In the previous post we covered API7:2019 Security Misconfiguration, which was the 7th post in this series. If you want to start from the beginning, go to the first post, API1:2019 Broken Object Level Authorization.
Injection has been part of the original ‘OWASP Top Ten Risks to Web Apps’ list since the very beginning. Injection happens when an attacker is able to trick an application (or API) into executing malicious code. It does this by adding code to a place in the application where data belongs, and the app becomes confused, and then executes it.
Think of a search field at the top of any website. Imagine if instead of entering in your search term, you added a bunch of code. Then imagine the application executes the code you added. That code would execute with the full authority of that application, and all the same access, behind your firewall. It could result in damage to a database, a web service, your LDAP system, and potentially even worse, assuming there are other vulnerabilities the attacker can combine with this one.
Unfortunately, APIs are subject to this vulnerability, just like a regular web app. No front end does not protect us from injection.
What types of injection exist?
If there’s code involved, someone will try to inject their own code. SQL, LDAP, or NoSQL queries, OS commands, XML parsers, and ORM are all potentially problematic (list provided by the OWASP API Security project team). Even Mongo DB databases, that don’t use the SQL language, are potentially vulnerable to NOSQL injection.
Special note on XSS: Cross Site Scripting (XSS) is also a form of code injection, but it has it’s own classification because of the following reasons:
- It occurs in the browser, as opposed to back on the server side like every other form of injection.
- Is incredibly prevalent, so much so that OWASP felt it was necessary to give it it’s own category.
- Has several defenses made just for this one vulnerability (cookie settings, and security headers).
- Does not work on APIs, because they have no GUI front end, meaning no browser.
What do we do?
Hopefully by this point you agree that injection is dangerous and should be remediated as soon as possible if you find it in one of your apps. But how do we find it? How do we fix it? How can we ensure this never happens? Dear reader, secure coding is my favorite topic! Let’s go!
First off you want to go through your APIs and figure out if you have injection. The most expensive way to do this would be to hire a PenTester to find and then test all the APIs. A cheaper and more sustainable way to do this would be to
- Buy a tool that can find all your APIs for you (No, I am not going recommend one at this time, there are several on the market of various qualities, and prices). This function is called “inventory” or “enumeration”.
- Run a SAST (static application security testing tool) on all of the APIs, ideally a next gen one, that has low false positives. Fix anything that says injection.
- Use a linter on your API, ensure you have completed your API definition file, as per the linter’s instructions. If you can find an API-specific linter, all the better.
- Run a DAST tool on the APIs that is made for APIs OR, use an old school DAST but first ensure you’ve linted your API perfectly, so it can hopefully do a good job. It will be easier and faster if you have an API-specific testing tool. Fix anything that says injection.
“That’s nice you told me to fix it. Exactly HOW do I do that?”
The first defense for injection is thorough input validation on any input to your app. This means data in the parameters, in a data field, in a hidden field, from an API you called, from the database, any input to your app needs to be validated that it is what you are expecting. What type is it? What size? What’s the content? Is it what we are expecting? If not, reject.
This is functionality is best performed using an approved list, on the server side. By ‘approved list’, we mean using a list of stuff you know is good, rather than a list of what you know is bad. It’s easy for malicious actions to get around a block list, using encoding, obfuscation, and other tactics. But if you give a regular expression and say “if it’s not in here, I’m just not having it”, bad things cannot get in.
As an example, imagine you have a username. It likely accepts numbers and letters. You could use a regular expression (REGEX) like this to say what is okay: [a-z,A-Z,0-9]. That’s an approved list or ‘accept list’. If instead you try to block bad characters such as <, >, ‘, “ and more, you (and your app) are in for a world of hurt.
Input validation is defense number #1. Other defenses include:
- Always using parameterized queries when making requests to any database (even non-SQL databases long mongo DB). It takes away the ability for it to be interpreted as code.
- Use output encoding when you put stuff onto the screen. Some frameworks do this for you by default. It takes away and superpowers of the characters, before it puts them on screen, making XSS impossible. Okay, maybe this is only for XSS and doesn’t apply to injection in general, but I would still do it if I were you.
If we want to prevent injection (and a myriad of other vulnerabilities), follow this advice:
- Have your development team take a secure coding course. It can be free or paid, formal or informal, live or recorded, interactive or lecture, the only important part is that they learn. Do the type of training that works best for you and your team.
- Follow a secure system development life cycle (S-SDLC). Add security steps to each part of your SDLC, such as security requirements, code review, or threat modelling.
- Ensure your application has thorough testing, which can mean any or several of the following: static analysis, dynamic analysis, manual code review, penetration testing, stress testing, performance testing, unit testing or any other testing you can think of!
- Never stop learning. Keep reading, studying, learning and hacking.
How To Prevent: OWASP API Security Top Ten Team Advice!
Preventing injection requires keeping data separate from commands and queries.
- Perform data validation using a single, trustworthy, and actively maintained library.
- Validate, filter, and sanitize all client-provided data, or other data coming from integrated systems.
- Special characters should be escaped using the specific syntax for the target interpreter.
- Prefer a safe API that provides a parameterized interface.
- Always limit the number of returned records to prevent mass disclosure in case of injection.
- Validate incoming data using sufficient filters to only allow valid values for each input parameter.
- Define data types and strict patterns for all string parameters.
In the next blog post we will be talking about API9:2019 Improper Assets Management.
Tags: API Security, owasp, AppSec