuth Core 1.0 Revision A, addresses several errata reported since that time, and makes numerous editorial clarifications. While this specification is not an item of the IETF's OAuth Working Group, which at the time of writing is working on an OAuth version that can be appropriate for publication on the standards track, it has been transferred to the IETF for change control by authors of the original work. In the traditional client-server authentication model, the client uses its credentials to access its resources hosted by the server. With the increasing use of distributed web services and cloud computing, third-party applications require access to these server- hosted resources. OAuth introduces a third role to the traditional client-server authentication model: the resource owner. In the OAuth model, the client (which is not the resource owner, but is acting on its behalf) requests access to resources controlled by the resource owner, but hosted by the server. In addition, OAuth allows the server to verify not only the resource owner authorization, but also the identity of the client making the request. OAuth provides a method for clients to access server resources on behalf of a resource owner (such as a different client or an end- user). It also provides a process for end-users to authorize third- party access to their server resources without sharing their credentials (typically, a username and password pair), using user- agent redirections. For example, a web user (resource owner) can grant a printing service (client) access to her private photos stored at a photo sharing service (server), without sharing her username and password with the printing service. Instead, she authenticates directly with the photo sharing service which issues the printing service delegation-specific credentials. Hammer-Lahav Informational [Page 3] RFC 5849 OAuth 1.0 April 2010 In order for the client to access resources, it first has to obtain permission from the resource owner. This permission is expressed in the form of a token and matching shared-secret. The purpose of the token is to make it unnecessary for the resource owner to share its credentials with the client. Unlike the resource owner credentials, tokens can be issued with a restricted scope and limited lifetime, and revoked independently. This specification consists of two parts. The first part defines a redirection-based user-agent process for end-users to authorize client access to their resources, by authenticating directly with the server and provisioning tokens to the client for use with the authentication method. The second part defines a method for making authenticated HTTP [RFC2616] requests using two sets of credentials, one identifying the client making the request, and a second identifying the resource owner on whose behalf the request is being made. The use of OAuth with any transport protocol other than [RFC2616] is undefined. 1.1. Terminology client An HTTP client (per [RFC2616]) capable of making OAuth- authenticated requests (Section 3). server An HTTP server (per [RFC2616]) capable of accepting OAuth- authenticated requests (Section 3). protected resource An access-restricted resource that can be obtained from the server using an OAuth-authenticated request (Section 3). resource owner An entity capable of accessing and controlling protected resources by using credentials to authenticate with the server. credentials Credentials are a pair of a unique identifier and a matching shared secret. OAuth defines three classes of credentials: client, temporary, and token, used to identify and authenticate the client making the request, the authorization request, and the access grant, respectively. Hammer-Lahav Informational [Page 4] RFC 5849 OAuth 1.0 April 2010 token A unique identifier issued by the server and used by the client to associate authenticated requests with the resource owner whose authorization is requested or has been obtained by the client. Tokens have a matching shared-secret that is used by the client to establish its ownership of the token, and its authority to represent the resource owner. The original community specification used a somewhat different terminology that maps to this specifications as follows (original community terms provided on left): Consumer: client Service Provider: server User: resource owner Consumer Key and Secret: client credentials Request Token and Secret: temporary credentials Access Token and Secret: token credentials 1.2. Example Jane (resource owner) has recently uploaded some private vacation photos (protected resources) to her photo sharing site 'photos.example.net' (server). She would like to use the 'printer.example.com' website (client) to print one of these photos. Typically, Jane signs into 'photos.example.net' using her username and password. However, Jane does not wish to share her username and password with the 'printer.example.com' website, which needs to access the photo in order to print it. In order to provide its users with better service, 'printer.example.com' has signed up for a set of 'photos.example.net' client credentials ahead of time: Client Identifier dpf43f3p2l4k3l03 Client Shared-Secret: kd94hf93k423kf44 The 'printer.example.com' website has also configured its application to use the protocol endpoints listed in the 'photos.example.net' API documentation, which use the "HMAC-SHA1" signature method: Hammer-Lahav Informational [Page 5] RFC 5849 OAuth 1.0 April 2010 Temporary Credential Request https://photos.example.net/initiate Resource Owner Authorization URI: https://photos.example.net/authorize Token Request URI: https://photos.example.net/token Before 'printer.example.com' can ask Jane to grant it access to the photos, it must first establish a set of temporary credentials with 'photos.example.net' to identify the delegation request. To do so, the client sends the following HTTPS [RFC2818] request to the server: POST /initiate HTTP/1.1 Host: photos.example.net Authorization: OAuth realm="Photos", oauth_consumer_key="dpf43f3p2l4k3l03", oauth_signature_method="HMAC-SHA1", oauth_timestamp="137131200", oauth_nonce="wIjqoS", oauth_callback="http%3A%2F%2Fprinter.example.com%2Fready", oauth_signature="74KNZJeDHnMBp0EMJ9ZHt%2FXKycU%3D" The server validates the request and replies with a set of temporary credentials in the body of the HTTP response (line breaks are for display purposes only): HTTP/1.1 200 OK Content-Type: application/x-www-form-urlencoded oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03& oauth_callback_confirmed=true The client redirects Jane's user-agent to the server's Resource Owner Authorization endpoint to obtain Jane's approval for accessing her private photos: https://photos.example.net/authorize?oauth_token=hh5s93j4hdidpola The server requests Jane to sign in using her username and password and if successful, asks her to approve granting 'printer.example.com' access to her private photos. Jane approves the request and her user-agent is redirected to the callback URI provided by the client in the previous request (line breaks are for display purposes only): http://printer.example.com/ready? oauth_token=hh5s93j4hdidpola&oauth_verifier=hfdp7dh39dks9884 Hammer-Lahav Informational [Page 6] RFC 5849 OAuth 1.0 April 2010 The callback request informs the client that Jane completed the authorization process. The client then requests a set of token credentials using its temporary credentials (over a secure Transport Layer Security (TLS) channel): POST /token HTTP/1.1 Host: photos.example.net Authorization: OAuth realm="Photos", 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="Photos", 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' server validates the request and responds with the requested photo. 