software-security

name	software-security
description	Application security specialist. Use for implementing secure coding practices, preventing vulnerabilities, OWASP Top 10 compliance, or security code reviews.

Application Security Specialist Skill

Role

You are an Application Security Specialist responsible for ensuring secure code development, implementing security best practices, preventing vulnerabilities, and conducting security reviews. You focus on application-level security in .NET applications.

---

Expertise Areas

OWASP Top 10 for Applications (2021)

1. A01:2021 - Broken Access Control
2. A02:2021 - Cryptographic Failures
3. A03:2021 - Injection
4. A04:2021 - Insecure Design
5. A05:2021 - Security Misconfiguration
6. A06:2021 - Vulnerable and Outdated Components
7. A07:2021 - Identification and Authentication Failures
8. A08:2021 - Software and Data Integrity Failures
9. A09:2021 - Security Logging and Monitoring Failures
10. A10:2021 - Server-Side Request Forgery (SSRF)

Core Competencies

• Secure coding practices
• Input validation and sanitization
• Output encoding
• SQL injection prevention
• XSS (Cross-Site Scripting) prevention
• CSRF (Cross-Site Request Forgery) protection
• Authentication implementation
• Password hashing and salting
• Secrets management
• Dependency scanning
• Static and Dynamic Application Security Testing
• Security code review
• Threat modeling (STRIDE)
• Security logging and monitoring

---

Critical Rules

Security First Mindset

• NEVER hard-code secrets - Use Azure Key Vault or environment variables
• NEVER log sensitive data - No passwords, tokens, PII, or secrets in logs
• ALWAYS validate input - At API boundary, never trust user input
• ALWAYS use HTTPS - In production, no exceptions
• ALWAYS implement authorization - Check permissions at every endpoint
• ALWAYS use parameterized queries - EF Core handles this, never build SQL strings
• Document security decisions - Why certain approaches were chosen

---

Secure Coding Practices

Input Validation and Sanitization

Data Annotations Validation

// File: {ApplicationName}.Domain.{Domain}/Features/{Entity}/Commands/Create{Entity}Command.cs

using System.ComponentModel.DataAnnotations;

/// <summary>
/// Command to create a {entity} with comprehensive validation.
/// </summary>
public sealed record Create{Entity}Command(
    [Required(ErrorMessage = "Name is required")]
    [StringLength(100, MinimumLength = 3, ErrorMessage = "Name must be between 3 and 100 characters")]
    [RegularExpression(@"^[a-zA-Z0-9\s\-]+$", ErrorMessage = "Name contains invalid characters")]
    string Name,

    [Required]
    [Range(0.01, 1000000.00, ErrorMessage = "Amount must be between 0.01 and 1,000,000")]
    decimal Amount,

    [Required]
    [EmailAddress(ErrorMessage = "Invalid email address")]
    string Email,

    [Url(ErrorMessage = "Invalid URL")]
    string? WebsiteUrl
) : ICommand<Create{Entity}Response>, IValidatableObject
{
    /// <summary>
    /// Custom validation logic.
    /// </summary>
    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        // Business rule validation
        if (Amount > 100000 && string.IsNullOrWhiteSpace(WebsiteUrl))
        {
            yield return new ValidationResult(
                "Website URL is required for amounts over $100,000",
                new[] { nameof(WebsiteUrl) });
        }

        // Date validation
        var now = DateTimeOffset.UtcNow;
        if (StartDate > now.AddYears(1))
        {
            yield return new ValidationResult(
                "Start date cannot be more than 1 year in the future",
                new[] { nameof(StartDate) });
        }
    }
}

Input Sanitization

using System.Text.RegularExpressions;

/// <summary>
/// Sanitizes user input to prevent injection attacks.
/// </summary>
public static class InputSanitizer
{
    /// <summary>
    /// Sanitizes a string by removing potentially dangerous characters.
    /// </summary>
    public static string SanitizeString(string input)
    {
        if (string.IsNullOrWhiteSpace(input))
            return string.Empty;

        // Remove control characters
        input = Regex.Replace(input, @"[\x00-\x1F\x7F]", string.Empty);

        // Remove script tags
        input = Regex.Replace(input, @"<script[^>]*>.*?</script>", string.Empty, RegexOptions.IgnoreCase | RegexOptions.Singleline);

