Preventing cross-site scripting (XSS)

1. Output Encoding

• HTML Encoding: Convert special HTML characters (like <, >) into HTML entities (<, >) to prevent them from being interpreted as HTML or JavaScript by the browser.

• URL Encoding: Use URL encoding for special characters in URLs to prevent XSS in URL parameters.

• Special Cases: Be cautious of corner cases like UTF-7 encoding; ensure the browser renders pages in UTF-8 to avoid issues.

2. JavaScript-Specific Considerations

• Escape Characters in JavaScript: When rendering user data as part of JavaScript, escape special characters like single quotes (') to prevent code injection.

• Avoid innerHTML: Using innerHTML can lead to XSS if user input is directly inserted. Instead, use textContent or innerText to insert text safely.

• Avoid eval(): The eval() function is dangerous as it executes arbitrary code, making it vulnerable to injection attacks. If necessary, scrutinize the input rigorously.

3. Input Validation

• Whitelisting: Implement a whitelist of acceptable characters or strings, rejecting all others. This limits the input to known safe values.

• Blacklisting: Avoid blacklisting as it's ineffective; attackers can easily bypass it with various techniques.

• Client-Side Validation: While useful for user experience, client-side validation is not secure as attackers can bypass it using proxies or other tools.

4. Defense in Depth

• Use multiple layers of defense, such as combining input validation with output encoding. This increases the chances of catching any potential security flaws.

5. Use Proven Libraries

• Avoid writing custom functions for encoding or validation. Instead, use established libraries that have been tested and are widely used in the industry.

6. Framework Implementation

• Centralize your validation and encoding logic in a framework to ensure consistent and thorough protection across your application.