Preface

If you don’t know what XSS (Cross-site Scripting) is, in simple terms, it is when a hacker can execute JavaScript code on your website. Since it can be executed, it is possible to steal the user’s token, impersonate the user’s identity to log in, even if the token cannot be stolen, it can still modify the page content, or redirect the user to a phishing website, and so on.

To prevent XSS, it is necessary to prevent hackers from executing code on the website, and there are many ways to defend against it. For example, CSP (Content-Security-Policy) can be used as an HTTP response header to prevent the execution of inline scripts or restrict the domains from which scripts can be loaded. Trusted Types can also be used to prevent some potential attacks and specify rules, or use some libraries that filter XSS, such as DOMPurify and js-xss.

But is it enough to use these? Yes and no.

If used correctly, of course, there is no problem, but if there are incorrect settings, there may still be XSS vulnerabilities.

Recently, I just transferred from a company to a cybersecurity team, Cymetrics, and when I was researching some websites, I found a ready-made case. Therefore, this article uses this ready-made case to illustrate what is called incorrect settings and what impact this setting will have.

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Preface

Recently, my interest is playing CTF, and I only play web problems inside, for a simple reason, because I don’t know anything about other fields… Currently, I am more interested in web things, so I solve problems as a leisure activity.

This article is a summary of the BambooFox CTF 2021, and I only solved three problems.

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Introduction

As a front-end engineer, it is natural to know a lot about front-end-related knowledge, such as HTML or JS-related things, but those knowledge is usually related to “use”. For example, I know that when writing HTML, I should be semantic and use the correct tags; I know how to use JS. However, some knowledge related to web pages, although related to web pages, is not something that front-end engineers usually come into contact with.

What I mean by “some knowledge” actually refers to “knowledge related to information security”. Some concepts commonly found in information security, although related to web pages, are things that we are not familiar with, and I think understanding these is actually very important. Because you must know how to attack in order to defend, you must first understand the attack methods and principles before you know how to defend against these attacks.

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