GuardAPI Logo
GuardAPI
Automated Security Protocol

How to fix Broken User Authentication
in .NET 8 Web API

Executive Summary

Broken User Authentication (OWASP A07:2021) remains a primary vector for account takeover. In .NET 8, the most common failures involve 'rolling your own' JWT logic, weak password hashing, and lack of account lockout mechanisms. To secure a Web API, you must abandon manual token generation in favor of the battle-tested ASP.NET Core Identity framework and robust JWT Bearer middleware.

The Vulnerable Pattern

VULNERABLE CODE
[HttpPost("login")]
public IActionResult Login([FromBody] LoginModel model)
{
    // VULNERABILITY: Hardcoded secret, weak hashing, and no expiration
    var user = _db.Users.FirstOrDefault(u => u.Username == model.Username && u.Password == model.Password);
    if (user == null) return Unauthorized();

var tokenHandler = new JwtSecurityTokenHandler();
var key = Encoding.ASCII.GetBytes("super_secret_key_123"); // Hardcoded & weak
var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(new[] { new Claim("id", user.Id.ToString()) }),
    SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};
var token = tokenHandler.CreateToken(tokenDescriptor);
return Ok(new { Token = tokenHandler.WriteToken(token) });


}

The Secure Implementation

The secure implementation fixes four critical flaws: 1. It replaces manual password checks with 'CheckPasswordSignInAsync', which uses PBKDF2 hashing and implements account lockout to prevent brute-force attacks. 2. It moves the JWT signing key to secure configuration (Environment Variables or Key Vault) instead of hardcoding. 3. It enforces strict 'TokenValidationParameters', ensuring the API validates the issuer, audience, and expiration (preventing replay attacks). 4. It leverages the standard IdentityUser model which handles secure salt-and-hash storage out of the box.

SECURE CODE
// Program.cs configuration
builder.Services.AddIdentity(options => {
    options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(15);
    options.Lockout.MaxFailedAccessAttempts = 5;
})
.AddEntityFrameworkStores();

builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
.AddJwtBearer(options => {
options.TokenValidationParameters = new TokenValidationParameters {
ValidateIssuer = true,
ValidateAudience = true,
ValidateLifetime = true,
ValidateIssuerSigningKey = true,
ValidIssuer = builder.Configuration[“Jwt:Issuer”],
ValidAudience = builder.Configuration[“Jwt:Audience”],
IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(builder.Configuration[“Jwt:Key”]))
};
});


// Secure Login Logic
[HttpPost(“login”)]
public async Task Login([FromBody] LoginRequest model)
{
var user = await _userManager.FindByNameAsync(model.Username);
if (user == null) return Unauthorized();


var result = await _signInManager.CheckPasswordSignInAsync(user, model.Password, lockoutOnFailure: true);
if (result.Succeeded)
{
    var token = GenerateSecureJwt(user);
    return Ok(new { Token = token });
}
if (result.IsLockedOut) return StatusCode(423, "Account locked");
return Unauthorized();


}

System Alert • ID: 9796
Target: .NET 8 Web API API
Potential Vulnerability

Your .NET 8 Web API API might be exposed to Broken User Authentication

74% of .NET 8 Web API apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

RUN FREE SECURITY DIAGNOSTIC
GuardLabs Engine: ONLINE

Free Tier • No Credit Card • Instant Report

Verified by Ghost Labs Security Team

This content is continuously validated by our automated security engine and reviewed by our research team. Ghost Labs analyzes over 500+ vulnerability patterns across 40+ frameworks to provide up-to-date remediation strategies.

About GuardLabs → Follow us on X