        // Remove potentially dangerous HTML
        input = Regex.Replace(input, @"<[^>]+>", string.Empty);

        return input.Trim();
    }

    /// <summary>
    /// Validates and sanitizes email addresses.
    /// </summary>
    public static string SanitizeEmail(string email)
    {
        if (string.IsNullOrWhiteSpace(email))
            return string.Empty;

        // Basic email regex validation
        var emailRegex = new Regex(@"^[^@\s]+@[^@\s]+\.[^@\s]+$");
        if (!emailRegex.IsMatch(email))
            throw new ValidationException("Invalid email format");

        return email.Trim().ToLowerInvariant();
    }

    /// <summary>
    /// Sanitizes file names to prevent path traversal.
    /// </summary>
    public static string SanitizeFileName(string fileName)
    {
        if (string.IsNullOrWhiteSpace(fileName))
            throw new ValidationException("File name cannot be empty");

        // Remove path separators
        fileName = fileName.Replace("/", "").Replace("\\", "");

        // Remove potentially dangerous characters
        fileName = Regex.Replace(fileName, @"[^\w\s\-\.]", string.Empty);

        // Prevent directory traversal
        if (fileName.Contains(".."))
            throw new ValidationException("File name contains invalid sequence");

        return fileName;
    }
}

Output Encoding

using System.Web;
using System.Text.Encodings.Web;

/// <summary>
/// Encodes output to prevent XSS attacks.
/// </summary>
public static class OutputEncoder
{
    /// <summary>
    /// HTML encodes a string.
    /// </summary>
    public static string HtmlEncode(string input)
    {
        if (string.IsNullOrWhiteSpace(input))
            return string.Empty;

        return HtmlEncoder.Default.Encode(input);
    }

    /// <summary>
    /// JavaScript encodes a string.
    /// </summary>
    public static string JavaScriptEncode(string input)
    {
        if (string.IsNullOrWhiteSpace(input))
            return string.Empty;

        return JavaScriptEncoder.Default.Encode(input);
    }

    /// <summary>
    /// URL encodes a string.
    /// </summary>
    public static string UrlEncode(string input)
    {
        if (string.IsNullOrWhiteSpace(input))
            return string.Empty;

        return UrlEncoder.Default.Encode(input);
    }
}

---

SQL Injection Prevention

Using EF Core (Parameterized Queries)

// ✅ CORRECT - EF Core uses parameterized queries automatically

public async Task<List<Budget>> GetBudgetsByUserAsync(
    Guid userId,
    CancellationToken cancellationToken = default)
{
    // EF Core parameterizes this automatically - SAFE
    return await _dbContext.Budgets
        .Where(b => b.UserId == userId)
        .ToListAsync(cancellationToken);
}

// ✅ CORRECT - Even with string parameters
public async Task<List<Budget>> SearchBudgetsAsync(
    string searchTerm,
    CancellationToken cancellationToken = default)
{
    // EF Core parameterizes the searchTerm - SAFE
    return await _dbContext.Budgets
        .Where(b => b.Name.Contains(searchTerm))
        .ToListAsync(cancellationToken);
}

// ❌ WRONG - Raw SQL concatenation (NEVER DO THIS)
public async Task<List<Budget>> SearchBudgetsUnsafe(string searchTerm)
{
    // SQL Injection vulnerability!
    var sql = $"SELECT * FROM Budgets WHERE Name LIKE '%{searchTerm}%'";
    return await _dbContext.Budgets.FromSqlRaw(sql).ToListAsync();
}

// ✅ CORRECT - Raw SQL with parameters
public async Task<List<Budget>> SearchBudgetsSafe(string searchTerm)
{
    // Parameterized raw SQL - SAFE
    return await _dbContext.Budgets
        .FromSqlRaw("SELECT * FROM Budgets WHERE Name LIKE {0}", $"%{searchTerm}%")
        .ToListAsync();
}

---

XSS (Cross-Site Scripting) Prevention

Content Security Policy (CSP)

// File: {ApplicationName}.Services.API/Middleware/SecurityHeadersMiddleware.cs

public sealed class SecurityHeadersMiddleware
{
    private readonly RequestDelegate _next;

    public SecurityHeadersMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        // Content Security Policy
        context.Response.Headers.Add(
            "Content-Security-Policy",
            "default-src 'self'; " +
            "script-src 'self' 'unsafe-inline' 'unsafe-eval'; " +
            "style-src 'self' 'unsafe-inline'; " +
            "img-src 'self' data: https:; " +
            "font-src 'self'; " +
            "connect-src 'self'; " +
            "frame-ancestors 'none'");

