Tuesday, March 19, 2013

OAuth - A great way to cripple your API


 A few years ago, the big social networking sites were looking for a secure way to allow their users to safely use any and all untrusted software to access their own personal accounts. So that a user could have their account on one social networking site safely interact with their account on another social networking site. They also wanted to allow for users to be able to allow their accounts to interact safely with various untrusted web applications and web sites.

In order to safely allow untrusted software to access a user's account, a few points needed to be kept in mind.
  • Untrusted software should not have access to a user's credentials, in case the software is compromised, passwords will not be stolen, as user's passwords are not stored by the software.
  • Untrusted software should not have full access to a user's account, but only limited access as defined by the user. In the same vein, giving the software user's personal credentials will allow unlimited access to a user's account, which could be used maliciously.
  • A user should be able to revoke the permission they granted to allow particular untrusted software to work, in case they turn malicious, despite the limited access they have.
A solution that was developed for this use-case was created - OAuth. OAuth's primary use-case is the one described above.


OAuth is generally implemented in a fashion where the untrusted software is accessed by a user via a standard web browser. In order for a user to authorize that software, the software will redirect the user's browser to a page on the social networking site, with a couple of parameters sent to it. Now that the user is on his social networking site, the untrusted software no longer has control over what the user is doing, as the user left the untrusted software, allowing the user to safely log in to his social networking account. Then, the social networking site will see the parameters it was passed, and ask the user if he or she wants to authorize the software in question, and what kind of access to the user's account it should be given.

If and when the user has authorized the untrusted software, the social networking site can then report back to the untrusted software that it was granted a certain amount of access, and give it some credentials to use. These credentials are unique to the user and the software in question. The social networking site allows for a user to later on see a list of software authorized, and revoke the unique credentials given to any one of them, or modify the amount of access a particular set of credentials has.


There is no standard

Now above, I said OAuth is generally implemented in this fashion. I say generally, because unlike standard HTTP authentication schemes (Basic, Digest, and others), OAuth is a big grab bag of ideas which can be mixed and matched in an infinite amount of ways, and also allows for developers to make their own unique tweaks to their personal implementations.

With the standard HTTP authentication schemes, once a developer knows the protocol, and implemented it with one web site, that exact same knowledge can be reused to logging into any other web site that supports the standard. Likewise, software libraries can be made to handle HTTP authentication, and all a third party developer needs to do is specify to the library which credentials should be used, and then everything works as expected.

With OAuth, once a developer learns how to authenticate with it to one web site, it is likely that the same developer will need to relearn how to connect to every other web site using OAuth. This further means that every web site which supports OAuth needs to document exactly how it is implementing it, and what tweaks are in use. It also means that no library can be written which can simply support every OAuth implementation out there. This places a great burden on developers on both sides. It can also greatly increase frustration for less able users, when their favorite library works great with one site they support, but are unable to extend their software to another site, because their library lacks some aspect of OAuth for this new site, or a unique OAuth tweak on this new site renders the library incompatible.

Here are some choice quotes from the official RFC:
  • However, 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.
  • This framework was designed with the clear expectation that future work will define prescriptive profiles and extensions necessary to achieve full web-scale interoperability.

Another issue is that while various standard HTTP authentication schemes have well understood security margins, OAuth is entirely variable. On one web site, its OAuth implementation may be secure, while on another, its implementation may be Swiss cheese. Even though a security consultant should generally look over any authorization implementation to ensure mistakes were not made, laymen can have a good understanding how reliable and secure their standardized authentication scheme is. A manager can ask their developers if some bullet points were adhered to, and then be reasonably confident in their security. Whereas with OAuth, the entire (complex) implementation needs to be reviewed from top to bottom by a top security professional to ensure it is secure. A manager has no adequate set of bullet points to discuss with their developers, as unique implementation details will drastically change the applicability of various points, with many important details still missing. At best, a manager can only get a false sense of security when OAuth is in use. The designers of OAuth respond to this point with a 71 page document of security issues that need to be dealt with!