'printer.example.com' is able to continue accessing Jane's private photos using the same set of token credentials for the duration of Jane's authorization, or until Jane revokes access. 1.3. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Hammer-Lahav Informational [Page 7] RFC 5849 OAuth 1.0 April 2010 2. Redirection-Based Authorization OAuth uses tokens to represent the authorization granted to the client by the resource owner. Typically, token credentials are issued by the server at the resource owner's request, after authenticating the resource owner's identity (usually using a username and password). There are many ways in which a server can facilitate the provisioning of token credentials. This section defines one such way, using HTTP redirections and the resource owner's user-agent. This redirection- based authorization method includes three steps: 1. The client obtains a set of temporary credentials from the server (in the form of an identifier and shared-secret). The temporary credentials are used to identify the access request throughout the authorization process. 2. The resource owner authorizes the server to grant the client's access request (identified by the temporary credentials). 3. The client uses the temporary credentials to request a set of token credentials from the server, which will enable it to access the resource owner's protected resources. The server MUST revoke the temporary credentials after being used once to obtain the token credentials. It is RECOMMENDED that the temporary credentials have a limited lifetime. Servers SHOULD enable resource owners to revoke token credentials after they have been issued to clients. In order for the client to perform these steps, the server needs to advertise the URIs of the following three endpoints: Temporary Credential Request The endpoint used by the client to obtain a set of temporary credentials as described in Section 2.1. Resource Owner Authorization The endpoint to which the resource owner is redirected to grant authorization as described in Section 2.2. Token Request The endpoint used by the client to request a set of token credentials using the set of temporary credentials as described in Section 2.3. Hammer-Lahav Informational [Page 8] RFC 5849 OAuth 1.0 April 2010 The three URIs advertised by the server MAY include a query component as defined by [RFC3986], Section 3, but if present, the query MUST NOT contain any parameters beginning with the "oauth_" prefix, to avoid conflicts with the protocol parameters added to the URIs when used. The methods in which the server advertises and documents its three endpoints are beyond the scope of this specification. Clients should avoid making assumptions about the size of tokens and other server- generated values, which are left undefined by this specification. In addition, protocol parameters MAY include values that require encoding when transmitted. Clients and servers should not make assumptions about the possible range of their values. 2.1. Temporary Credentials The client obtains a set of temporary credentials from the server by making an authenticated (Section 3) HTTP "POST" request to the Temporary Credential Request endpoint (unless the server advertises another HTTP request method for the client to use). The client constructs a request URI by adding the following REQUIRED parameter to the request (in addition to the other protocol parameters, using the same parameter transmission method): oauth_callback: An absolute URI back to which the server will redirect the resource owner when the Resource Owner Authorization step (Section 2.2) is completed. If the client is unable to receive callbacks or a callback URI has been established via other means, the parameter value MUST be set to "oob" (case sensitive), to indicate an out-of-band configuration. Servers MAY specify additional parameters. When making the request, the client authenticates using only the client credentials. The client MAY omit the empty "oauth_token" protocol parameter from the request and MUST use the empty string as the token secret value. Since the request results in the transmission of plain text credentials in the HTTP response, the server MUST require the use of a transport-layer mechanisms such as TLS or Secure Socket Layer (SSL) (or a secure channel with equivalent protections). Hammer-Lahav Informational [Page 9] RFC 5849 OAuth 1.0 April 2010 For example, the client makes the following HTTPS request: POST /request_temp_credentials HTTP/1.1 Host: server.example.com Authorization: OAuth realm="Example", oauth_consumer_key="jd83jd92dhsh93js", oauth_signature_method="PLAINTEXT", oauth_callback="http%3A%2F%2Fclient.example.net%2Fcb%3Fx%3D1", oauth_signature="ja893SD9%26" The server MUST verify (Section 3.2) the request and if valid, respond back to the client with a set of temporary credentials (in the form of an identifier and shared-secret). The temporary credentials are included in the HTTP response body using the "application/x-www-form-urlencoded" content type as defined by [W3C.REC-html40-19980424] with a 200 status code (OK). The response contains the following REQUIRED parameters: oauth_token The temporary credentials identifier. oauth_token_secret The temporary credentials shared-secret. oauth_callback_confirmed MUST be present and set to "true". The parameter is used to differentiate from previous versions of the protocol. Note that even though the parameter names include the term 'token', these credentials are not token credentials, but are used in the next two steps in a similar manner to token credentials. For example (line breaks are for display purposes only): HTTP/1.1 200 OK 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 REQUIRED query parameter to the Resource Owner Authorization endpoint URI: Hammer-Lahav Informational [Page 10] RFC 5849 OAuth 1.0 April 2010 oauth_token The temporary credentials identifier obtained in Section 2.1 in the "oauth_token" parameter. Servers MAY declare this parameter as OPTIONAL, in which case they MUST provide a way for the resource owner to indicate the identifier through other means. Servers MAY specify additional parameters. The client directs the resource owner to the constructed URI using an HTTP redirection response, or by other means available to it via the resource owner's user-agent. The request MUST use the HTTP "GET" method. For example, the client redirects the resource owner's user-agent to make the following HTTPS request: GET /authorize_access?oauth_token=hdk48Djdsa HTTP/1.1 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 REQUIRED parameters to the callback URI query component: oauth_token The temporary credentials identifier received from the client. Hammer-Lahav Informational [Page 11] RFC 5849 OAuth 1.0 April 2010 oauth_verifier The verification code. If the callback URI already includes a query component, the server MUST append the OAuth parameters to the end of the existing query. For example, the server redirects the resource owner's user-agent to make the following HTTP request: GET /cb?x=1&oauth_token=hdk48Djdsa&oauth_verifier=473f82d3 HTTP/1.1 Host: client.example.net If the client did not provide a callback URI, the server SHOULD display the value of the verification code, and instruct the resource owner to manually inform the client that authorization is completed. If the server knows a client to be running on a limited device, it SHOULD ensure that the verifier value is suitable for manual entry. 