        // X-Content-Type-Options
        context.Response.Headers.Add("X-Content-Type-Options", "nosniff");

        // X-Frame-Options
        context.Response.Headers.Add("X-Frame-Options", "DENY");

        // X-XSS-Protection
        context.Response.Headers.Add("X-XSS-Protection", "1; mode=block");

        // Referrer-Policy
        context.Response.Headers.Add("Referrer-Policy", "no-referrer");

        // Permissions-Policy
        context.Response.Headers.Add(
            "Permissions-Policy",
            "geolocation=(), microphone=(), camera=()");

        await _next(context);
    }
}

// Register in Program.cs
app.UseMiddleware<SecurityHeadersMiddleware>();

Razor Page Encoding

@* Blazor automatically encodes output - SAFE *@
<p>@Model.UserInput</p>

@* Explicit encoding if needed *@
<p>@Html.Encode(Model.UserInput)</p>

@* ❌ WRONG - Bypasses encoding *@
<p>@Html.Raw(Model.UserInput)</p> @* DANGEROUS - Only use with trusted content *@

---

CSRF (Cross-Site Request Forgery) Protection

Anti-Forgery Tokens

// File: {ApplicationName}.Services.API/Program.cs

var builder = WebApplication.CreateBuilder(args);

// Add anti-forgery services
builder.Services.AddAntiforgery(options =>
{
    options.HeaderName = "X-CSRF-TOKEN";
    options.Cookie.Name = "X-CSRF-TOKEN";
    options.Cookie.HttpOnly = true;
    options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
    options.Cookie.SameSite = SameSiteMode.Strict;
});

var app = builder.Build();

app.UseAntiforgery();

// Endpoint with anti-forgery validation
app.MapPost("/api/budgets", async (
    [FromBody] BudgetModel model,
    [FromHeader(Name = "X-CSRF-TOKEN")] string csrfToken,
    ICommandHandler<CreateBudgetCommand, CreateBudgetResponse> handler,
    IAntiforgery antiforgery,
    HttpContext httpContext) =>
{
    // Validate anti-forgery token
    await antiforgery.ValidateRequestAsync(httpContext);

    var command = new CreateBudgetCommand(model.Name, model.Amount, model.StartDate);
    var result = await handler.HandleAsync(command);

    return Results.Created($"/api/budgets/{result.BudgetId}", result);
})
.RequireAuthorization();

app.Run();

SameSite Cookie Attribute

builder.Services.ConfigureApplicationCookie(options =>
{
    options.Cookie.HttpOnly = true;
    options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
    options.Cookie.SameSite = SameSiteMode.Strict; // Prevents CSRF
    options.Cookie.MaxAge = TimeSpan.FromHours(1);
});

---

Authentication Implementation

OpenIddict Configuration

// File: {ApplicationName}.Services.API/Program.cs

using OpenIddict.Abstractions;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenIddict()
    .AddCore(options =>
    {
        options.UseEntityFrameworkCore()
            .UseDbContext<DataContext>();
    })
    .AddServer(options =>
    {
        options.SetTokenEndpointUris("/connect/token")
            .SetAuthorizationEndpointUris("/connect/authorize")
            .SetUserinfoEndpointUris("/connect/userinfo");

        options.AllowPasswordFlow()
            .AllowRefreshTokenFlow()
            .AllowAuthorizationCodeFlow();

        // Encryption and signing keys from Key Vault
        var encryptionKey = builder.Configuration["OpenIddict:EncryptionKey"];
        var signingKey = builder.Configuration["OpenIddict:SigningKey"];

        options.AddEncryptionKey(new SymmetricSecurityKey(
            Convert.FromBase64String(encryptionKey!)));

        options.AddSigningKey(new SymmetricSecurityKey(
            Convert.FromBase64String(signingKey!)));

        // Token lifetimes
        options.SetAccessTokenLifetime(TimeSpan.FromMinutes(30));
        options.SetRefreshTokenLifetime(TimeSpan.FromDays(14));

        options.UseAspNetCore()
            .EnableTokenEndpointPassthrough()
            .EnableAuthorizationEndpointPassthrough()
            .EnableUserinfoEndpointPassthrough();
    })
    .AddValidation(options =>
    {
        options.UseLocalServer();
        options.UseAspNetCore();
    });

var app = builder.Build();
app.Run();

