OAuth is an open standard for authorization, commonly used as a way for Internet users to authorize websites or applications to access their information on other websites but without giving them the passwords. This mechanism is used by companies such as Google, Facebook, Microsoft and Twitter to permit the users to share information about their accounts with third party applications or websites.
Generally, OAuth provides to clients a "secure delegated access" to server resources on behalf of a resource owner. It specifies a process for resource owners to authorize third-party access to their server resources without sharing their credentials. Designed specifically to work with Hypertext Transfer Protocol (HTTP), OAuth essentially allows access tokens to be issued to third-party clients by an authorization server, with the approval of the resource owner. The third party then uses the access token to access the protected resources hosted by the resource server.
OAuth is a service that is complementary to and distinct from OpenID. OAuth is also distinct from OATH, which is a reference architecture for authentication, not a standard for authorization. However, OAuth is directly related to OpenID Connect (OIDC) since OIDC is an authentication layer built on top of OAuth 2.0. OAuth is also distinct from XACML, which is an authorization policy standard. OAuth can be used in conjunction with XACML where OAuth is used for ownership consent and access delegation whereas XACML is used to define the authorization policies (e.g. managers can view documents in their region).
OAuth began in November 2006 when Blaine Cook was developing the Twitter OpenID implementation. Meanwhile, Ma.gnolia needed a solution to allow its members with OpenIDs to authorize Dashboard Widgets to access their service. Cook, Chris Messina and Larry Halff from Magnolia met with David Recordon to discuss using OpenID with the Twitter and Ma.gnolia APIs to delegate authentication. They concluded that there were no open standards for API access delegation.
The OAuth discussion group was created in April 2007, for the small group of implementers to write the draft proposal for an open protocol. DeWitt Clinton from Google learned of the OAuth project, and expressed his interest in supporting the effort. In July 2007, the team drafted an initial specification. Eran Hammer joined and coordinated the many OAuth contributions creating a more formal specification. On December 4, 2007, the OAuth Core 1.0 final draft was released.
At the 73rd Internet Engineering Task Force (IETF) meeting in Minneapolis in November 2008, an OAuth BoF was held to discuss bringing the protocol into the IETF for further standardization work. The event was well attended and there was wide support for formally chartering an OAuth working group within the IETF.
Since August 31, 2010, all third party Twitter applications have been required to use OAuth.
OAuth 2.0 is not backwards compatible with OAuth 1.0. OAuth 2.0 provides specific authorization flows for web applications, desktop applications, mobile phones, and living room devices. The specification and associated RFCs are developed by the IETF OAuth WG; the main framework was published in October 2012.
Facebook's Graph API only supports OAuth 2.0. Google supports OAuth 2.0 as the recommended authentication mechanism for all of its APIs. Microsoft also supports OAuth 2.0 for various APIs and its Azure Active Directory service, which is used to secure many Microsoft and third party APIs.
On April 23, 2009, a session fixation security flaw in the 1.0 protocol was announced. It affects the OAuth authorization flow (also known as "3-legged OAuth") in OAuth Core 1.0 Section 6. Version 1.0a of the OAuth Core protocol was issued to address this issue.
OAuth 2.0 has had numerous security flaws exposed in implementations. The protocol itself has been described as inherently insecure by security experts and a primary contributor to the specification stated that implementation mistakes are almost inevitable.
In January 2013, the Internet Engineering Task Force published a number of threat models for OAuth 2.0. Among them was one called "Open Redirector"; in the spring of 2014, this was described under the name "Covert Redirect" by Wang Jing.
Possibly the most devastating OAuth security failure is phishing vulnerability: every web site using OAuth is visually (but not technically) asking end users for their username and password of their master identity, which prevents ordinary users from understanding that they should not type those in should they encounter an attacker's web site that visually emulates this process to steal credentials. Two-factor authentication does not prevent this attack, because the phishing site can steal that as well (and use it right away).
Because OAuth 2.0 is more of a framework than a defined protocol, one OAuth 2.0 implementation is less likely to be naturally interoperable with another OAuth 2.0 implementation. Further deployment profiling and specification is required for any interoperability.
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OAuth can be used as an authorizing mechanism to consume secured RSS/ATOM feeds. Consumption of RSS/ATOM feeds that require authentication has always been an issue. For example, an RSS feed from a secured Google Site cannot be consumed using Google Reader. Instead, three-legged OAuth can be used to authorize Google Reader to access the RSS feed from that Google Site.
OAuth and other standards
OpenID vs. pseudo-authentication using OAuth
OAuth is an authorization protocol, rather than an authentication protocol. Using OAuth on its own as an authentication method may be referred to as pseudo-authentication. The following diagrams highlight the differences between using OpenID (specifically designed as an authentication protocol) and OAuth for authentication.
