Implementation of a robust and secure OTP (One-Time Password) authentication mechanism
public class RegistrationController : Controller
{
[HttpPost]
public IActionResult Register(UserModel model)
{
if (ModelState.IsValid)
{
// Register the user
var newUser = new User { Username = model.Username, Password = model.Password };
_context.Users.Add(newUser);
_context.SaveChanges();
// Send OTP
var otp = new Random().Next(100000, 999999);
_smsService.Send(model.PhoneNumber, $"Your verification code is {otp}");
// Redirect to OTP verification page
return RedirectToAction("Verify", new { userId = newUser.Id });
}
return View(model);
}
[HttpPost]
public IActionResult Verify(int userId, string otp)
{
// Here the OTP is not validated
var user = _context.Users.Find(userId);
if (user != null)
{
user.IsVerified = true;
_context.SaveChanges();
return RedirectToAction("Index", "Home");
}
return NotFound();
}
}
The code above is a simplified example of a registration process in an ASP.NET Core application. Here, a user can register by providing a username, password, and phone number. After registration, a 6-digit One-Time Password (OTP) is generated and sent to the user's phone number.
The vulnerability lies in the
Verify method, which is supposed to confirm the OTP entered by the user. However, the provided OTP is not checked against the one that was sent to the user. This means that any OTP, even one that doesn't match the sent OTP or doesn't comply with the 6-digit requirement, would be considered valid. As a result, an attacker could bypass this verification step by providing any OTP.
public class RegistrationController : Controller
{
[HttpPost]
public IActionResult Register(UserModel model)
{
if (ModelState.IsValid)
{
// Register the user
var newUser = new User { Username = model.Username, Password = model.Password };
_context.Users.Add(newUser);
_context.SaveChanges();
// Generate OTP
var otp = new Random().Next(100000, 999999);
newUser.OTP = otp;
_context.SaveChanges();
// Send OTP
_smsService.Send(model.PhoneNumber, $"Your verification code is {otp}");
// Redirect to OTP verification page
return RedirectToAction("Verify", new { userId = newUser.Id });
}
return View(model);
}
[HttpPost]
public IActionResult Verify(int userId, string otp)
{
// Validate the OTP
var user = _context.Users.Find(userId);
if (user != null && user.OTP == otp)
{
user.IsVerified = true;
_context.SaveChanges();
return RedirectToAction("Index", "Home");
}
return NotFound();
}
}
The updated code now includes OTP validation in the
Verify method. When a user registers, an OTP is generated and stored in the user's record in the database. This OTP is then sent to the user's phone number.
When the user enters the OTP on the verification page, the
Verify method retrieves the user's record from the database and compares the entered OTP with the stored OTP. If the OTPs match, the user's account is marked as verified and the user is redirected to the home page. If the OTPs do not match or if the user's record cannot be found, the method returns a
NotFound result.
This update addresses the vulnerability by ensuring that the entered OTP matches the one sent to the user's phone number, thereby preventing authentication mechanism evasion. It also uses a secure and reliable OTP generation algorithm to generate the verification code and stores the generated OTP securely.
Additional security measures such as rate limiting, IP whitelisting, or CAPTCHA can be implemented to protect against brute-force or automated attacks. Regular reviews and updates of the authentication mechanism can also help to address any new vulnerabilities or emerging threats.