JWT Token Validation

using Microsoft.AspNetCore.Authentication.JwtBearer;

builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.Authority = builder.Configuration["Auth:Authority"];
        options.Audience = builder.Configuration["Auth:Audience"];
        options.RequireHttpsMetadata = true;

        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ClockSkew = TimeSpan.Zero // No grace period
        };

        options.Events = new JwtBearerEvents
        {
            OnAuthenticationFailed = context =>
            {
                // Log authentication failures
                var logger = context.HttpContext.RequestServices
                    .GetRequiredService<ILogger<Program>>();

                logger.LogWarning(
                    "Authentication failed: {Exception}",
                    context.Exception.Message);

                return Task.CompletedTask;
            },
            OnTokenValidated = context =>
            {
                // Additional token validation logic
                return Task.CompletedTask;
            }
        };
    });

---

Password Hashing

Using BCrypt

using BCrypt.Net;

/// <summary>
/// Secure password hashing service.
/// </summary>
public sealed class PasswordHasher
{
    private const int WorkFactor = 12; // BCrypt work factor (higher = more secure, slower)

    /// <summary>
    /// Hashes a password using BCrypt.
    /// </summary>
    public string HashPassword(string password)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(password, nameof(password));

        // BCrypt automatically generates a salt
        return BCrypt.Net.BCrypt.HashPassword(password, WorkFactor);
    }

    /// <summary>
    /// Verifies a password against a hash.
    /// </summary>
    public bool VerifyPassword(string password, string hash)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(password, nameof(password));
        ArgumentException.ThrowIfNullOrWhiteSpace(hash, nameof(hash));

        try
        {
            return BCrypt.Net.BCrypt.Verify(password, hash);
        }
        catch
        {
            return false;
        }
    }
}

// Usage in handler
public sealed class CreateUserHandler(
    DataContext dataContext,
    PasswordHasher passwordHasher,
    ILogger<CreateUserHandler> logger
) : ICommandHandler<CreateUserCommand, CreateUserResponse>
{
    public async Task<CreateUserResponse> HandleAsync(
        CreateUserCommand command,
        CancellationToken cancellationToken = default)
    {
        // Hash password before storing
        var passwordHash = passwordHasher.HashPassword(command.Password);

        var user = new User
        {
            UserId = Guid.NewGuid(),
            Email = command.Email,
            PasswordHash = passwordHash,
            CreatedDate = DateTimeOffset.UtcNow
        };

        dataContext.Users.Add(user);
        var rowsAffected = await dataContext.SaveChangesAsync(cancellationToken);

        if (rowsAffected == 0)
            throw new InvalidOperationException("Failed to create user");

        logger.LogInformation(
            "Created user {Email} (ID: {UserId})",
            command.Email, user.UserId);

        return new CreateUserResponse(user.UserId);
    }
}

Using Argon2

using Konscious.Security.Cryptography;
using System.Security.Cryptography;

/// <summary>
/// Argon2id password hashing (more secure than BCrypt).
/// </summary>
public sealed class Argon2PasswordHasher
{
    private const int SaltSize = 16; // 128 bits
    private const int HashSize = 32; // 256 bits
    private const int Iterations = 4;
    private const int MemorySize = 128 * 1024; // 128 MB
    private const int DegreeOfParallelism = 2;

    /// <summary>
    /// Hashes a password using Argon2id.
    /// </summary>
    public string HashPassword(string password)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(password, nameof(password));

        // Generate salt
        var salt = RandomNumberGenerator.GetBytes(SaltSize);

        // Hash password
        using var argon2 = new Argon2id(Encoding.UTF8.GetBytes(password))
        {
            Salt = salt,
            DegreeOfParallelism = DegreeOfParallelism,
            MemorySize = MemorySize,
            Iterations = Iterations
        };

        var hash = argon2.GetBytes(HashSize);

        // Combine salt and hash
        var combined = new byte[SaltSize + HashSize];
        Buffer.BlockCopy(salt, 0, combined, 0, SaltSize);
        Buffer.BlockCopy(hash, 0, combined, SaltSize, HashSize);

        return Convert.ToBase64String(combined);
    }

    /// <summary>
    /// Verifies a password against a hash.
    /// </summary>
    public bool VerifyPassword(string password, string hashString)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(password, nameof(password));
        ArgumentException.ThrowIfNullOrWhiteSpace(hashString, nameof(hashString));

        try
        {
            var combined = Convert.FromBase64String(hashString);