The communication flow in both processes is similar:
- (Not pictured) The user requests a resource or site login from the application.
- The site sees that the user is not authenticated. It formulates a request for the identity provider, encodes it, and sends it to the user as part of a redirect URL.
- The user's browser requests the redirect URL for the identity provider, including the application's request
- If necessary, the identity provider authenticates the user (perhaps by asking them for their username and password)
- Once the identity provider is satisfied that the user is sufficiently authenticated, it processes the application's request, formulates a response, and sends that back to the user along with a redirect URL back to the application.
- The user's browser requests the redirect URL that goes back to the application, including the identity provider's response
- The application decodes the identity provider's response, and carries on accordingly.
- (OAuth only) The response includes an access token which the application can use to gain direct access to the identity provider's services on the user's behalf.
The crucial difference is that in the OpenID authentication use case, the response from the identity provider is an assertion of identity; while in the OAuth authorization use case, the identity provider is also an API provider, and the response from the identity provider is an access token that may grant the application ongoing access to some of the identity provider's APIs, on the user's behalf. The access token acts as a kind of "valet key" that the application can include with its requests to the identity provider, which prove that it has permission from the user to access those APIs.
Because the identity provider typically (but not always) authenticates the user as part of the process of granting an OAuth access token, it's tempting to view a successful OAuth access token request as an authentication method itself. However, because OAuth was not designed with this use case in mind, making this assumption can lead to major security flaws.
OAuth and XACML
- An access control architecture described here.
- A policy language with which to express a wide range of access control policies including policies that can use consents handled / defined via OAuth.
- A request / response scheme to send and receive authorization requests.
XACML and OAuth can be combined together to deliver a more comprehensive approach to authorization. OAuth does not provide a policy language with which to define access control policies. XACML can be used for its policy language.
Where OAuth focuses on delegated access (I, the user, grant Twitter access to my Facebook wall), and identity-centric authorization, XACML takes an attribute-based approach which can consider attributes of the user, the action, the resource, and the context (who, what, where, when, how). With XACML it is possible to define policies such as
- Managers can view documents in their department
- Managers can edit documents they own in draft mode
XACML provides more fine-grained access control than OAuth does. OAuth is limited in granularity to the coarse functionality (the scopes) exposed by the target service. As a result, it often makes sense to combine OAuth and XACML together where OAuth will provide the delegated access use case and consent management and XACML will provide the authorization policies that work on the applications, processes, and data.
Lastly, XACML can work transparently across multiple stacks (APIs, web SSO, ESBs, home-grown apps, databases...). OAuth focuses exclusively on HTTP-based apps.
In July 2012, Eran Hammer resigned his role of lead author for the OAuth 2.0 project, withdrew from the IETF working group, and removed his name from the specification. Hammer pointed to a conflict between the web and enterprise cultures, citing the IETF as a community that is "all about enterprise use cases", that is "not capable of simple." What is now offered is a blueprint for an authorization protocol, he says, and "that is the enterprise way", providing a "whole new frontier to sell consulting services and integration solutions."
In comparing OAuth 2.0 with 1.0, Hammer points out that it has become "more complex, less interoperable, less useful, more incomplete, and most importantly, less secure." He explains how architectural changes for 2.0 unbound tokens from clients, removed all signatures and cryptography at a protocol level and added expiring tokens (because tokens couldn't be revoked) while complicating the processing of authorization. Numerous items were left unspecified or unlimited in the specification because "as has been the nature of this working group, no issue is too small to get stuck on or leave open for each implementation to decide."
Hammer later gave a presentation at &Yet elaborating on his views.
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- "RFC 6749 - The OAuth 2.0 Authorization Framework". Retrieved 2016-05-15.
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- "RFC6749 - The OAuth 2.0 Authorization Framework". Dick Hardt. October 2012. Retrieved 10 October 2012.
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- Dick Hardt, ed. "Interoperability". The OAuth 2.0 Authorization Framework. IETF. sec. 1.8. RFC 6749. https://tools.ietf.org/html/rfc6749#section-1.8. Retrieved 8 May 2013. "as a rich and highly extensible framework with many optional components, on its own, this specification is likely to produce a wide range of non-interoperable implementations."
- "End User Authentication with OAuth 2.0". OAuth Community Site. Retrieved 8 March 2016.
- "OAuth 2.0 and the Road to Hell". Eran Hammer. 28 July 2012. Retrieved 16 August 2012.
- "OAuth 2.0 - Looking Back and Moving On". Eran Hammer. 23 October 2012. Retrieved 22 November 2012.
- The OAuth 1.0 Protocol (RFC 5849)
- The OAuth 2.0 Authorization Framework (RFC 6749)
- The OAuth 2.0 Authorization Framework: Bearer Token Usage (RFC 6750)
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- oAuth End points cheat sheet