2.3. Token Credentials The client obtains a set of token credentials from the server by making an authenticated (Section 3) HTTP "POST" request to the Token Request endpoint (unless the server advertises another HTTP request method for the client to use). The client constructs a request URI by adding the following REQUIRED parameter to the request (in addition to the other protocol parameters, using the same parameter transmission method): oauth_verifier The verification code received from the server in the previous step. When making the request, the client authenticates using the client credentials as well as the temporary credentials. The temporary credentials are used as a substitute for token credentials in the authenticated request and transmitted using the "oauth_token" parameter. Since the request results in the transmission of plain text credentials in the HTTP response, the server MUST require the use of a transport-layer mechanism such as TLS or SSL (or a secure channel with equivalent protections). Hammer-Lahav Informational [Page 12] RFC 5849 OAuth 1.0 April 2010 For example, the client makes the following HTTPS request: POST /request_token HTTP/1.1 Host: server.example.com Authorization: OAuth realm="Example", oauth_consumer_key="jd83jd92dhsh93js", oauth_token="hdk48Djdsa", oauth_signature_method="PLAINTEXT", oauth_verifier="473f82d3", oauth_signature="ja893SD9%26xyz4992k83j47x0b" The server MUST verify (Section 3.2) the validity of the request, ensure that the resource owner has authorized the provisioning of token credentials to the client, and ensure that the temporary credentials have not expired or been used before. The server MUST also verify the verification code received from the client. If the request is valid and authorized, the token credentials are included in the HTTP response body using the "application/x-www-form-urlencoded" content type as defined by [W3C.REC-html40-19980424] with a 200 status code (OK). The response contains the following REQUIRED parameters: oauth_token The token identifier. oauth_token_secret The token shared-secret. For example: HTTP/1.1 200 OK Content-Type: application/x-www-form-urlencoded oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk The server must retain the scope, duration, and other attributes approved by the resource owner, and enforce these restrictions when receiving a client request made with the token credentials issued. Once the client receives and stores the token credentials, it can proceed to access protected resources on behalf of the resource owner by making authenticated requests (Section 3) using the client credentials together with the token credentials received. Hammer-Lahav Informational [Page 13] RFC 5849 OAuth 1.0 April 2010 3. Authenticated Requests The HTTP authentication methods defined by [RFC2617] enable clients to make authenticated HTTP requests. Clients using these methods gain access to protected resources by using their credentials (typically, a username and password pair), which allow the server to verify their authenticity. Using these methods for delegation requires the client to assume the role of the resource owner. OAuth provides a method designed to include two sets of credentials with each request, one to identify the client, and another to identify the resource owner. Before a client can make authenticated requests on behalf of the resource owner, it must obtain a token authorized by the resource owner. Section 2 provides one such method through which the client can obtain a token authorized by the resource owner. The client credentials take the form of a unique identifier and an associated shared-secret or RSA key pair. Prior to making authenticated requests, the client establishes a set of credentials with the server. The process and requirements for provisioning these are outside the scope of this specification. Implementers are urged to consider the security ramifications of using client credentials, some of which are described in Section 4.6. Making authenticated requests requires prior knowledge of the server's configuration. OAuth includes multiple methods for transmitting protocol parameters with requests (Section 3.5), as well as multiple methods for the client to prove its rightful ownership of the credentials used (Section 3.4). The way in which clients discover the required configuration is outside the scope of this specification. 3.1. Making Requests An authenticated request includes several protocol parameters. Each parameter name begins with the "oauth_" prefix, and the parameter names and values are case sensitive. Clients make authenticated requests by calculating the values of a set of protocol parameters and adding them to the HTTP request as follows: 1. The client assigns value to each of these REQUIRED (unless specified otherwise) protocol parameters: Hammer-Lahav Informational [Page 14] RFC 5849 OAuth 1.0 April 2010 oauth_consumer_key The identifier portion of the client credentials (equivalent to a username). The parameter name reflects a deprecated term (Consumer Key) used in previous revisions of the specification, and has been retained to maintain backward compatibility. oauth_token The token value used to associate the request with the resource owner. If the request is not associated with a resource owner (no token available), clients MAY omit the parameter. oauth_signature_method The name of the signature method used by the client to sign the request, as defined in Section 3.4. oauth_timestamp The timestamp value as defined in Section 3.3. The parameter MAY be omitted when using the "PLAINTEXT" signature method. oauth_nonce The nonce value as defined in Section 3.3. The parameter MAY be omitted when using the "PLAINTEXT" signature method. oauth_version OPTIONAL. If present, MUST be set to "1.0". Provides the version of the authentication process as defined in this specification. 2. The protocol parameters are added to the request using one of the transmission methods listed in Section 3.5. Each parameter MUST NOT appear more than once per request. 