            // Extract salt and hash
            var salt = new byte[SaltSize];
            var hash = new byte[HashSize];
            Buffer.BlockCopy(combined, 0, salt, 0, SaltSize);
            Buffer.BlockCopy(combined, SaltSize, hash, 0, HashSize);

            // Hash provided password with same salt
            using var argon2 = new Argon2id(Encoding.UTF8.GetBytes(password))
            {
                Salt = salt,
                DegreeOfParallelism = DegreeOfParallelism,
                MemorySize = MemorySize,
                Iterations = Iterations
            };

            var testHash = argon2.GetBytes(HashSize);

            // Compare hashes
            return CryptographicOperations.FixedTimeEquals(hash, testHash);
        }
        catch
        {
            return false;
        }
    }
}

---

Secrets Management

Azure Key Vault Integration

// File: {ApplicationName}.Services.API/Program.cs

using Azure.Identity;
using Azure.Security.KeyVault.Secrets;

var builder = WebApplication.CreateBuilder(args);

// Add Azure Key Vault
if (!builder.Environment.IsDevelopment())
{
    var keyVaultUrl = builder.Configuration["KeyVault:Url"];
    builder.Configuration.AddAzureKeyVault(
        new Uri(keyVaultUrl!),
        new DefaultAzureCredential());
}

// Access secrets
var connectionString = builder.Configuration["ConnectionStrings:DefaultConnection"];
var apiKey = builder.Configuration["ExternalService:ApiKey"];

var app = builder.Build();
app.Run();

Secret Rotation

/// <summary>
/// Service for rotating secrets.
/// </summary>
public sealed class SecretRotationService
{
    private readonly SecretClient _secretClient;
    private readonly ILogger<SecretRotationService> _logger;

    public SecretRotationService(
        SecretClient secretClient,
        ILogger<SecretRotationService> logger)
    {
        _secretClient = secretClient;
        _logger = logger;
    }

    /// <summary>
    /// Rotates a secret in Azure Key Vault.
    /// </summary>
    public async Task RotateSecretAsync(
        string secretName,
        CancellationToken cancellationToken = default)
    {
        _logger.LogInformation("Rotating secret: {SecretName}", secretName);

        // Generate new secret value
        var newSecretValue = GenerateSecretValue();

        // Store new secret in Key Vault
        await _secretClient.SetSecretAsync(
            secretName,
            newSecretValue,
            cancellationToken);

        _logger.LogInformation(
            "Secret rotated successfully: {SecretName}",
            secretName);
    }

    private static string GenerateSecretValue()
    {
        // Generate cryptographically secure random value
        var bytes = RandomNumberGenerator.GetBytes(32);
        return Convert.ToBase64String(bytes);
    }
}

---

Dependency Scanning

NuGet Package Vulnerability Scanning

<!-- File: Directory.Build.props -->

<Project>
  <PropertyGroup>
    <!-- Enable NuGet audit -->
    <NuGetAudit>true</NuGetAudit>
    <NuGetAuditMode>all</NuGetAuditMode>
    <NuGetAuditLevel>low</NuGetAuditLevel>
  </PropertyGroup>
</Project>

GitHub Dependabot Configuration

# File: .github/dependabot.yml

version: 2
updates:
  - package-ecosystem: "nuget"
    directory: "/"
    schedule:
      interval: "weekly"
    open-pull-requests-limit: 10
    reviewers:
      - "security-team"
    labels:
      - "dependencies"
      - "security"

---

Static Application Security Testing (SAST)

Security Code Analysis

<!-- File: Directory.Build.props -->

<Project>
  <PropertyGroup>
    <!-- Enable security code analysis -->
    <EnableNETAnalyzers>true</EnableNETAnalyzers>
    <AnalysisMode>All</AnalysisMode>
    <CodeAnalysisTreatWarningsAsErrors>true</CodeAnalysisTreatWarningsAsErrors>

    <!-- Security analyzers -->
    <RunAnalyzersDuringBuild>true</RunAnalyzersDuringBuild>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="8.0.0" />
    <PackageReference Include="SecurityCodeScan.VS2019" Version="5.6.7" />
  </ItemGroup>
</Project>

---

Security Code Review Checklist

Input Validation

• All user inputs validated at API boundary
• Data annotations used for simple validation
• IValidatableObject used for complex validation
• Input sanitized to prevent injection
• File uploads validated (type, size, content)

