Skip to content

Month: December 2020

Writeups on Free Challenges in BugBountyHunter

In this post, I will share some writeups on the free challenges in BugBountyHunter platform. I encourage everyone to check out the site

Cross-Site Scripting in Image Tag

How does the feature works? If you select the dropdown option, the image will be rendered with special effects using CSS. In the img tag, we can see the selected option value will appear in class.

What could go wrong? The POST request looks suspicious.


There is no server side validation of the imageClass value. You can send any value and it will be reflected in the response.


We can try sending a few payloads to test for XSS issue. First, we can modify the imageClass value to imageClass=helloworld" onload="alert(1)". The response return a class="1" in the tag. We can try imageClass=helloworld" onclick="alert(1)". The response also return class="1" in the tag.

It seems like there is some blacklisting of some keywords such as onload or onclick etc. Hence we need to find the event handler method that is not blacklisted. Fortunately, this payload was not blocked: img2" onpointerup="alert('xss').

When we click on the image in the browser, the malicious script is executed.

Learning JavaScript as Beginner?

Besides learning about Python ASYNCIO, For the last few weeks, I have been learning JavaScript for web development. My methodology is to consume the knowledge from multiple resources (book, blogs and MOOCs).

Why multiple resources that explain the same concepts?
If you use different resources, you will be exposed to the concept in different context. This is especially useful for beginners to not stuck in one context. You need to understand the concept in different situation.

Also, feel free to modify the tutorial steps. Add in anything that is interesting. Apply previously learned knowledge to the tutorial. Combine two different concepts. In short, be active in experimenting.

For this post, I want to share some resources that are useful in my journey of learning JavaScript.

The Modern JavaScript Bootcamp

If you are starting to learn JavaScript from the basics, please use this course. I find that there is a balanced mix of explanation and practical usage of concepts.

One particular thing that is useful is the challenges that the course instructor gave to the students. After the instructor demonstrated on a practical concept, you are expected to complete the variant of the demo.

Eloquent JavaScript

Eloquent JavaScript

Disclaimer: I cannot give a complete review since I completed only the earlier chapters (1-7). In future, I will read the remaining chapters again.
The book introduces foundational programming knowledge. If you are new to programming, you can consider reading chapter 1 – 7 to learn the fundamentals. The chapter on different JavaScript built-in functions for Arrays (e.g. forEach(..), filter(...) and map(...)) was useful later on when I studied with the other MOOCs.

I also advise beginners to try the few challenges that are available at the end of each chapters. I have consolidated my understanding by doing these challenges. Some of the challenges may be difficult. So you should feel free to refer to the code (this is not school).

The Complete React Developer Course

Before you take this React course, I suggest that you take Modern JavaScript Bootcamp. At the same time, you should create a few demo web applications. If you want to learn Web Development, then React is one of the JS framework that you need to learn. Why? Because of the wide adoption. One cool thing that I like about React is the speed of rendering and JSX (JavaScript XML).

Food for Thought: 
React seems like a powerful framework that allows the application to process and compute data for the client side. Does this mean that more application will start perform business logic workflows in the client side and forgets about backend validation?