Fix Broken User Authentication in ASP.NET Core

Executive Summary

Broken Authentication is the kill chain's favorite entry point. If you're rolling your own credential validation or failing to implement account lockouts, you're practically handing over the keys to the kingdom. In ASP.NET Core, manual string comparisons and lack of rate limiting are rookie mistakes that lead to credential stuffing and brute-force pwnage. We're ditching the 'creative' auth logic for hardened, industry-standard middleware.

The Vulnerable Pattern

VULNERABLE CODE

[HttpPost("login")]
public IActionResult Login(string username, string password) {
    // VULNERABILITY: Plaintext comparison and no lockout mechanism
    var user = _context.Users.FirstOrDefault(u => u.Username == username);
    if (user != null && user.Password == password) {
        var token = GenerateJwtToken(user);
        return Ok(new { token });
    }
    // Timing attack surface: returns quickly on wrong username
    return Unauthorized();
}

The Secure Implementation

The vulnerable code is a goldmine for attackers: it stores passwords in plaintext (or assumes they are pre-hashed) and performs a direct comparison, making it vulnerable to SQLi or simple data leaks. Worse, it lacks account lockout logic, allowing an attacker to fire infinite login attempts. The secure version leverages 'Microsoft.AspNetCore.Identity', which enforces salted PBKDF2 hashing by default and implements a 'lockoutOnFailure' flag. This triggers a cooling-off period after X failed attempts, effectively neutralizing automated credential stuffing attacks. It also abstracts the validation logic to prevent timing attacks that could reveal valid usernames.

SECURE CODE

[HttpPost("login")]
public async Task Login([FromBody] LoginRequest request) {
    // SECURE: Use ASP.NET Core Identity with built-in lockout and secure hashing
    var result = await _signInManager.PasswordSignInAsync(
        request.Email, 
        request.Password, 
        isPersistent: false, 
        lockoutOnFailure: true // MITIGATES BRUTE FORCE
    );
if (result.Succeeded) {
    return Ok("Authenticated");
}
if (result.IsLockedOut) {
    return StatusCode(423, "Account locked due to multiple failed attempts.");
}
return Unauthorized("Invalid credentials.");
}

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How to fix Broken User Authentication
in ASP.NET Core

Executive Summary

The Vulnerable Pattern

The Secure Implementation

Protect your ASP.NET Core API

Verified by Ghost Labs Security Team