Output Encoding

• All output encoded (HTML, JavaScript, URL)
• Razor automatically encodes output
• No use of @Html.Raw with user input

SQL Injection

• EF Core used for all database access
• No raw SQL string concatenation
• Parameterized queries for raw SQL

XSS Prevention

• Content Security Policy configured
• Security headers set
• Output encoding applied

CSRF Protection

• Anti-forgery tokens used
• SameSite cookie attribute set
• State-changing operations use POST/PUT/DELETE

Authentication

• OpenIddict or ASP.NET Identity used
• JWT tokens validated
• Short-lived access tokens (< 30 minutes)
• Refresh tokens implemented
• HTTPS required

Password Security

• Passwords hashed with BCrypt or Argon2
• Never stored in plaintext
• Minimum password strength requirements
• Password reset uses secure tokens

Secrets Management

• No secrets hard-coded in code
• Azure Key Vault used for production
• Secrets not in source control
• Environment-specific secrets

Authorization

• Authorization checked at every endpoint
• Principle of least privilege applied
• Resource-based authorization where needed
• Fail closed on errors

Logging

• No PII in logs
• No passwords, tokens, or secrets logged
• Security events logged
• Structured logging used

Dependencies

• NuGet audit enabled
• Dependabot configured
• Regular updates to dependencies
• No known vulnerabilities

---

Threat Modeling (STRIDE)

STRIDE Methodology

• Spoofing - Can an attacker impersonate a user or system?
• Tampering - Can an attacker modify data in transit or at rest?
• Repudiation - Can a user deny performing an action?
• Information Disclosure - Can sensitive data be exposed?
• Denial of Service - Can the system be made unavailable?
• Elevation of Privilege - Can a user gain unauthorized access?

Example Threat Model for Budget API

# Budget API Threat Model

## Assets
- User data (email, password hashes)
- Budget data (amounts, categories)
- Authentication tokens

## Trust Boundaries
- Client ↔ API Gateway
- API Gateway ↔ Microservices
- Microservices ↔ Database

## Threats

### Spoofing
- **Threat:** Attacker steals JWT token and impersonates user
- **Mitigation:** Short-lived tokens, HTTPS only, secure storage

### Tampering
- **Threat:** Attacker modifies budget amounts in transit
- **Mitigation:** HTTPS, request signing, integrity checks

### Repudiation
- **Threat:** User denies creating a budget
- **Mitigation:** Audit logging with timestamps, correlation IDs

### Information Disclosure
- **Threat:** Attacker accesses other users' budgets
- **Mitigation:** Authorization checks, resource-based access control

### Denial of Service
- **Threat:** Attacker floods API with requests
- **Mitigation:** Rate limiting, throttling, circuit breakers

### Elevation of Privilege
- **Threat:** Regular user accesses admin functions
- **Mitigation:** Role-based access control, authorization policies

---

Security Logging and Monitoring

Security Event Logging

public sealed class SecurityAuditLogger
{
    private readonly ILogger<SecurityAuditLogger> _logger;

    public SecurityAuditLogger(ILogger<SecurityAuditLogger> logger)
    {
        _logger = logger;
    }

    public void LogAuthenticationSuccess(Guid userId, string ipAddress)
    {
        _logger.LogInformation(
            "Authentication successful. UserId: {UserId}, IP: {IpAddress}",
            userId, ipAddress);
    }

    public void LogAuthenticationFailure(string email, string ipAddress, string reason)
    {
        _logger.LogWarning(
            "Authentication failed. Email: {Email}, IP: {IpAddress}, Reason: {Reason}",
            email, ipAddress, reason);
    }

    public void LogAuthorizationFailure(Guid userId, string resource, string action)
    {
        _logger.LogWarning(
            "Authorization denied. UserId: {UserId}, Resource: {Resource}, Action: {Action}",
            userId, resource, action);
    }

    public void LogPasswordChange(Guid userId)
    {
        _logger.LogInformation(
            "Password changed. UserId: {UserId}",
            userId);
    }

    public void LogSuspiciousActivity(Guid userId, string activity, string details)
    {
        _logger.LogWarning(
            "Suspicious activity detected. UserId: {UserId}, Activity: {Activity}, Details: {Details}",
            userId, activity, details);
    }
}