3. The client calculates and assigns the value of the "oauth_signature" parameter as described in Section 3.4 and adds the parameter to the request using the same method as in the previous step. 4. The client sends the authenticated HTTP request to the server. For example, to make the following HTTP request authenticated (the "c2&a3=2+q" string in the following examples is used to illustrate the impact of a form-encoded entity-body): POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1 Host: example.com Content-Type: application/x-www-form-urlencoded c2&a3=2+q Hammer-Lahav Informational [Page 15] RFC 5849 OAuth 1.0 April 2010 The client assigns values to the following protocol parameters using its client credentials, token credentials, the current timestamp, a uniquely generated nonce, and indicates that it will use the "HMAC-SHA1" signature method: oauth_consumer_key: 9djdj82h48djs9d2 oauth_token: kkk9d7dh3k39sjv7 oauth_signature_method: HMAC-SHA1 oauth_timestamp: 137131201 oauth_nonce: 7d8f3e4a The client adds the protocol parameters to the request using the OAuth HTTP "Authorization" header field: Authorization: OAuth realm="Example", oauth_consumer_key="9djdj82h48djs9d2", oauth_token="kkk9d7dh3k39sjv7", oauth_signature_method="HMAC-SHA1", oauth_timestamp="137131201", oauth_nonce="7d8f3e4a" Then, it calculates the value of the "oauth_signature" parameter (using client secret "j49sk3j29djd" and token secret "dh893hdasih9"), adds it to the request, and sends the HTTP request to the server: POST /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="Example", oauth_consumer_key="9djdj82h48djs9d2", oauth_token="kkk9d7dh3k39sjv7", oauth_signature_method="HMAC-SHA1", oauth_timestamp="137131201", oauth_nonce="7d8f3e4a", oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D" c2&a3=2+q 3.2. Verifying Requests Servers receiving an authenticated request MUST validate it by: o Recalculating the request signature independently as described in Section 3.4 and comparing it to the value received from the client via the "oauth_signature" parameter. Hammer-Lahav Informational [Page 16] RFC 5849 OAuth 1.0 April 2010 o If using the "HMAC-SHA1" or "RSA-SHA1" signature methods, ensuring that the combination of nonce/timestamp/token (if present) received from the client has not been used before in a previous request (the server MAY reject requests with stale timestamps as described in Section 3.3). o If a token is present, verifying the scope and status of the client authorization as represented by the token (the server MAY choose to restrict token usage to the client to which it was issued). o If the "oauth_version" parameter is present, ensuring its value is "1.0". If the request fails verification, the server SHOULD respond with the appropriate HTTP response status code. The server MAY include further details about why the request was rejected in the response body. The server SHOULD return a 400 (Bad Request) status code when receiving a request with unsupported parameters, an unsupported signature method, missing parameters, or duplicated protocol parameters. The server SHOULD return a 401 (Unauthorized) status code when receiving a request with invalid client credentials, an invalid or expired token, an invalid signature, or an invalid or used nonce. 3.3. Nonce and Timestamp The timestamp value MUST be a positive integer. Unless otherwise specified by the server's documentation, the timestamp is expressed in the number of seconds since January 1, 1970 00:00:00 GMT. A nonce is a random string, uniquely generated by the client to allow the server to verify that a request has never been made before and helps prevent replay attacks when requests are made over a non-secure channel. The nonce value MUST be unique across all requests with the same timestamp, client credentials, and token combinations. To avoid the need to retain an infinite number of nonce values for future checks, servers MAY choose to restrict the time period after which a request with an old timestamp is rejected. Note that this restriction implies a level of synchronization between the client's and server's clocks. Servers applying such a restriction MAY provide a way for the client to sync with the server's clock; alternatively, both systems could synchronize with a trusted time service. Details of clock synchronization strategies are beyond the scope of this specification. Hammer-Lahav Informational [Page 17] RFC 5849 OAuth 1.0 April 2010 3.4. Signature OAuth-authenticated requests can have two sets of credentials: those passed via the "oauth_consumer_key" parameter and those in the "oauth_token" parameter. In order for the server to verify the authenticity of the request and prevent unauthorized access, the client needs to prove that it is the rightful owner of the credentials. This is accomplished using the shared-secret (or RSA key) part of each set of credentials. OAuth provides three methods for the client to prove its rightful ownership of the credentials: "HMAC-SHA1", "RSA-SHA1", and "PLAINTEXT". These methods are generally referred to as signature methods, even though "PLAINTEXT" does not involve a signature. In addition, "RSA-SHA1" utilizes an RSA key instead of the shared- secrets associated with the client credentials. OAuth does not mandate a particular signature method, as each implementation can have its own unique requirements. Servers are free to implement and document their own custom methods. Recommending any particular method is beyond the scope of this specification. Implementers should review the Security Considerations section (Section 4) before deciding on which method to support. The client declares which signature method is used via the "oauth_signature_method" parameter. It then generates a signature (or a string of an equivalent value) and includes it in the "oauth_signature" parameter. The server verifies the signature as specified for each method. The signature process does not change the request or its parameters, with the exception of the "oauth_signature" parameter. 3.4.1. Signature Base String The signature base string is a consistent, reproducible concatenation of several of the HTTP request elements into a single string. The string is used as an input to the "HMAC-SHA1" and "RSA-SHA1" signature methods. The signature base string includes the following components of the HTTP request: o The HTTP request method (e.g., "GET", "POST", etc.). o The authority as declared by the HTTP "Host" request header field. Hammer-Lahav Informational [Page 18] RFC 5849 OAuth 1.0 April 2010 o The path and query components of the request resource URI. o The protocol parameters excluding the "oauth_signature". o Parameters included in the request entity-body if they comply with the strict restrictions defined in Section 3.4.1.3. The signature base string does not cover the entire HTTP request. Most notably, it does not include the entity-body in most requests, nor does it include most HTTP entity-headers. It is important to note that the server cannot verify the authenticity of the excluded request components without using additional protections such as SSL/ TLS or other methods. 3.4.1.1. String Construction The signature base string is constructed by concatenating together, in order, the following HTTP request elements: 1. The HTTP request method in uppercase. For example: "HEAD", "GET", "POST", etc. If the request uses a custom HTTP method, it MUST be encoded (Section 3.6). 2. An "&" character (ASCII code 38). 3. The base string URI from Section 3.4.1.2, after being encoded (Section 3.6). 4. An "&" character (ASCII code 38). 