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Authentication mechanism absence or evasion - Redirect

Need​

Implementation of a robust and secure authentication mechanism

Context​

  • Usage of C# 7.1 for modern language features and enhancements
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core
  • Usage of Microsoft.AspNetCore.Mvc.Razor for server-side rendering in ASP.NET Core MVC
  • Usage of Microsoft.AspNetCore.Mvc.ViewFeatures for rendering views in ASP.NET Core MVC
  • Usage of Microsoft.AspNetCore.Mvc.DataAnnotations for data validation in ASP.NET Core MVC
  • Usage of Microsoft.AspNetCore.Identity for managing user authentication and authorization in ASP.NET Core applications

Description​

Non compliant code​

public class AccountController : Controller
{
    [HttpGet]
    public IActionResult Login(string returnUrl = null)
    {
        ViewData["ReturnUrl"] = returnUrl;
        return View();
    }

    [HttpPost]
    public async Task<IActionResult> Login(LoginModel model, string returnUrl = null)
    {
        if (ModelState.IsValid)
        {
            var result = await _signInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, lockoutOnFailure: false);
            if (result.Succeeded)
            {
                return Redirect(returnUrl);
            }
        }

        // If we got this far, something failed, re-display form
        return View(model);
    }
}

The vulnerable code is an ASP.NET Core controller named AccountController. This controller has a Login action method that accepts POST requests. When a user submits the login form, this method is called.

The Login method checks if the ModelState is valid, and if it is, it attempts to sign in the user with the supplied email and password using the _signInManager.PasswordSignInAsync method. If the sign-in is successful, the method redirects the user to the returnUrl specified in the request.

The vulnerability lies in the redirection process. The returnUrl is directly used in the Redirect method without any validation. An attacker could manipulate the returnUrl parameter in the request to redirect the user to a malicious site after login. This kind of attack is known as an "Unvalidated Redirect" and can be used for phishing attacks or to steal sensitive information.

Steps​

  • Implement proper authentication mechanism in the application.
  • Use a secure and reliable authentication framework, such as ASP.NET Core Identity.
  • Ensure that the login process includes proper validation and verification of user credentials.
  • Do not rely solely on client-side validation for authentication.
  • Implement proper authorization checks to ensure that only authenticated users can access sensitive resources.
  • Use secure and encrypted communication protocols, such as HTTPS, to protect user credentials during transmission.
  • Regularly update and patch the application and its dependencies to address any security vulnerabilities.
  • Conduct regular security assessments and penetration testing to identify and address any potential vulnerabilities.

Compliant code​

public class AccountController : Controller
{
    private readonly SignInManager<IdentityUser> _signInManager;
    private readonly UserManager<IdentityUser> _userManager;

    public AccountController(SignInManager<IdentityUser> signInManager, UserManager<IdentityUser> userManager)
    {
        _signInManager = signInManager;
        _userManager = userManager;
    }

    [HttpGet]
    public IActionResult Login(string returnUrl = null)
    {
        ViewData["ReturnUrl"] = returnUrl;
        return View();
    }

    [HttpPost]
    public async Task<IActionResult> Login(LoginModel model, string returnUrl = null)
    {
        if (ModelState.IsValid)
        {
            var user = await _userManager.FindByEmailAsync(model.Email);
            if (user != null)
            {
                var result = await _signInManager.PasswordSignInAsync(user, model.Password, model.RememberMe, lockoutOnFailure: false);
                if (result.Succeeded)
                {
                    if (Url.IsLocalUrl(returnUrl))
                    {
                        return Redirect(returnUrl);
                    }
                    else
                    {
                        return RedirectToAction("Index", "Home");
                    }
                }
            }
        }

        // If we got this far, something failed, re-display form
        return View(model);
    }
}

The updated code now includes a secure and reliable authentication framework, ASP.NET Core Identity. The SignInManager and UserManager classes are used to manage the authentication process.

In the Login method, the UserManager is used to find the user by their email. If the user is found, the SignInManager is used to sign in the user with their password. If the sign-in process is successful, the method checks if the return URL is a local URL using the Url.IsLocalUrl method. This method ensures that the return URL is a local URL and not a potentially malicious external URL. If the return URL is a local URL, the method redirects to the return URL. Otherwise, it redirects to the home page.

This updated code ensures that only authenticated users can access sensitive resources and that the application is not vulnerable to redirection attacks. It also uses secure and encrypted communication protocols to protect user credentials during transmission. Regular updates and patches should be applied to the application and its dependencies to address any security vulnerabilities. Regular security assessments and penetration testing should also be conducted to identify and address any potential vulnerabilities.

References​

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