OWASP Top 10: The Ultimate Guide

If you are here, chances are that you want to learn web application security or the OWASP Top 10, but you don’t know where to start. Well, let me tell you that you came to the right place.

I’m a penetration tester who enjoys breaking into my clients’ infrastructure and web applications. Besides, I train developers on how to write secure code. Part of the training involves showing them what hackers can do and how to prevent them from exploiting security vulnerabilities.

I am bringing this knowledge to you so that you can benefit from it as well. In this exhaustive guide, you will not just learn the OWASP Top 10, but you will also practice them on hands-on examples from the best vulnerable web applications. On top of that, I’ve prepared online video tutorials which will help you follow the hacking process step by step. Once you’ve covered the OWASP Top 10, I will show you where to go next in your hacking journey.
Let’s start learning right now!

What is OWASP Top 10?

The OWASP Top 10 is a standard document which consists of the top ten of the most impactful web application security risks in the world. The Open Web Application Security Project foundation (OWASP) publishes a version every three years. 

OWASP collects data from companies which specialize in application security. It also collects data from individuals using industry surveys. All of the results get ranked based on impact and prevalence. At last, the top ten risks are then filtered.

OWASP Top ten doesn’t cover all the vulnerabilities, but it’s a solid start for security testers, developers and organizations who want to exploit vulnerabilities and implement measures to protect against the security risks.

Why is the OWASP Top 10 important when learning web application security?

You might ask why I chose OWASP Top 10 as a backbone for this guide. Great question! Well, there are two reasons for that.

Firstly, OWASP is a respected community of tens of thousands of members ranging from information security experts to security-focused developers. It has existed for almost two decades and has produced methodologies, documents and tools which help building secure code. For example, OWASP Zed Attack Proxy (ZAP) is a tool which we will use during this training to test for security vulnerabilities. OWASP also organizes events with high-quality subjects and speakers. For you, this means that you are in good hands, and you will be learning from mature and professional resources.

Secondly, the OWASP Top 10 covers all the basics you will need to kickstart your career in application security. In fact, each one of the top 10 security risks includes one or many security vulnerabilities. For example, the Injection security risk covers all sorts of security vulnerabilities which can lead to injections.

To summarize, let me say that the OWASP Top 10 is solid and efficient if you want to quickly and efficiently learn web application security. Convinced? Let’s see what is the best way to learn that thing!

What is the best way to learn OWASP Top 10?

You might be wondering: I want to start right away, but what is the best way to learn OWASP Top 10? The answer is simply one word: Practice!

Hacking is practical by nature. Surely, you need the theory behind how each security vulnerability works, which I cover in this guide. However, you can’t say that you’ve learned them until you can exploit them, practically! That’s why for most vulnerabilities we will discuss shortly, I’ve prepared a training tutorial which will help you get your hands dirty with different challenges.

OWASP Top 10 training syllabus

The OWASP Top ten training I’ve included in this guide contains two web applications which cover the OWASP Top 10 security risks. You can download the lab right here. Alternatively, I walk you through how to set it up if you want to build it yourself.

Kickstart your web hacking journey with this OWASP Top 10 lab.
Kickstart your web hacking journey with this OWASP Top 10 lab.

The web applications are OWASP Juice Shop and OWASP WebGoat. They are both mature and well-maintained projects. They are also written in different languages, which will expose you to hacking different technologies.

During this OWASP Top ten training, we will set up and configure the best web proxies that the hackers use to test for security vulnerabilities: OWASP ZAP and Burp Suite.

I’ve also recorded a Youtube playlist as a complement to the blog posts for you to see how I solve the hands-on challenges. So, you literally have all you need to build a solid knowledge of web application hacking.

Now that you know what you will expect from this OWASP Top 10 guide, let’s get into the meat and potatoes! Let’s learn those risks and practice on hands-on challenges! 

OWASP Top 10 vulnerabilities explained

This section provides you with the OWASP Top 10 summary of all the security risks. For each one of them, there are links to dedicated posts which detail the theory and help you practice on hands-on challenges.

I recommend you bookmark this page and learn each vulnerability at a time. Once you finish it to the end, you will have a solid understanding and will be ready to test the OWASP Top 10 vulnerabilities on your own. You can even look for what you’ve learned on bug bounty platforms and get paid!

Injection

An injection is a security risk that you can find on pretty much any target. Basically, it happens when a server-side interpreter processes untrusted user input as part of a command or a query. There are many vulnerabilities which cause injection. Here are some examples:

  • SQL injection: You can find a SQL injection when the developer runs a SQL query that takes a parameter you control as an input. If you successfully exploit it, you steal data from the database, edit it or delete it altogether. 
  • OS command injection: It happens when user input is used as part of an insecure call to operating system commands. If you find one, you can run arbitrary operating system commands on the vulnerable server.
  • XPATH injection: It targets the query language typically used in XML. When you can control part of the query. Therefore, you can bypass restrictions, read unauthorized XML nodes, etc. 
  • Server-Side Template Injection: This flaw affects applications which use template engines to render server-side data. If you can control variables passed into the template, you can achieve remote code execution. 
  • LDAP Injection: When your target insecurely uses some user input to query an LDAP directory, you can perform an injection to bypass restrictions, read unauthorized data, etc.

Broken authentication and session management

Authentication is a feature which verifies an identity’s claims. For example, when you login into an application, it uses your username and password to verify that you are indeed who you are claiming to be. Upon authentication, and due to the stateless nature of HTTP, the application provides you with a session representing your identity, which your web browser sends on your subsequent requests.