5. The request parameters as normalized in Section 3.4.1.3.2, after being encoded (Section 3.6). For example, the HTTP request: POST /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="Example", oauth_consumer_key="9djdj82h48djs9d2", oauth_token="kkk9d7dh3k39sjv7", oauth_signature_method="HMAC-SHA1", oauth_timestamp="137131201", oauth_nonce="7d8f3e4a", oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D" c2&a3=2+q Hammer-Lahav Informational [Page 19] RFC 5849 OAuth 1.0 April 2010 is represented by the following signature base string (line breaks are for display purposes only): POST&http%3A%2F%2Fexample.com%2Frequest&a2%3Dr%2520b%26a3%3D2%2520q %26a3%3Da%26b5%3D%253D%25253D%26c%2540%3D%26c2%3D%26oauth_consumer_ key%3D9djdj82h48djs9d2%26oauth_nonce%3D7d8f3e4a%26oauth_signature_m ethod%3DHMAC-SHA1%26oauth_timestamp%3D137131201%26oauth_token%3Dkkk 9d7dh3k39sjv7 3.4.1.2. Base String URI The scheme, authority, and path of the request resource URI [RFC3986] are included by constructing an "http" or "https" URI representing the request resource (without the query or fragment) as follows: 1. The scheme and host MUST be in lowercase. 2. The host and port values MUST match the content of the HTTP request "Host" header field. 3. The port MUST be included if it is not the default port for the scheme, and MUST be excluded if it is the default. Specifically, the port MUST be excluded when making an HTTP request [RFC2616] to port 80 or when making an HTTPS request [RFC2818] to port 443. All other non-default port numbers MUST be included. 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 that is often different from their original encoding scheme, and concatenated into a single string. Hammer-Lahav Informational [Page 20] RFC 5849 OAuth 1.0 April 2010 3.4.1.3.1. Parameter Sources The parameters from the following sources are collected into a single list of name/value pairs: o The query component of the HTTP request URI as defined by [RFC3986], Section 3.4. The query component is parsed into a list of name/value pairs by treating it as an "application/x-www-form-urlencoded" string, separating the names and values and decoding them as defined by [W3C.REC-html40-19980424], Section 17.13.4. o The OAuth HTTP "Authorization" header field (Section 3.5.1) if present. The header's content is parsed into a list of name/value pairs excluding the "realm" parameter if present. The parameter values are decoded as defined by Section 3.5.1. o The HTTP request entity-body, but only if all of the following conditions are met: * The entity-body is single-part. * The entity-body follows the encoding requirements of the "application/x-www-form-urlencoded" content-type as defined by [W3C.REC-html40-19980424]. * The HTTP request entity-header includes the "Content-Type" header field set to "application/x-www-form-urlencoded". The entity-body is parsed into a list of decoded name/value pairs as described in [W3C.REC-html40-19980424], Section 17.13.4. The "oauth_signature" parameter MUST be excluded from the signature base string if present. Parameters not explicitly included in the request MUST be excluded from the signature base string (e.g., the "oauth_version" parameter when omitted). Hammer-Lahav Informational [Page 21] RFC 5849 OAuth 1.0 April 2010 For example, the HTTP request: POST /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="Example", 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: +------------------------+------------------+ | Name | Value | +------------------------+------------------+ | b5 | =%3D | | a3 | a | | c@ | | | a2 | r b | | oauth_consumer_key | 9djdj82h48djs9d2 | | oauth_token | kkk9d7dh3k39sjv7 | | oauth_signature_method | HMAC-SHA1 | | oauth_timestamp | 137131201 | | oauth_nonce | 7d8f3e4a | | c2 | | | a3 | 2 q | +------------------------+------------------+ Note that the value of "b5" is "=%3D" and not "==". Both "c@" and "c2" have empty values. While the encoding rules specified in this specification for the purpose of constructing the signature base string exclude the use of a "+" character (ASCII code 43) to represent an encoded space character (ASCII code 32), this practice is widely used in "application/x-www-form-urlencoded" encoded values, and MUST be properly decoded, as demonstrated by one of the "a3" parameter instances (the "a3" parameter is used twice in this request). Hammer-Lahav Informational [Page 22] RFC 5849 OAuth 1.0 April 2010 3.4.1.3.2. Parameters Normalization The parameters collected in Section 3.4.1.3 are normalized into a single string as follows: 1. First, the name and value of each parameter are encoded (Section 3.6). 2. The parameters are sorted by name, using ascending byte value ordering. If two or more parameters share the same name, they are sorted by their value. 3. The name of each parameter is concatenated to its corresponding value using an "=" character (ASCII code 61) as a separator, even if the value is empty. 4. The sorted name/value pairs are concatenated together into a single string by using an "&" character (ASCII code 38) as separator. For example, the list of parameters from the previous section would be normalized as follows: Encoded: +------------------------+------------------+ | Name | Value | +------------------------+------------------+ | b5 | %3D%253D | | a3 | a | | c%40 | | | a2 | r%20b | | oauth_consumer_key | 9djdj82h48djs9d2 | | oauth_token | kkk9d7dh3k39sjv7 | | oauth_signature_method | HMAC-SHA1 | | oauth_timestamp | 137131201 | | oauth_nonce | 7d8f3e4a | | c2 | | | a3 | 2%20q | +------------------------+------------------+ Hammer-Lahav Informational [Page 23] RFC 5849 OAuth 1.0 April 2010 Sorted: +------------------------+------------------+ | Name | Value | +------------------------+------------------+ | a2 | r%20b | | a3 | 2%20q | | a3 | a | | b5 | %3D%253D | | c%40 | | | c2 | | | oauth_consumer_key | 9djdj82h48djs9d2 | | oauth_nonce | 7d8f3e4a | | oauth_signature_method | HMAC-SHA1 | | oauth_timestamp | 137131201 | | oauth_token | kkk9d7dh3k39sjv7 | +------------------------+------------------+ Concatenated Pairs: +-------------------------------------+ | Name=Value | +-------------------------------------+ | a2=r%20b | | a3=2%20q | | a3=a | | b5=%3D%253D | | c%40= | | c2= | | oauth_consumer_key=9djdj82h48djs9d2 | | oauth_nonce=7d8f3e4a | | oauth_signature_method=HMAC-SHA1 | | oauth_timestamp=137131201 | | oauth_token=kkk9d7dh3k39sjv7 | +-------------------------------------+ and concatenated together into a single string (line breaks are for display purposes only): a2=r%20b&a3=2%20q&a3=a&b5=%3D%253D&c%40=&c2=&oauth_consumer_key=9dj dj82h48djs9d2&oauth_nonce=7d8f3e4a&oauth_signature_method=HMAC-SHA1 &oauth_timestamp=137131201&oauth_token=kkk9d7dh3k39sjv7 Hammer-Lahav Informational [Page 24] RFC 5849 OAuth 1.0 April 2010 3.4.2. HMAC-SHA1 The "HMAC-SHA1" signature method uses the HMAC-SHA1 signature algorithm as defined in [RFC2104]: digest = HMAC-SHA1 (key, text) The HMAC-SHA1 function variables are used in following way: text is set to the value of the signature base string from Section 3.4.1.1. key is set to the concatenated values of: 1. The client shared-secret, after being encoded (Section 3.6). 