csharp
// Web application using the vulnerable code
using System.Web.UI;

public partial class UploadFile : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            var handler = new FileUpload();
            handler.ProcessRequest(HttpContext.Current);
        }
    }
}

csharp
// Vulnerable code
using System;
using System.IO;
using System.Web;

public class FileUpload : IHttpHandler {
    
    public void ProcessRequest (HttpContext context) {
        HttpPostedFile file = context.Request.Files["file"];
        string savePath = context.Server.MapPath("~/uploads/");

        if (file != null && file.ContentLength > 0) {
            string fileName = Path.GetFileName(file.FileName);
            file.SaveAs(savePath + fileName);
        }
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }
}

csharp
// Solution to fix the vulnerable code
using System;
using System.IO;
using System.Web;

public class SafeFileUpload : IHttpHandler {
    
    public void ProcessRequest (HttpContext context) {
        HttpPostedFile file = context.Request.Files["file"];
        string savePath = context.Server.MapPath("~/uploads/");

        if (file != null && file.ContentLength > 0) {
            string fileName = Path.GetFileName(file.FileName);
            string fileExtension = Path.GetExtension(file.FileName).ToLower();

            // Only allow specific file types
            string[] allowedExtensions = { ".jpg", ".jpeg", ".png", ".gif" };
            if (Array.IndexOf(allowedExtensions, fileExtension) >= 0) {
                file.SaveAs(savePath + fileName);
            } else {
                context.Response.Write("Invalid file type. Only .jpg, .jpeg, .png, and .gif are allowed.");
            }
        }
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }
}