well-tested authorization design patterns

Aug 1, 2016  

Authorization is a strange beast. In theory, it appears to be rather straight-forward: a user should not be able to create, read, update, or delete data that it does not have access to. However, from our experience, theory tends to deviate from practice. Missing or incorrect access controls are a dime a dozen for applications we test and this very rarely stems from a complete lack of access controls. More often then not, authorization issues spring up during assessments where the application manages a complex authorization model and an incorrect assumption was made or an edge case was missed. Conversely, we have seen applications that have incredibly complicated authorization models that have zero access control problems.

Authorization may seem like a trivial engineering problem, but consider a mobile phone. Many smart phones support configuring a lock screen that should prevent unauthorized users from accessing the device. This seems straight-forward, but allowing this leads to interesting edge cases. What features should be exposed to users without a lock screen code? Most smart phones can still place emergency calls while locked. What about access to the phone’s camera? If these features are allowed, access controls must be handled properly. Even if an application begins with simple authorization models, as features are added, the once simple access control mechanism must handle complex logic.

Requirements for access control mechanisms can vary greatly, so there is no catch-all implementation. Even so, we observe that applications free from authorization flaws still follow certain design patterns or principles. To best understand and evaluate our rationale behind these why we recommend these design principles, we must first form a solid understanding of the threats that access controls are designed to thwart.

The Threat Model: Horizontal and Vertical Privilege Escalation

There are two primary classes of bugs that access controls attempt to prevent: horizontal and vertical privilege escalation. Vertical privilege escalation is fairly intuitive: a user should not be able to perform actions above her privilege level (e.g. a low-privilege user should not be able to perform administrative actions). The somewhat unintuitively named “horizontal privilege escalation” is when a user can perform actions at her privilege level that are not typically allowed. This typically happens when user is able to act on another user’s behalf (e.g. a customer of an online bank transfers money from another customer’s account).

Understanding the distinction between these two classes of vulnerabilities are crucial: doing so allows us to better reason about the security of our access control mechanism. If you are interested in reading more on the subject, I recommend checking out Wikipedia’s page on privilege escalation.

Authorization Design Patterns

From assessing a significant number of authorization schemes, we have compiled a list of key design principles which successful schemes follow. While requirements for any particular scheme may vary, and not all principles may be relevant to your particular needs, following the following design principles will assist in avoiding common pitfalls.

Key Principle 1: Design an access control model.

This is likely the least interesting component of designing a decent access control mechanism, and I can hear the booing already, but access controls don’t really mean much unless some sort of access control model is defined. We highly recommend that clients formally document their access controls if they have not already. This document will be useful for some of the later key principles.

Doing this right:

  • Consider following one of the models suggested by OWASP. How the document looks is up to your team.

  • Be sure this document is within reach of all developers.

  • Update it frequently.

Key Principle 2: Don’t trust client-provided data when authenticating or authorizing a user.

In order to make access control decisions, we must first correctly identify the user making a request. This should be solely dependent on the application’s method of maintaining a user’s authenticated session (e.g. a session identifier, an authenticated claim). When access control decisions are made it is of critical importance that client-provided data is not trusted without verification.

This description is a bit general because identifying a user can be done in several ways, but for the sake of clarity, one particularly egregious example of using meaningful data would be including a parameter “admin=False” in requests. If this “admin” parameter is used to determine whether the user has administrative permissions, a malicious user could easily exploit a vertical privilege escalation flaw.

Relying on obscurity should also be avoided: if access control decisions are based on a static identifiers that should only be known by users at that privilege level, it is a matter of time before those secret values are leaked in some fashion.

Doing this right:

  • Identify users strictly by their session identifier.

  • A user’s session identifier should be directly tied to whatever permissions they may have (however that is represented by your system).

  • This session identifier should be the only item that is used when authorizing a user.

  • Session identifiers should follow current best practices.

Key Principle 3: Deny access by default.

Access controls should deny access by default. This is critical, because sometimes developers forget to include an access control check. In the event this mistake happens, the application should not allow a user to gain unfettered access to the application. Furthermore, in more complex access controls, if a user finds herself (or intentionally puts herself) in a state that is not currently handled by the access control logic, it is best not to default to allowing access.

Doing this right:

  • This is highly dependent on implementation! However your access control mechanism is built, be sure to handle both the forgetful developer case and the unhandled state case.

Key Principle 4: Be abstract and centralized.

If your access control checks take place within more than one conditional (e.g. if, switch) statement I would reconsider the design of the access control mechanism. Consider a simple CRUD API for a widget transaction. In this example, we are working with a single object (the widget transaction), that has at least 4 actions (create, read, update, delete). A likely method of implementing access controls would be at the action-level. This means there are 4 separate conditional statements that authorize a user’s action. This may work for the short-term but this mechanism will quickly grow to be complex and error-prone.

There are design patterns that can be leveraged to abstract access control checks that are less problematic than conditional statements throughout the codebase. We have seen many cases where conditional statements have preceding logic that affect access control decisions and complicate or cause authorization flaws. Additionally, conditional statements could be easily forgotten (Hopefully key principle 2 is obeyed). Wouldn’t life be so much better if you didn’t have to write a potentially nasty switch statement within every function that need access controls? We recommend that all access control logic is centralized and abstract. This allows for a cleaner implementation and easier bug fixes.

Doing this right:

  • If you are using a framework that provides an access control API that obeys the listed key principles, that should be leveraged as much as possible.

  • Generally, the decorator design pattern is something we have seen work well as a method of verify a user’s level of access.

Authorization Bonus Points

There are also other commendable access control principles that we recommend. These may not prevent authorization flaws, but they may help identify or limit issues considerably.

Bonus Principle: Avoid complexity

Avoid uneccessary complexity if you can! “Security‚Äôs worst enemy is complexity”. Easier said than done, but important to keep in mind.

Bonus Principle: Write access control tests

Write tests to validate that your model from Key Principle 0 is implemented correctly. This won’t catch all flaws but it will likely catch simple bugs and regressions. This is much easier when you follow Key Principle 3.

Bonus Principle: Log access control events.

Logging can help identify strange behavior from users or highlight flaws in the implementation. If you are not following Key Principle 3 this will be a nightmare. We also recommend logging both access control failures (e.g. “User A tried to access User B’s profile so we stopped her!“) and successes (e.g. “We let User A view her time sheet.“). Logging successes may add a bit of noise, but success events also add context that may be useful. We suggest accounting for noise, and distinguishing between failure and success events in a way that still allows the events to be coupled if necessary.

Testing For Authorization Flaws

Authorization testing is too important to pass up but is error-prone (and a bit boring) to test manually. Fortunately, in many cases, access control testing can be trivially automated. We are partial to Burp so I wrote a plugin to automate authorization testing. I am calling it Otter. There are a few Burp plugins that have a similar premise but they didn’t quite satisfy our needs (namely, less-than-stellar UX and atypical assumptions about sessions). In a nutshell, Otter browses the target web application alongside the web browser. While browsing, Otter is transparently capturing requests and replaying them with the session information of another user. Otter logs all of these requests and records information that could be used to find differences between the ordinary request and the modified one. If differences are noted between the two requests, this is suggestive of authorization flaws. In short, Otter allows testers to find authorization flaws in applications with the same amount of effort it takes to browse the application. If this sounds appealing, please check out Otter on Github.