What are the prevention measures for cross-site scripting (XSS)?

Prevention measures for Cross-Site Scripting (XSS) involve a combination of secure coding practices, input validation, and output encoding. Here’s a breakdown with examples:

1. Input Validation:

   • Validate and sanitize all user inputs on both client and server sides. Reject or escape any unexpected data.
   • Example: If a form field expects an email, ensure the input matches a valid email format using regex.

2. Output Encoding:

   • Encode data before rendering it in the browser to prevent scripts from executing. Use context-specific encoding (HTML, JavaScript, URL, etc.).
   • Example: Convert < to < and > to > when displaying user-generated content in HTML.

3. Content Security Policy (CSP):

   • Implement CSP headers to restrict the sources of executable scripts. This mitigates the impact of XSS by blocking unauthorized scripts.
   • Example: Set a CSP header like Content-Security-Policy: default-src 'self'; script-src 'self' to allow scripts only from your domain.

4. HttpOnly and Secure Flags for Cookies:

   • Mark cookies as HttpOnly to prevent JavaScript access and Secure to ensure they’re transmitted only over HTTPS.
   • Example: Configure cookies in server responses with Set-Cookie: sessionId=abc123; HttpOnly; Secure.

5. Use Frameworks with Built-in XSS Protection:

   • Modern frameworks (e.g., React, Vue) automatically escape outputs, reducing XSS risks.
   • Example: In React, rendering <div>{userInput}</div> automatically escapes HTML.

6. Avoid Inline Scripts and Event Handlers:

   • Refrain from using inline JavaScript (onclick, javascript: URLs) as they are common XSS vectors.
   • Example: Replace <button onclick="doSomething()"> with event listeners in external scripts.

For cloud-based applications, Tencent Cloud Web Application Firewall (WAF) can help detect and block XSS attacks by filtering malicious traffic before it reaches your servers. Additionally, Tencent Cloud’s Content Security Service provides advanced detection for malicious scripts and content.