Of course, you need to be able to sign up, log in, reset your password or enable Multi-Factor authentication. That’s why authentication is hard to implement without making any mistakes. Any flaw in one of those features can lead to broken authentication. We cover this in detail in a dedicated blog post.

Sensitive data exposure

If your IT assets disclose data which is not meant to be publicly accessible, they suffer from sensitive data exposure. On the one hand, this data can be at rest, like your databases or files. On the other hand, it can be in transit, especially if you are using unencrypted or weak encryption for your data transmission.

Apart from exposing your customers’ data which is a scandal, you will also get fines for exposing them. Think of the GDPR regulation where fines can go up to 20 Million Euros.

XML-External Entity (XXE)

XXE is a flaw in the way XML parsers get configured. Specifically, this vulnerability happens when the XML parser can evaluate DTDs and external entities. It allows an attacker to achieve many exploits, like listing directories and reading files from the server. It can even provoke a Denial of Service.

Broken access control

Broken access control happens when the application allows a user to perform unauthorized actions. There are many vulnerabilities which contribute to this risk, For instance, if the developer forgets to validate permissions when dealing with identifiers, the application becomes vulnerable to Insecure Direct Object Reference (IDOR).

Other vulnerabilities include Cross-site Request Forgery (CSRF), Cross-Origin Resource Sharing (CORS) misconfigurations and forced browsing. Read more about them in the dedicated blog post.

Security misconfiguration

Security misconfigurations, as the name suggests, expose vulnerabilities due to weak configurations of an IT asset. It doesn’t affect web assets only. Any component which requires a configuration is subject to this vulnerability. This means that network devices, hardware, email services, etc. can suffer from this vulnerability. For instance, your smart door lock can have a predefined default administration PIN code. If you don’t change it, anyone can access and change your device configuration.

In the context of web applications, you can find things like directory listing enabled, which would allow you to list all files and directories. Or maybe the developer forgot to disable the debug mode, allowing you to get more insights on the inner-workings of the vulnerable application.

Cross-site Scripting (XSS)

This is one of the famous client-side vulnerabilities. It allows an attacker to run arbitrary Javascript code on the victim’s web browser. XSS becomes possible when user input ends up inside an HTML page or a piece of Javascript code without proper encoding.

There are basically three types of XSS, all of them along with hands-on tutorials are explained further:

  • Stored XSS happens when the user input gets stored in the application’s datastore, then retrieved back and rendered in a page without proper encoding. 
  • Reflected XSS happens when user input gets directly returned into the HTML page without proper encoding.
  • DOM XSS happens when user input gets inside a Javascript code. Here, it is possible to exploit XSS even if there is no request made to the server.

Insecure deserialization

Insecure deserialization happens when the developer doesn’t check serialized data that a user sends to the application. This is another vulnerability where a lack of user input validation can lead to serious security problems. It is hard to exploit, but when it works, it can lead to either remote code execution or denial of service.

Using components with known vulnerabilities

You might have totally secured your own code, but what about the dependencies you are using? Have you checked them or just imported them into your code? There is a high chance that one or more of them are vulnerable. 

Unfortunately, using components with known vulnerabilities had led to many serious breaches in the past, and will still cause many breaches to come. But you already have the tools to check for them. For more in-depth knowledge of that, head to this dedicated article.

Insufficient logging and monitoring

When a hacker infiltrates a network, IT systems will generate traffic which usually doesn’t correspond to the normal one, unless you are dealing with highly skilled hackers who have time and money to go after your IT infrastructure. If you can’t detect this abnormal behaviour as soon as possible, you are essentially giving them enough time to achieve their goal. Read more about this in this blog post.

Logging and monitoring should be part of your essential security infrastructure because you simply cannot defend what you don’t know.

Beyond the OWASP Top 10

If you have reached this far, let me congratulate you for taking real steps towards your goal! I really mean it because not everyone is willing to put the effort as you did! I hope you’ve enjoyed the journey as much as I did preparing this OWASP Top 10 training.

If you want to take your skills steps further, here are some leads you can follow.

Read infosec books

Reading information security books is rewarding because you will be learning from experts in the field. I recommend the Web Application Hacker Handbook edition 2. It’s a huge book, but it gives you so many details on different security vulnerabilities and explains how to detect and exploit each one of them.

Learn other OWASP top ten projects

OWASP publishes new documents are covering new topics. For instance, the OWASP API security project covers security risks related to APIs. Also, you have the OWASP Top 10 Mobile security risks which covers security risks in mobile applications.

Learn infrastructure penetration testing

Testing web applications for security vulnerabilities is good, but what if you want to test the security of a client’s infrastructure. In this case, you have to expand your knowledge and skills further. You can learn about networking, Active Directory and other services. Then, learn how to perform enumeration, scanning, exploitation, privilege escalation and persistence. There are many courses which cover that, I recommend the Offensive Security PWK course.

Practice on hacking platforms and CTFs

When I started my journey in ethical hacking, I’ve learned so much from hacking platforms like root-me and VulnHub. There are many new emerging platforms, which means that the market is expanding and the demand for security testing skills is growing. Some platforms like hackthebox also give you exposure to hiring companies. So it’s a great opportunity for you to learn, practice, have fun and get a job in information security, all at the same time!

Earn money while hacking ethically

If you want to monetize your knowledge, you can get paid when finding security vulnerabilities with the rise of bug bounty hunting platforms like Hackerone, Bugcrowd, YesWeHack, Intigriti among many others. I personally hack on Hackerone.

I hope this OWASP Top 10 guide has opened your eyes to see how wonderful the industry of information security is! You now have a solid foundation of web application security.

You are now ready to tackle new challenges in your information security career. Good luck!