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HTTP parameter pollution

Need​

Prevention of HTTP parameter pollution

Context​

  • Usage of C# for developing robust and efficient applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications using the ASP.NET Core MVC framework
  • Usage of Newtonsoft.Json for JSON serialization and deserialization

Description​

Non compliant code​

[HttpPost]
public IActionResult Login(string username, string password)
{
    // Authenticate the user
    User user = AuthenticateUser(username, password);

    if (user != null)
    {
        // Create a new session
        Session session = new Session(user);
        HttpContext.Session.SetString("UserSession", JsonConvert.SerializeObject(session));

        return RedirectToAction("Index", "Home");
    }

    return View();
}

In this piece of code, the Login action method in a controller is vulnerable to HTTP Parameter Pollution (HPP). The method takes two parameters, username and password, from the HTTP request and uses them to authenticate the user.

The problem is that the application does not validate the parameters or protect against additional, unexpected parameters. This means an attacker could inject extra parameters into the HTTP request, causing unexpected behavior.

For example, an attacker could inject an additional username parameter into the request. If the server simply takes the last username parameter it receives, the attacker could log in as any user. Similarly, injecting an additional password parameter could allow an attacker to bypass password checks.

This vulnerability could be exploited to perform actions as other users, gain unauthorized access to data, or cause other unexpected server behavior.

Steps​

  • Validate and sanitize user input before using it in the code.
  • Use a whitelist approach to validate and accept only expected parameters.
  • Implement input validation and parameter binding techniques provided by the ASP.NET Core framework.
  • Avoid directly using user input in sensitive operations without proper validation and sanitization.
  • Consider using a secure authentication mechanism, such as token-based authentication, to prevent unauthorized access.
  • Regularly update and patch the ASP.NET Core framework to benefit from the latest security enhancements.
  • Perform security testing and code reviews to identify and fix any potential vulnerabilities.

Compliant code​

[HttpPost]
public IActionResult Login([FromForm] LoginModel model)
{
    if (ModelState.IsValid)
    {
        // Authenticate the user
        User user = AuthenticateUser(model.Username, model.Password);

        if (user != null)
        {
            // Create a new session
            Session session = new Session(user);
            HttpContext.Session.SetString("UserSession", JsonConvert.SerializeObject(session));

            return RedirectToAction("Index", "Home");
        }
    }

    return View();
}

public class LoginModel
{
    [Required]
    public string Username { get; set; }

    [Required]
    public string Password { get; set; }
}

The updated code now uses the ASP.NET Core model binding feature to bind HTTP POST request parameters to a model. This model, LoginModel, contains properties for Username and Password which are marked as required using the [Required] attribute. This ensures that these parameters are present in the request, otherwise, the model state will be invalid.

The Login action now accepts a LoginModel object as a parameter instead of separate username and password parameters. This helps to prevent HTTP parameter pollution as only the parameters defined in the LoginModel will be bound from the request.

Before authenticating the user, the code checks if the model state is valid using ModelState.IsValid. This ensures that the required parameters are present and in the correct format.

The rest of the code remains the same. If the user is authenticated successfully, a new session is created and the user is redirected to the home page. If the user is not authenticated, the login view is returned.

This approach helps to mitigate the HTTP parameter pollution vulnerability by ensuring that only expected parameters are accepted and validated before use. It also provides a clear and explicit mapping of expected parameters, making the code easier to understand and maintain.

References​

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