CSS Injection

👉 Overview

👀 What ?

CSS Injection is a type of web vulnerability that occurs when an application includes user input in its CSS without properly validating or escaping it. This allows attackers to inject malicious CSS code into a web page, which can lead to a variety of attacks such as defacing websites, stealing user data, or executing malicious scripts.

🧐 Why ?

Understanding CSS Injection is important because it is a common web application security flaw that can have severe consequences if exploited. It can lead to information disclosure, unauthorized actions on behalf of the user, or even complete takeover of the user's session. Therefore, both developers and security professionals should be aware of this vulnerability, how it can be exploited, and how to prevent it.

⛏️ How ?

To exploit a CSS Injection vulnerability, an attacker typically needs to find a point of input within the application that is included in CSS. This could be a user profile field, a search box, or any other user input that is reflected in the CSS of the page. The attacker can then inject malicious CSS code into this input, which will be executed when the page is loaded. To prevent CSS Injection, developers should properly validate and escape all user input that is included in CSS. Additionally, using Content Security Policy (CSP) can help to mitigate the impact of a potential CSS Injection attack.

⏳ When ?

CSS Injection has been a known vulnerability since the early days of the web, but it has become more prevalent with the rise of complex web applications that heavily rely on CSS for styling and layout.

⚙️ Technical Explanations

At a technical level, CSS Injection works by exploiting the fact that CSS allows for arbitrary code execution through certain features such as the 'expression' function in Internet Explorer, or the 'url' function in other browsers. If an attacker can control the input to these functions, they can execute arbitrary CSS code. This code can manipulate the layout and appearance of the web page, but it can also be used to execute JavaScript and other scripting languages in some circumstances. Therefore, even though CSS is mostly a styling language, it can be used as a vector for more serious attacks if not properly secured.