How do you prevent cross-site scripting attacks?

1 Use Content Security Policy

Content Security Policy (CSP) is a HTML5 feature that lets you define a whitelist of sources and scripts that are allowed to run on your web pages. CSP can block any inline or external scripts that are not explicitly authorized by you. This can prevent most XSS attacks that rely on injecting script tags or attributes into your HTML code. To use CSP, you need to add a <meta> tag or a HTTP header with the content-security-policy attribute and the policy rules.

2 Escape User Input

One of the main causes of XSS attacks is unescaped user input that is rendered as HTML. For example, if you have a form that accepts user comments and displays them on your website, an attacker can enter a script tag or an HTML attribute with JavaScript code and execute it on your page. To prevent this, you need to escape user input before inserting it into your HTML. Escaping means replacing special characters like <, >, ", and ' with their HTML entities like &lt;, &gt;, &quot;, and &apos;. This way, the browser will not interpret them as HTML code, but as plain text.

3 Validate User Input

Another way to prevent XSS attacks is to validate user input before processing it. Validation means checking if the user input matches the expected format, type, length, and range of values. For example, if you have a form that accepts an email address, you can use a regular expression to check if the input is a valid email format. If not, you can reject it or ask the user to correct it. Validation can help you filter out malicious input that does not match your requirements and avoid injecting it into your HTML.

4 Use HTTPOnly Cookies

Cookies are small pieces of data that are stored by the browser and sent to the server with every request. Cookies can store user information, preferences, or session tokens. However, cookies can also be accessed by JavaScript code on the web page, which can expose them to XSS attacks. For example, an attacker can inject a script that reads the cookie value and sends it to a malicious server. To prevent this, you can use HTTPOnly cookies, which are cookies that cannot be accessed by JavaScript. To use HTTPOnly cookies, you need to set the HttpOnly flag in the cookie header or attribute.

5 Sanitize HTML Output

Sometimes, you may want to allow some user input to contain HTML tags, such as bold, italic, or links. For example, if you have a blog or a forum that supports rich text formatting. However, this also opens the door for XSS attacks, as an attacker can insert malicious HTML tags or attributes into their input. To prevent this, you need to sanitize HTML output, which means removing or replacing any unwanted or dangerous HTML elements or attributes. For example, you can use a library like DOMPurify or sanitize-html to sanitize HTML output in JavaScript.

6 Update Your Libraries and Frameworks

Finally, you need to keep your libraries and frameworks updated to prevent XSS attacks. Libraries and frameworks are code packages that provide useful features and functions for web development. However, they can also contain bugs or vulnerabilities that can be exploited by XSS attackers. For example, jQuery, Angular, React, and Bootstrap are popular libraries and frameworks that have had XSS issues in the past. To prevent this, you need to regularly check for updates and patches for your libraries and frameworks and apply them as soon as possible.