How to secure cascading style sheets?

Web development in this modern era heavily relies on Cascading Style Sheets (CSS), which play a crucial role in the visual appearance or designing of the UI and layout of web pages. The CSS enables website designers to create responsive and captivating websites that are easy on the eyes, regardless of the device used to access them. Nonetheless, CSS, like any other technology, has vulnerabilities, making it susceptible to security threats.

However, with the rise of cyber threats and security breaches, securing CSS has become a significant concern for web developers. The attackers can exploit vulnerabilities in CSS code to inject malicious code, steal sensitive information, or perform other malicious activities. Therefore, it is crucial for web developers to implement proper security measures to protect their CSS code and website.

In this article, we will learn the essential tips on how to secure your cascading style sheets (CSS) and ensure that your web applications are immune to potential attacks.

What are Possible Threats in CSS?

1. CSS Injection

CSS Injection is a common vulnerability that occurs when an attacker injects malicious code into a website's CSS file. This code can be used to redirect users to phishing sites, steal sensitive information, or perform other malicious activities.


The possible threat is that the below example code can be used which redirects the users to a phishing site or serve malicious content.


background-image: url('');

2. DoS Attack

A CSS DoS (Denial of Service) Attack is a technique used by attackers to overload a website's resources by using large or complex CSS files. This can cause the website to become unresponsive or crash.


The threat here is that the code can overload the website's resources and cause it to become unresponsive or crash.


* {
   animation-name: dos-attack;
   animation-duration: 20s;
@keyframes dos-attack {
   from { color: green; }
   to { color: blue; }

3. CSS Keylogger

A CSS Keylogger is a technique used by attackers to steal user keystrokes by using CSS code to track user input on a website. This technique can be used to steal sensitive information, such as passwords and credit card numbers.


The possible threat can be that the below example can be used by the attacker to track user input and steal sensitive information, such as passwords and credit card numbers.


input[type="text"]:focus {
   background-image: url('' + document.getElementById(“you-input-field”).value);

4. Cross-Site Scripting

Cross-Site Scripting (XSS) occurs when an attacker injects malicious code into a website's HTML or JavaScript, which is then executed by the victim's browser. In some cases, CSS can also be used to execute XSS attacks.


The possible threat here is that the below code example can execute malicious JavaScript code on the victim's browser, which can be used to steal sensitive information or perform other malicious activities.


background-image: url('javascript:alert("XSS Attack")');

How to Secure Cascading Style Sheet (CSS)?

1. Use HTTPS

It's safer to use because it encrypts your data. This is why it should be the default for all websites. Without it, your private information and login credentials are vulnerable to being intercepted by hackers. So, if you see a website that doesn't use HTTPS, avoid it like the plague!

To intensify the security of your CSS, it's crucial to have HTTPS in place. The reason behind this is that CSS files merge with various web resources, such as images and JavaScript, and so, are prone to security hazards. With HTTPS, you can ensure that every web resource, including your CSS, is safeguarded and transmitted securely through the internet.

Your website can utilize HTTPS if you get an SSL/TLS certificate and configure your web server accordingly. There are several available methods for this, including using Let's Encrypt or your web hosting provider's control panel.

2. Minimal Use of External CSS Dependencies

To increase the security of your CSS, consider reducing your reliance on external CSS dependencies. These dependencies are CSS files that are imported from sources outside of your website, like CDNs (content delivery networks). Although CDNs can positively affect website speed, they can also introduce vulnerabilities by becoming compromised or delivering harmful content.

Consider hosting your CSS files on your web server to minimize external dependencies. Local hosting gives you control over your CSS files, which can reduce the risk of compromise.

3. Keep Your CSS Files Up to Date

Finally, it is essential to keep your CSS files up to date to prevent security vulnerabilities. This means regularly checking for updates to your CSS libraries and frameworks and applying any security patches as soon as they become available. Additionally, you should be cautious when using third-party CSS libraries and frameworks, as they may not be regularly updated or may contain security vulnerabilities.

4. Validate User Input

One of the most common security risks associated with CSS is the injection of malicious code through user input. This can occur when a user submits a form or enters data into a text field, and the input is not properly sanitized before being included in a CSS file.

To prevent CSS injection attacks, you should always validate user input before using it in a CSS file. This can be done using server-side input validation, which checks user input for known patterns and blocks any input that contains potentially malicious code. Additionally, you can use client-side input validation, which provides immediate feedback to users when they enter invalid data, helping to prevent them from submitting malicious input.

5. Use Content Security Policy (CSP)

Content Security Policy (CSP) is a security standard that allows website owners to control the types of content that can be loaded onto their websites. By using CSP, you can prevent malicious code from being injected into your CSS files by specifying which domains are allowed to load content on your website.

To use CSP, you will need to add a Content-Security-Policy header to your website's HTTP response. This header specifies the rules for content loading on your website and can be customized to fit your specific security needs. For example, you can specify which domains are allowed to load images, scripts, and stylesheets on your website, and block any content that does not meet these criteria.


Cascading Style Sheets (CSS) are a very significant part of web development, as they play a vital role in designing the layout and visual appearance like the UI of web pages. However, they are not immune to security threats, which can lead to significant risks, including data breaches, malware injection, CSS keylogger, cross-site scripting (XSS), and denial of service attacks. There can be other potential threats also leading to CSS failure. To secure CSS, the developers can implement various security measures, including using HTTPS, using fewer external dependencies, keeping CSS files up to date, validating user input, and implementing Content Security Policy (CSP). By following the above points, developers can ensure their web applications are immune to potential attacks and keep their user data safe and secure.

Updated on: 02-May-2023


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