2. An "&" character (ASCII code 38), which MUST be included even when either secret is empty. 3. The token shared-secret, after being encoded (Section 3.6). digest is used to set the value of the "oauth_signature" protocol parameter, after the result octet string is base64-encoded per [RFC2045], Section 6.8. 3.4.3. RSA-SHA1 The "RSA-SHA1" signature method uses the RSASSA-PKCS1-v1_5 signature algorithm as defined in [RFC3447], Section 8.2 (also known as PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5. To use this method, the client MUST have established client credentials with the server that included its RSA public key (in a manner that is beyond the scope of this specification). The signature base string is signed using the client's RSA private key per [RFC3447], Section 8.2.1: S = RSASSA-PKCS1-V1_5-SIGN (K, M) Where: K is set to the client's RSA private key, M is set to the value of the signature base string from Section 3.4.1.1, and Hammer-Lahav Informational [Page 25] RFC 5849 OAuth 1.0 April 2010 S is the result signature used to set the value of the "oauth_signature" protocol parameter, after the result octet string is base64-encoded per [RFC2045] section 6.8. The server verifies the signature per [RFC3447] section 8.2.2: RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S) Where: (n, e) is set to the client's RSA public key, M is set to the value of the signature base string from Section 3.4.1.1, and S is set to the octet string value of the "oauth_signature" protocol parameter received from the client. 3.4.4. PLAINTEXT The "PLAINTEXT" method does not employ a signature algorithm. It MUST be used with a transport-layer mechanism such as TLS or SSL (or sent over a secure channel with equivalent protections). It does not utilize the signature base string or the "oauth_timestamp" and "oauth_nonce" parameters. The "oauth_signature" protocol parameter is set to the concatenated value of: 1. The client shared-secret, after being encoded (Section 3.6). 2. An "&" character (ASCII code 38), which MUST be included even when either secret is empty. 3. The token shared-secret, after being encoded (Section 3.6). 3.5. Parameter Transmission When making an OAuth-authenticated request, protocol parameters as well as any other parameter using the "oauth_" prefix SHALL be included in the request using one and only one of the following locations, listed in order of decreasing preference: 1. The HTTP "Authorization" header field as described in Section 3.5.1. 2. The HTTP request entity-body as described in Section 3.5.2. Hammer-Lahav Informational [Page 26] RFC 5849 OAuth 1.0 April 2010 3. The HTTP request URI query as described in Section 3.5.3. In addition to these three methods, future extensions MAY define other methods for including protocol parameters in the request. 3.5.1. Authorization Header Protocol parameters can be transmitted using the HTTP "Authorization" header field as defined by [RFC2617] with the auth-scheme name set to "OAuth" (case insensitive). For example: Authorization: OAuth realm="Example", 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 field as follows: 1. Parameter names and values are encoded per Parameter Encoding (Section 3.6). 2. Each parameter's name is immediately followed by an "=" character (ASCII code 61), a """ character (ASCII code 34), the parameter value (MAY be empty), and another """ character (ASCII code 34). 3. Parameters are separated by a "," character (ASCII code 44) and OPTIONAL linear whitespace per [RFC2617]. 4. The OPTIONAL "realm" parameter MAY be added and interpreted per [RFC2617] section 1.2. Servers MAY indicate their support for the "OAuth" auth-scheme by returning the HTTP "WWW-Authenticate" response header field upon client requests for protected resources. As per [RFC2617], such a response MAY include additional HTTP "WWW-Authenticate" header fields: For example: WWW-Authenticate: OAuth realm="http://server.example.com/" Hammer-Lahav Informational [Page 27] RFC 5849 OAuth 1.0 April 2010 The realm parameter defines a protection realm per [RFC2617], Section 1.2. 3.5.2. Form-Encoded Body Protocol parameters can be transmitted in the HTTP request entity- body, but only if the following REQUIRED conditions are met: o The entity-body is single-part. o The entity-body follows the encoding requirements of the "application/x-www-form-urlencoded" content-type as defined by [W3C.REC-html40-19980424]. o The HTTP request entity-header includes the "Content-Type" header field set to "application/x-www-form-urlencoded". 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 The entity-body MAY include other request-specific parameters, in which case, the protocol parameters SHOULD be appended following the request-specific parameters, properly separated by an "&" character (ASCII code 38). 3.5.3. Request URI Query Protocol parameters can be transmitted by being added to the HTTP request URI as a query parameter as defined by [RFC3986], Section 3. 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 The request URI MAY include other request-specific query parameters, in which case, the protocol parameters SHOULD be appended following the request-specific parameters, properly separated by an "&" character (ASCII code 38). Hammer-Lahav Informational [Page 28] RFC 5849 OAuth 1.0 April 2010 3.6. Percent Encoding Existing percent-encoding methods do not guarantee a consistent construction of the signature base string. The following percent- encoding method is not defined to replace the existing encoding methods defined by [RFC3986] and [W3C.REC-html40-19980424]. It is used only in the construction of the signature base string and the "Authorization" header field. This specification defines the following method for percent-encoding strings: 1. Text values are first encoded as UTF-8 octets per [RFC3629] if they are not already. This does not include binary values that are not intended for human consumption. 2. The values are then escaped using the [RFC3986] percent-encoding (%XX) mechanism as follows: * Characters in the unreserved character set as defined by [RFC3986], Section 2.3 (ALPHA, DIGIT, "-", ".", "_", "~") MUST NOT be encoded. * All other characters MUST be encoded. * The two hexadecimal characters used to represent encoded characters MUST be uppercase. This method is different from the encoding scheme used by the "application/x-www-form-urlencoded" content-type (for example, it encodes space characters as "%20" and not using the "+" character). It MAY be different from the percent-encoding functions provided by web-development frameworks (e.g., encode different characters, use lowercase hexadecimal characters). 4. Security Considerations As stated in [RFC2617], the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use. Implementers are strongly encouraged to assess how this protocol addresses their security requirements. 