Basic ASP.NET Web Form Security Practices

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The following are some of the basic ways one can ensure security in the case of ASP.NET web forms:

1. **Input Validation**: Always validate inputs coming from users to prevent injection attacks, like SQL injection and cross-site scripting. Use out-of-the-box validation controls provided by ASP.NET or custom logic by using RequiredFieldValidator, RangeValidator, and RegularExpressionValidator.

2. **AntiForgery Tokens**: This will avoid Cross-Site Request Forgery (CSRF) attacks; use `@Html.AntiForgeryToken()` provided by ASP.NET in your form. It helps in ensuring requests are coming from the legitimate user.

3. **Output Encoding**: Encode all output data to avoid Cross-Site Scripting (XSS) attacks. ASP.NET provides methods like `Server.HtmlEncode()` to ensure data displayed on the page is safe.

4. **Authentication and Authorization**: Secure each sensitive page by authenticating it, for instance, using ASP.NET Identity or Forms Authentication, followed by using role-based authorization.

5. **Use HTTPS**: Always make use of HTTPS to encrypt data sent from the client to the server, hence avoiding man-in-the-middle attacks.

6. **Session Management**: Follow best practices in managing sessions, including setting the session timeout and using secure cookies (`HttpOnly`, `Secure` attributes) to keep session data secure.

7. **Limit File Uploads**: In file upload handling, restrict types of files uploaded and limit file size. Never trust the file extension; always validate it on the server side.

8. **XSS Protection**: Use `HttpOnly` cookies and properly encode/escape user inputs in order to mitigate XSS attacks.

9. **Password Storage**: Let your password, always be in a secure store, hashed by algorithms like SHA-256 or bcrypt. Never store passwords as plain text.

10. **Security Headers**: Make use of HTTP security headers like `X-Content-Type-Options`, `Strict-Transport-Security (HSTS)`, `X-Frame-Options`, and `Content-Security-Policy (CSP)` to mitigate against different attack vectors.

The above practices will greatly improve the security of your ASP.Net web applications.