What all this boils down to, is that OAuth is really just a set of ideas and recommendations on how to implement a unique authorization scheme. OAuth does not allow for interoperability as standards (usually) guarantee. Essentially, OAuth ensures that API authentication with your web site will be confusing, and will only work for the exact use-case for social networking sites described above.

Crippling Design - APUI

The common OAuth use-case described above is to allow for a user on one web site to allow their software to communicate for them with another web site. The workflow described above requires that a user navigate between web sites using their browser and manually enter their credentials and authorization to various aspects of their account as part of the overall software authorization process.

This means that OAuth (at least how it's normally implemented) only works between two web sites, with individual users, and that user intervention is required in order for software to authenticate with another web site. The fact that user intervention is required with manual input means that any API behind OAuth is not an API - Application Programming Interface, but an APUI - Application Programming User Interface, meaning user intervention is required for functionality. All in all, this cripples your API, or APUI as it should now properly be called.

Let us now focus on what cannot be properly done with OAuth in place:
  • Third party software which is not part of a web site cannot interact with an OAuth APUI.
  • One user in third party software cannot act on behalf of another user via an OAuth APUI.
  • Third party software cannot run automated processes on an OAuth APUI.
  • Organizations cannot ensure tight integration between various software and services they use.
Let us review an example case where a company launches a new online service for coordinating schedules and personal calendars between people. We'll call this new online service Calendar. The developers of Calendar create an APUI for it using OAuth, so third party software can integrate with Calendar.

As described above, a user needs to navigate between one web site and another in order to authorize software, with the two web sites sending each other information. What if one wants to integrate something which isn't a web site with Calendar? There's tons of desktop calendaring applications, Microsoft Outlook, Mozilla Lightning, Evolution, KOrganizer, Kontact, and more. If their developers want to integrate with the Calendar service, they need to embed a web browser in their applications now?

Furthermore, that may not even help, if the OAuth workflow requires information be sent to an existing web site, what web site is associated with a user's personal copy of software? Developers of that software are unlikely to create a web site with user accounts just for this integration. Even if they did, if the site goes down (temporarily), then the integration stops working. Even though, in theory, there should be no dependance of some extra web site between Calendar and the software trying to integrate with it.


Also, as a security consideration, if an application does embed a web browser in it so it can authenticate with OAuth, the first requirement is no longer met - Untrusted software should not have access to a user's credentials. When using a standard web browser, once a user leaves the third party software's site and redirects to Calendar's site, the third party software cannot steal the information the user is entering. But when the third party software itself is the one which browses to Calendar's site, then it has access to everything the user is entering, including passwords.

Actually, this attack works on OAuth in every circumstance, including web site to web site, and I've actually used it in practice. A web site can embed a web browser via a Java Applet or similar, or have a web browser server side which presents the OAuth log in page to the user, but slightly modified to have all the data entered pass through the third party site. Therefore OAuth doesn't even fulfill its own primary security objective!

Incompatible with Enterprise

Next, once we go to the enterprise level, OAuth starts becoming much worse. Say a boss has his secretary manage his calendar for him, which is a very common scenario. In many OAuth setups, he cannot enter his Calendar credentials into the company-wide calendaring software running on a secure company server, which the secretary can then access. Rather, he would need to enter it directly in the browser the secretary uses, and stay there checking off many options. Also it is common that OAuth implementations are using security tokens which expire, meaning the boss will need to keep reentering his Calendar credentials again and again. Most bosses will just get fed up and give his secretary his credentials, especially if he's not physically near his secretary. It is also likely that the secretary will then write the password down. This gets compounded if multiple secretaries manage his schedule.

Now say an organization has software for which it would like to run background processes for all its users with Calendar. Perhaps every week it would like to automatically analyze the Calendar accounts of all department chiefs to find a good time for a weekly meeting. How would this work exactly? Every department chief would now have to go and enter their credentials into this software? Then do it again each time security tokens expire from past authentications?