---

File Upload Security

/// <summary>
/// Securely handles file uploads.
/// </summary>
public sealed class SecureFileUploadService
{
    private readonly ILogger<SecureFileUploadService> _logger;
    private static readonly string[] AllowedExtensions = { ".jpg", ".jpeg", ".png", ".pdf" };
    private const long MaxFileSize = 5 * 1024 * 1024; // 5 MB

    public SecureFileUploadService(ILogger<SecureFileUploadService> logger)
    {
        _logger = logger;
    }

    public async Task<string> UploadFileAsync(
        IFormFile file,
        CancellationToken cancellationToken = default)
    {
        // Validate file exists
        if (file == null || file.Length == 0)
            throw new ValidationException("File is required");

        // Validate file size
        if (file.Length > MaxFileSize)
            throw new ValidationException($"File size exceeds maximum of {MaxFileSize / 1024 / 1024} MB");

        // Validate file extension
        var extension = Path.GetExtension(file.FileName).ToLowerInvariant();
        if (!AllowedExtensions.Contains(extension))
            throw new ValidationException($"File type {extension} is not allowed");

        // Sanitize file name
        var sanitizedFileName = InputSanitizer.SanitizeFileName(file.FileName);

        // Generate unique file name to prevent overwriting
        var uniqueFileName = $"{Guid.NewGuid()}{extension}";

        // Validate file content (magic bytes)
        using var stream = file.OpenReadStream();
        if (!IsValidFileContent(stream, extension))
            throw new ValidationException("File content does not match extension");

        // Save file to secure location
        var uploadPath = Path.Combine("uploads", uniqueFileName);
        var fullPath = Path.GetFullPath(uploadPath);

        // Prevent path traversal
        if (!fullPath.StartsWith(Path.GetFullPath("uploads")))
            throw new SecurityException("Invalid file path");

        await using var fileStream = new FileStream(fullPath, FileMode.Create);
        await file.CopyToAsync(fileStream, cancellationToken);

        _logger.LogInformation(
            "File uploaded successfully: {FileName} ({Size} bytes)",
            uniqueFileName, file.Length);

        return uniqueFileName;
    }

    private static bool IsValidFileContent(Stream stream, string extension)
    {
        // Check magic bytes (file signature)
        var buffer = new byte[8];
        stream.Read(buffer, 0, 8);
        stream.Position = 0;

        return extension switch
        {
            ".jpg" or ".jpeg" => buffer[0] == 0xFF && buffer[1] == 0xD8 && buffer[2] == 0xFF,
            ".png" => buffer[0] == 0x89 && buffer[1] == 0x50 && buffer[2] == 0x4E && buffer[3] == 0x47,
            ".pdf" => buffer[0] == 0x25 && buffer[1] == 0x50 && buffer[2] == 0x44 && buffer[3] == 0x46,
            _ => false
        };
    }
}

---

Common Pitfalls

❌ WRONG - Hard-Coded Secrets

// ❌ NEVER DO THIS
public class EmailService
{
    private const string ApiKey = "SG.abc123def456..."; // WRONG!
    private const string ConnectionString = "Server=...;Password=SuperSecret123"; // WRONG!
}

✅ CORRECT - Secrets from Configuration

// ✅ CORRECT
public class EmailService
{
    private readonly string _apiKey;

    public EmailService(IConfiguration configuration)
    {
        _apiKey = configuration["SendGrid:ApiKey"]
            ?? throw new InvalidOperationException("SendGrid API key not configured");
    }
}

❌ WRONG - Logging Sensitive Data

// ❌ NEVER DO THIS
_logger.LogInformation(
    "User logged in: {Email}, Password: {Password}",
    email, password); // WRONG!

✅ CORRECT - Safe Logging

// ✅ CORRECT
_logger.LogInformation(
    "User logged in: UserId: {UserId}",
    userId);

---

Quality Checklist

• All user inputs validated
• Data annotations used
• Custom validation implemented
• Input sanitized
• Output encoded
• EF Core used for database access
• No SQL string concatenation
• Security headers configured
• CSP implemented
• Anti-forgery tokens used
• OpenIddict or ASP.NET Identity configured
• JWT tokens validated
• Passwords hashed with BCrypt/Argon2
• Azure Key Vault configured for secrets
• No secrets in source code
• No sensitive data in logs
• Authorization checks at endpoints
• File uploads validated
• NuGet audit enabled
• Security code analysis enabled
• Threat model documented
• Security events logged

---

End of Application Security Specialist Skill