4.1. RSA-SHA1 Signature Method Authenticated requests made with "RSA-SHA1" signatures do not use the token shared-secret, or any provisioned client shared-secret. This means the request relies completely on the secrecy of the private key used by the client to sign requests. Hammer-Lahav Informational [Page 29] RFC 5849 OAuth 1.0 April 2010 4.2. Confidentiality of Requests While this protocol provides a mechanism for verifying the integrity of requests, it provides no guarantee of request confidentiality. Unless further precautions are taken, eavesdroppers will have full access to request content. Servers should carefully consider the kinds of data likely to be sent as part of such requests, and should employ transport-layer security mechanisms to protect sensitive resources. 4.3. Spoofing by Counterfeit Servers This protocol makes no attempt to verify the authenticity of the server. A hostile party could take advantage of this by intercepting the client's requests and returning misleading or otherwise incorrect responses. Service providers should consider such attacks when developing services using this protocol, and should require transport-layer security for any requests where the authenticity of the server or of request responses is an issue. 4.4. Proxying and Caching of Authenticated Content The HTTP Authorization scheme (Section 3.5.1) is optional. However, [RFC2616] relies on the "Authorization" and "WWW-Authenticate" header fields to distinguish authenticated content so that it can be protected. Proxies and caches, in particular, may fail to adequately protect requests not using these header fields. For example, private authenticated content may be stored in (and thus retrievable from) publicly accessible caches. Servers not using the HTTP "Authorization" header field should take care to use other mechanisms, such as the "Cache-Control" header field, to ensure that authenticated content is protected. 4.5. Plaintext Storage of Credentials The client shared-secret and token shared-secret function the same way passwords do in traditional authentication systems. In order to compute the signatures used in methods other than "RSA-SHA1", the server must have access to these secrets in plaintext form. This is in contrast, for example, to modern operating systems, which store only a one-way hash of user credentials. If an attacker were to gain access to these secrets -- or worse, to the server's database of all such secrets -- he or she would be able to perform any action on behalf of any resource owner. Accordingly, it is critical that servers protect these secrets from unauthorized access. Hammer-Lahav Informational [Page 30] RFC 5849 OAuth 1.0 April 2010 4.6. Secrecy of the Client Credentials In many cases, the client application will be under the control of potentially untrusted parties. For example, if the client is a desktop application with freely available source code or an executable binary, an attacker may be able to download a copy for analysis. In such cases, attackers will be able to recover the client credentials. Accordingly, servers should not use the client credentials alone to verify the identity of the client. Where possible, other factors such as IP address should be used as well. 4.7. Phishing Attacks Wide deployment of this and similar protocols may cause resource owners to become inured to the practice of being redirected to websites where they are asked to enter their passwords. If resource owners are not careful to verify the authenticity of these websites before entering their credentials, it will be possible for attackers to exploit this practice to steal resource owners' passwords. Servers should attempt to educate resource owners about the risks phishing attacks pose, and should provide mechanisms that make it easy for resource owners to confirm the authenticity of their sites. Client developers should consider the security implications of how they interact with a user-agent (e.g., separate window, embedded), and the ability of the end-user to verify the authenticity of the server website. 4.8. Scoping of Access Requests By itself, this protocol does not provide any method for scoping the access rights granted to a client. However, most applications do require greater granularity of access rights. For example, servers may wish to make it possible to grant access to some protected resources but not others, or to grant only limited access (such as read-only access) to those protected resources. When implementing this protocol, servers should consider the types of access resource owners may wish to grant clients, and should provide mechanisms to do so. Servers should also take care to ensure that resource owners understand the access they are granting, as well as any risks that may be involved. Hammer-Lahav Informational [Page 31] RFC 5849 OAuth 1.0 April 2010 4.9. Entropy of Secrets Unless a transport-layer security protocol is used, eavesdroppers will have full access to authenticated requests and signatures, and will thus be able to mount offline brute-force attacks to recover the credentials used. Servers should be careful to assign shared-secrets that are long enough, and random enough, to resist such attacks for at least the length of time that the shared-secrets are valid. For example, if shared-secrets are valid for two weeks, servers should ensure that it is not possible to mount a brute force attack that recovers the shared-secret in less than two weeks. Of course, servers are urged to err on the side of caution, and use the longest secrets reasonable. It is equally important that the pseudo-random number generator (PRNG) used to generate these secrets be of sufficiently high quality. Many PRNG implementations generate number sequences that may appear to be random, but that nevertheless exhibit patterns or other weaknesses that make cryptanalysis or brute force attacks easier. Implementers should be careful to use cryptographically secure PRNGs to avoid these problems. 4.10. Denial-of-Service / Resource-Exhaustion Attacks This specification includes a number of features that may make resource exhaustion attacks against servers possible. For example, this protocol requires servers to track used nonces. If an attacker is able to use many nonces quickly, the resources required to track them may exhaust available capacity. And again, this protocol can require servers to perform potentially expensive computations in order to verify the signature on incoming requests. An attacker may exploit this to perform a denial-of-service attack by sending a large number of invalid requests to the server. Resource Exhaustion attacks are by no means specific to this specification. However, implementers should be careful to consider the additional avenues of attack that this protocol exposes, and design their implementations accordingly. For example, entropy starvation typically results in either a complete denial of service while the system waits for new entropy or else in weak (easily guessable) secrets. When implementing this protocol, servers should consider which of these presents a more serious risk for their application and design accordingly. Hammer-Lahav Informational [Page 32] RFC 5849 OAuth 1.0 April 2010 4.11. SHA-1 Cryptographic Attacks SHA-1, the hash algorithm used in "HMAC-SHA1" and "RSA-SHA1" signature methods, has been shown to have a number of cryptographic weaknesses that significantly reduce its resistance to collision attacks. While these weaknesses do not seem to affect the use of SHA-1 with the Hash-based Message Authentication Code (HMAC) and should not affect the "HMAC-SHA1" signature method, it may affect the use of the "RSA-SHA1" signature method. NIST has announced that it will phase out use of SHA-1 in digital signatures by 2010 [NIST_SHA-1Comments]. Practically speaking, these weaknesses are difficult to exploit, and by themselves do not pose a significant risk to users of this protocol. They may, however, make more efficient attacks possible, and servers should take this into account when considering whether SHA-1 provides an adequate level of security for their applications. 