Of course this is only the beginning. Since OAuth was designed for the likes of Twitter and Facebook which cater to individual personal accounts, the common implementations do not allow for hierarchical permissions as enterprise organizations need.

In enterprise infrastructure, software is already used which has well defined roles and access rights for ever single user. This infrastructure should be deciding who can do what, and to what level one user can act on behalf of another user. Once an organization purchases accounts for all their employees from Calendar, no employee should be able to turn off or limit what the enterprise management software can do with their Calendar account. The enterprise management software is the one who needs to make such decisions. This flies in the face of Untrusted software should not have full access to a user's account, but only limited access as defined by the user and A user should be able to revoke the permission they granted to allow particular untrusted software to work.

The amount of work involved to tightly integrate Calendar with existing infrastructure is also enormous from a user perspective. Every single account now has to have a user navigate across web pages and check all kinds of boxes until all the users are integrated. OAuth implementations generally forget to have administrator accounts which can do everything on behalf of other users in their organization.

It should be obvious at this point whether enterprise organizations would be willing to purchase accounts for their entire staff with Calendar, when integration into their existing infrastructure or new endeavors with the service will be difficult if not outright impossible. Imagine how much money Calendar sales now stand to lose from being unattractive to enterprise clients.

Repel third party developers

OAuth implementations also generally require that any software which uses their APUIs be preregistered in advance. This places extra burden on third party developers. The OAuth implementations also commonly require that the third party web site integrating be preregistered in advance, so you can say goodbye to your application being compatible with staging servers which may be at another URL. It also makes software much less attractive when one party sells software to another party, as every new client now needs to go through an application registration process for the URLs they plan to setup the software they are purchasing. Therefore third party developers are less likely to be interested in selling software which integrates with Calendar, as there's hassle involved with every single sale.


  • OAuth is not a standard, but a set of ideas, which does not allow third party developers to reuse knowledge, and places extra documentation burden on implementers.
  • OAuth as a whole has undefined security characteristics.
  • OAuth doesn't properly fulfill its primary security objectives.
  • OAuth doesn't work well outside social networking web site use-cases.
  • OAuth services are unattractive to enterprise organizations looking to integrate such services into their infrastructure.
  • OAuth services are less likely to have professional third parties sell software based upon them.


If you're looking to implement authorization for your API, I recommend to sticking with well understood secure designs, such as HTTP Basic Authentication over SSL/TLS (or HTTP Digest Authentication).

In order to achieve a situation where users can securely authorize third party software, without giving over their personal credentials (passwords), I recommend that these services have a page where they can generate new credentials (keys) which the user can copy and paste. They can then name these keys themselves (avoiding application registration hassle), and set permissions upon them themselves. Since the user is the one initiating the key creation, and copying and pasting it themselves, they cannot fall prey to a man-in-the-middle attack where the third party software initiates the authorization process.

But remember the use-cases described here, and ensure that organizations have a way to access all user accounts company-wide, and without individual users being able to disable or limit that access.


If you want your service to be used by everyone out there, be well supported by third parties, and to have them create all kinds of interesting software with it, do not use OAuth.

Even the original social networking sites behind OAuth decided they really need other options for different use-cases, such as Twitter's xAuth, or Yahoo offering Direct OAuth, which turns the entire scheme into a more complicated version of HTTP Basic Authentication, with no added benefits. Perhaps the most damaging point against OAuth, is that the original designer behind it decided to remove his name from the specification, and is washing his hands clean of it.

I find it really amazing at how blind many big players are these days to all the problems with OAuth. When I first heard that IBM, a major enterprise player started offering services only accessible with typical utterly crippled OAuth, I was overcome with disbelief. Yet at the same time, I hear that they're wondering why they're not seeing the sales they used to with other services they offer, or compared to the competition.

I'm even more amazed to see other big companies throwing away their currently working authentication systems for OAuth. Followed by them wondering why many third party developers are not upgrading to support the new authentication scheme, and clients jumping ship to inferior services.

It seems many developers and managers out there simply don't get it. If you know anyone like that, show them this article.