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Coders Conquer Security OWASP Top 10 API Series - Missing Function Level Access Control

7th October 2020

This series of blogs will focus on some of the worst vulnerabilities as they relate to Application Programming Interfaces (APIs). These are so bad that they made the Open Web Application Security Project (OWASP) list of top API vulnerabilities. Given how important APIs are to modern computing infrastructures, these are critical problems that you need to keep out of your applications and programs at all costs.

The missing function level access control vulnerability allows users to perform functions that should be restricted, or lets them access resources that should be protected. Normally, functions and resources are directly protected in the code or by configuration settings, but it’s not always easy to do correctly. Implementing proper checks can be difficult because modern applications often contain many types of roles and groups, plus a complex user hierarchy.

But first, why not jump in and play our gamified challenge to see where you’re at with navigating this tricky class of bug?

Let’s take a more in-depth look:

APIs are especially vulnerable to this flaw because they are highly structured. Attackers who understand code can make educated guesses about how to implement commands that should be restricted to them. That is one of the main reasons why the function/resource level access control vulnerability made the OWASP top ten.

How can attackers exploit the function level access control vulnerability?

Attackers who suspect that functions or resources are not properly protected must first gain access to the system they want to attack. To exploit this vulnerability, they must have permission to send legitimate API calls to the endpoint. Perhaps there is a low-level guest access function or some way to join anonymously as part of the application’s function. Once that access has been established, they can start changing commands in their legitimate API calls. For example, they might swap out GET with PUT, or change the USERS string in the URL to ADMINS. Again, because APIs are structured, it’s easy to guess which commands might be allowed, and where to put them in the string.

OWASP gives an example of this vulnerability of a registration process set up to allow new users to join a website. It would probably use an API GET call, like this:

GET /api/invites/{invite_guid}

The malicious user would get back a JSON with details about the invite, including the user’s role and email. They could then change GET to POST and also elevate their invite from a user to an admin using the following API call:

POST /api/invites/new 
{“email”:“shadyguy@targetedsystem.com”,“role”:“admin”}

Only admins should be able to send POST commands, but if they are not properly secured, the API will accept them as legitimate and execute whatever the attacker wants. In this case, the malicious user would be invited to join the system as a new administrator. After that, they could see and do anything that a legitimate administrator could, which would not be good.

Eliminating the function level access control vulnerability

Preventing this API vulnerability is especially important because it’s not difficult for an attacker to find functions that are unprotected within a structured API. So long as they can get some level of access to an API, they can begin to map the structure of the code and create calls that will eventually be followed.

As such, all business-level functions must be protected using a role-based authorization method. Most frameworks offer centralized routines to make that happen. If your chosen framework doesn’t, or if the routine it has is difficult to implement, there are many external modules that are built specifically for easy use. Whatever method you ultimately choose, be sure to implement the authorization on the server. Never try to secure functions from the client side.

When working to create function and resource level permissions, keep in mind that users should only be given permissions to do what they need and nothing more. As is always the case when coding APIs or anything else, practice the least privilege methodology. It will secure your environment and head off a lot of cybersecurity-related trouble down the road.

Check out the Secure Code Warrior blog pages for more insight about this vulnerability and how to protect your organization and customers from the ravages of other security flaws. You can also try a demo of the Secure Code Warrior training platform to keep all your cybersecurity skills honed and up-to-date.

Matias is the co-founder and CTO of Secure Code Warrior. He has over a decade of hands-on software security experience, holding a Ph.D. in computer engineering from Ghent University.

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