Best Practices to Keep Website Secure from Cross-Site Scripting (XSS)

Introduction:

Cross-site scripting (XSS) is a computer security vulnerability typically found in web applications that enable malicious attacks through injected client-side scripts. Malicious users can inject scripts into legitimate sites, intercept data or misrepresent HTML/CSS. XSS attacks are considered a serious security risk and can enable hackers to access sensitive data such as usernames, passwords, and credit card numbers.

They are often used with attacks such as SQL injection (SQLi) and cross-site request forgery (CSRF). This allows attackers to access database servers and their sensitive information, like passwords or credit card numbers.

In this article, we will discuss what cross-site scripting (XSS) is and why it's so dangerous. The following best practices will assist you in protecting your web application from XSS attacks.

What is Cross-Site Scripting?

Web applications are most commonly vulnerable to cross-site scripting (XSS). Cross-site scripting flaws can be used to steal cookie credentials, compromise user sessions, and launch phishing attacks.

XSS was the second most prevalent kind of online application vulnerability, making up 14% of all vulnerabilities, according to the most recent annual Verizon Data Breach Investigations Report (DBIR), which was published in May 2021.

In simple words, XSS is when a website's code is manipulated so that malicious code gets reflected onto the site's pages and executed on the visitor's browser (usually without their knowledge). This can happen when you load an external script from another domain into your page without properly validating it first — which leaves room for hackers to exploit this weakness and insert their code into the page being served up by your server.

XSS attacks can be divided into three categories: stored, reflected, and DOM-based.

1.    Stored Cross-Site Scripting (XSS) occurs when user input is stored without proper validation or escaping and later displayed back to the user without being properly encoded. This type of attack mostly comes under SQL Injection Attacks.

2.    Reflected Cross-Site Scripting (XSS) occurs when user input containing malicious code is sent to an application via HTTP requests. This type of attack uses HTTP response headers to send the malicious code back to the client browser that displays it on their screen.

3.    DOM-based Cross-Site Scripting (XSS) occurs when malicious code is injected into DOM elements of a webpage at runtime and then executed by the browser. This type of attack uses DOM properties and methods to execute malicious code on a webpage.

How Does Cross-Site Scripting Work?

A cross-site scripting attack, or XSS attack, is a type of malicious attack that involves injecting a client-side script into a target website. Scripts can be written in JavaScript or VBScript, but CSS or HTML5 can also be used.

A cross-site scripting vulnerability injects the script into a legitimate website or application. The vulnerabilities are most commonly found on third-party-developed websites — for example, if you sell products online through an eCommerce platform, attackers may be able to access your database and store credit card information illegally if the platform contains vulnerabilities.

Cross-site scripting attacks can be used to steal sensitive data, such as passwords and credit card numbers, from users. 

Since they occur on the server side rather than the client side (i.e., in your browser), cross-site scripting attacks are also called "server-side injections".

What are Status Codes?

Status codes are responses sent by a server in response to a request. They are used to indicate whether or not the request was successful and can also be used to provide additional information about the request.

Status codes fall into five categories:

Informational (1xx): The request was received, but there is nothing else to say about it; for example, 200 OK.

Success (2xx): The request was received and understood; for example, 200 OK. Sometimes, a client may receive multiple successful responses in sequence (for example, when downloading several image files).

Redirection (3xx): The resource has been moved temporarily or permanently; for example, 301 Moved Permanently or 302 Found.

Client Error (4xx): The request contains bad syntax or cannot be fulfilled at this server; for example, 404 Not Found. No indication is given as to whether this condition is temporary or permanent. A response may be received from the web server after several attempts at accessing a website that no longer exists.

5xx Server Error: The server failed to fulfill a valid request (e.g., 500 Internal Server Error).

Best Practices to Keep Website Secure from Cross-Site Scripting (XSS)

Here are some best practices from OWASP to keep your website secure from XSS:

1. Input Validation

Input validation is an important security measure that needs to be implemented on all pages of your website. Input validation ensures that each input parameter in a web form is validated before it's sent to the server-side code for processing. The goal of input validation is to prevent attackers from sending malicious data that could harm your website or application.

2. Encode Output

Use HTML entities (such as &), character references, and/or escaping instead of embedding actual characters in your output. This will ensure that any script elements are encoded before being rendered into the page.

3. Make use of the Content Security Policy (CSP)

CSP is an HTTP header that allows you to reduce the risk of XSS attacks by allowing you to specify which resources a website can load and from where. You can also use CSP to ban inline JavaScript on your pages.

4. Use HTTP-only Cookies

HTTP-only cookies don't transmit over non-secure connections. They cannot be accessed by code running in non-secure contexts, such as JavaScript loaded from another domain or subdomain (such as iframes). An XSS attacker cannot log into your account without your permission using an existing cookie.

5. Install a Web Application Firewall (WAF)

A WAF protects against malicious code injection in a web application firewall (WAF). To detect malicious activity, a WAF inspects all incoming requests and response injections or cross-site scripting (XSS). In addition, a good WAF prevents attackers from scanning your site for vulnerabilities using automated tools.

6. Update Frameworks and Web Applications Regularly

Regularly updating web applications and frameworks prevents attackers from exploiting bugs. This includes running regular scans for known vulnerabilities in popular websites and frameworks such as WordPress, Drupal, and Magento.

7. Implementing Authentication and Authorization Mechanisms

One way to avoid XSS attacks is by implementing authentication mechanisms like CAPTCHA, which prevents bots from accessing forms on websites where sensitive information is entered, such as password reset pages or login forms.

8. Providing Users and Developers With Security Best Practices Education

Make sure all users who have access to your website's code understand how XSS attacks work, what they look like, how they're prevented, and what they look like when they occur — so they can report them if they do happen on your site.

9. Sanitize User-Generated Content

The best way to protect yourself from XSS attacks is to sanitize user-generated content before you display it on your website. Sanitization removes malicious code from user input by filtering out unsafe characters like <script> tags and double quotes (") from strings. We recommend using the built-in sanitizer provided by WordPress core or an add-on such as AntiSamy or HTML Purifier.

10. Limit the Use of Client-Side Scripting (JavaScript)

JavaScript is used for creating interactive website content and can be used for malicious purposes if not properly managed. This can lead to cross-site scripting and other security issues, so limit its use as much as possible.

Conclusion

Webmasters can do their best to protect their sites from XSS, but they should also be familiar with what can happen after a security breach. Hopefully, knowing these tips will help you get up and running quickly if you ever find yourself the victim of a cross-site scripting attack. And if your site is secure against XSS, that's one more website that's safer in our increasingly dangerous digital world.