4.12. Signature Base String Limitations The signature base string has been designed to support the signature methods defined in this specification. Those designing additional signature methods, should evaluated the compatibility of the signature base string with their security requirements. Since the signature base string does not cover the entire HTTP request, such as most request entity-body, most entity-headers, and the order in which parameters are sent, servers should employ additional mechanisms to protect such elements. 4.13. Cross-Site Request Forgery (CSRF) Cross-Site Request Forgery (CSRF) is a web-based attack whereby HTTP requests are transmitted from a user that the website trusts or has authenticated. CSRF attacks on authorization approvals can allow an attacker to obtain authorization to protected resources without the consent of the User. Servers SHOULD strongly consider best practices in CSRF prevention at all the protocol authorization endpoints. CSRF attacks on OAuth callback URIs hosted by clients are also possible. Clients should prevent CSRF attacks on OAuth callback URIs by verifying that the resource owner at the client site intended to complete the OAuth negotiation with the server. The methods for preventing such CSRF attacks are beyond the scope of this specification. Hammer-Lahav Informational [Page 33] RFC 5849 OAuth 1.0 April 2010 4.14. User Interface Redress Servers should protect the authorization process against user interface (UI) redress attacks (also known as "clickjacking"). As of the time of this writing, no complete defenses against UI redress are available. Servers can mitigate the risk of UI redress attacks using the following techniques: o JavaScript frame busting. o JavaScript frame busting, and requiring that browsers have JavaScript enabled on the authorization page. o Browser-specific anti-framing techniques. o Requiring password reentry before issuing OAuth tokens. 4.15. Automatic Processing of Repeat Authorizations Servers may wish to automatically process authorization requests (Section 2.2) from clients that have been previously authorized by the resource owner. When the resource owner is redirected to the server to grant access, the server detects that the resource owner has already granted access to that particular client. Instead of prompting the resource owner for approval, the server automatically redirects the resource owner back to the client. If the client credentials are compromised, automatic processing creates additional security risks. An attacker can use the stolen client credentials to redirect the resource owner to the server with an authorization request. The server will then grant access to the resource owner's data without the resource owner's explicit approval, or even awareness of an attack. If no automatic approval is implemented, an attacker must use social engineering to convince the resource owner to approve access. Servers can mitigate the risks associated with automatic processing by limiting the scope of token credentials obtained through automated approvals. Tokens credentials obtained through explicit resource owner consent can remain unaffected. Clients can mitigate the risks associated with automatic processing by protecting their client credentials. Hammer-Lahav Informational [Page 34] RFC 5849 OAuth 1.0 April 2010 5. Acknowledgments This specification is directly based on the OAuth Core 1.0 Revision A community specification, which in turn was modeled after existing proprietary protocols and best practices that have been independently implemented by various companies. The community specification was edited by Eran Hammer-Lahav and authored by: Mark Atwood, Dirk Balfanz, Darren Bounds, Richard M. Conlan, Blaine Cook, Leah Culver, Breno de Medeiros, Brian Eaton, Kellan Elliott-McCrea, Larry Halff, Eran Hammer-Lahav, Ben Laurie, Chris Messina, John Panzer, Sam Quigley, David Recordon, Eran Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy Smith. The editor would like to thank the following individuals for their invaluable contribution to the publication of this edition of the protocol: Lisa Dusseault, Justin Hart, Avshalom Houri, Chris Messina, Mark Nottingham, Tim Polk, Peter Saint-Andre, Joseph Smarr, and Paul Walker. Hammer-Lahav Informational [Page 35] RFC 5849 OAuth 1.0 April 2010 Appendix A. Differences from the Community Edition This specification includes the following changes made to the original community document [OAuthCore1.0_RevisionA] in order to correct mistakes and omissions identified since the document was originally published at <http://oauth.net>. o Changed using TLS/SSL when sending or requesting plain text credentials from SHOULD to MUST. This change affects any use of the "PLAINTEXT" signature method, as well as requesting temporary credentials (Section 2.1) and obtaining token credentials (Section 2.3). o Adjusted nonce language to indicate it is unique per token/ timestamp/client combination. o Removed the requirement for timestamps to be equal to or greater than the timestamp used in the previous request. o Changed the nonce and timestamp parameters to OPTIONAL when using the "PLAINTEXT" signature method. o Extended signature base string coverage that includes "application/x-www-form-urlencoded" entity-body parameters when the HTTP method used is other than "POST" and URI query parameters when the HTTP method used is other than "GET". o Incorporated corrections to the instructions in each signature method to encode the signature value before inserting it into the "oauth_signature" parameter, removing errors that would have caused double-encoded values. o Allowed omitting the "oauth_token" parameter when empty. o Permitted sending requests for temporary credentials with an empty "oauth_token" parameter. o Removed the restrictions from defining additional "oauth_" parameters. Hammer-Lahav Informational [Page 36] RFC 5849 OAuth 1.0 April 2010 6. References 6.1. Normative References [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [W3C.REC-html40-19980424] Hors, A., Raggett, D., and I. Jacobs, "HTML 4.0 Specification", World Wide Web Consortium Recommendation REC-html40-19980424, April 1998, <http://www.w3.org/TR/1998/REC-html40-19980424>. Hammer-Lahav Informational [Page 37] RFC 5849 OAuth 1.0 April 2010 6.2. Informative References [NIST_SHA-1Comments] Burr, W., "NIST Comments on Cryptanalytic Attacks on SHA-1", <http://csrc.nist.gov/groups/ST/hash/statement.html>. [OAuthCore1.0_RevisionA] OAuth Community, "OAuth Core 1.0 Revision A", <http://oauth.net/core/1.0a>. Author's Address Eran Hammer-Lahav (editor) EMail: eran@hueniverse.com URI: http://hueniverse.com Hammer-Lahav Informational [Page 38]