Security Review — Backend (.NET)

Security checklist for ASP.NET Core and EF Core backend code.

Reference: OWASP Top 10:2025

When to Activate

• Implementing authentication or authorization
• Handling user input or file uploads
• Creating new API endpoints
• Working with secrets or credentials
• Before deploying to production
• Integrating third-party APIs

1. Secrets Management

Never do this

var connectionString = "Server=prod-db;Password=secret123";
var apiKey = "sk-proj-xxxxx";

Always do this

var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
var apiKey = builder.Configuration["ExternalApi:Key"];

// Verify at startup
if (string.IsNullOrEmpty(apiKey))
    throw new InvalidOperationException("ExternalApi:Key not configured");

Azure Key Vault (Production)

// Azure Key Vault integration
builder.Configuration.AddAzureKeyVault(
    new Uri($"https://{vaultName}.vault.azure.net/"),
    new DefaultAzureCredential());

Checklist:

• No hardcoded connection strings, API keys, or passwords
• Secrets in User Secrets (dev), Azure Key Vault / env vars (prod)
• Azure Key Vault configured for production deployments
• appsettings.Development.json in .gitignore
• No secrets in git history (git log -p -S "password")

2. Input Validation

Result-Based Validation (Results)

public static Result<CreateOrderRequest> ValidateOrder(CreateOrderRequest request)
{
    return ValidationBuilder.Create()
        .ValidateNotNullOrWhiteSpace(request.Items?.FirstOrDefault()?.Sku, "Items[0].Sku")
        .ValidateRange(request.Items?.Count ?? 0, 1, int.MaxValue, "Items.Count")
        .Build()
        .Bind(_ => ValidateItems(request));
}

private static Result<CreateOrderRequest> ValidateItems(CreateOrderRequest request)
{
    var results = request.Items.Select(item =>
        ValidationBuilder.Create()
            .ValidateNotNullOrWhiteSpace(item.Sku, nameof(item.Sku))
            .ValidateRange(item.Quantity, 1, 1000, nameof(item.Quantity))
            .ValidateRange(item.Price, 0.01m, decimal.MaxValue, nameof(item.Price))
            .Build());

    return ResultCombinators.Combine(results).Map(_ => request);
}

// In endpoint — converts Result to TypedResults automatically
app.MapPost("/api/orders", (CreateOrderRequest request) =>
    ValidateOrder(request)
        .Bind(r => orderService.CreateAsync(r))
        .ToCreatedHttpResult(o => $"/api/orders/{o.Id}"));

### File Upload Validation
```csharp
[HttpPost("upload")]
public async Task<IActionResult> Upload(IFormFile file)
{
    if (file.Length > 5 * 1024 * 1024)
        return BadRequest("File too large (max 5MB)");

    var allowedTypes = new[] { "image/jpeg", "image/png", "application/pdf" };
    if (!allowedTypes.Contains(file.ContentType))
        return BadRequest("Invalid file type");

    var ext = Path.GetExtension(file.FileName).ToLowerInvariant();
    if (!new[] { ".jpg", ".jpeg", ".png", ".pdf" }.Contains(ext))
        return BadRequest("Invalid extension");

    // Use a generated filename, never the user-provided one
    var safeName = $"{Guid.NewGuid()}{ext}";
    // ...
}

Checklist:

• All API inputs validated with Result-based validation (Results) or DataAnnotations
• File uploads restricted (size, type, extension)
• Never trust client-side validation alone — always validate server-side
• Error messages don't leak internal details

3. SQL Injection Prevention

Note: If using Event Sourcing (e.g., EventSourcing with Azure Blob Storage), SQL injection is not applicable. Focus on blob key validation and event stream access control instead.

Never do this

var sql = $"SELECT * FROM Users WHERE Email = '{email}'";
await context.Database.ExecuteSqlRawAsync(sql);

Always do this

// EF Core — parameterized automatically
var user = await context.Users.FirstOrDefaultAsync(u => u.Email == email);

// If raw SQL is needed — parameterized
await context.Database.ExecuteSqlInterpolatedAsync(
    $"SELECT * FROM Users WHERE Email = {email}");

// Or explicit parameters
await context.Database.ExecuteSqlRawAsync(
    "SELECT * FROM Users WHERE Email = @p0", email);

Checklist:

• All queries use EF Core LINQ or parameterized SQL
• No string concatenation in any SQL
• ExecuteSqlInterpolatedAsync over ExecuteSqlRawAsync
• For Event Sourcing: blob keys validated and sanitized
• For Event Sourcing: event stream access scoped per tenant/user

4. Authentication & Authorization

// Enforce auth on endpoints
app.MapGet("/api/orders", GetOrders).RequireAuthorization();

// Role-based
app.MapDelete("/api/orders/{id}", DeleteOrder).RequireAuthorization("AdminOnly");

// Policy-based authorization
builder.Services.AddAuthorizationBuilder()
    .AddPolicy("AdminOnly", p => p.RequireRole("Admin"))
    .AddPolicy("CanManageOrders", p => p.RequireClaim("permission", "orders:manage"));

Cookie Configuration

builder.Services.AddAuthentication().AddCookie(options =>
{
    options.Cookie.HttpOnly = true;
    options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
    options.Cookie.SameSite = SameSiteMode.Strict;
    options.ExpireTimeSpan = TimeSpan.FromHours(1);
    options.SlidingExpiration = true;
});

Checklist:

• All endpoints require authentication (opt-out, not opt-in)
• Authorization checks before sensitive operations
• Tokens in HttpOnly cookies (not exposed to JavaScript)
• Cookie: HttpOnly, Secure, SameSite=Strict

5. Security Headers

app.Use(async (context, next) =>
{
    context.Response.Headers.Append("X-Content-Type-Options", "nosniff");
    context.Response.Headers.Append("X-Frame-Options", "DENY");
    context.Response.Headers.Append("X-XSS-Protection", "0"); // Use CSP instead
    context.Response.Headers.Append("Content-Security-Policy",
        "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'");
    await next();
});

Checklist:

• CSP headers configured
• X-Content-Type-Options: nosniff
• X-Frame-Options: DENY
• HTTPS enforced (UseHttpsRedirection())

6. CSRF Protection

// Enabled by default for Razor Pages and MVC
// For minimal APIs with cookie auth:
builder.Services.AddAntiforgery();
app.UseAntiforgery();

Checklist:

• Anti-forgery tokens on state-changing operations
• SameSite=Strict on all cookies

7. Rate Limiting

builder.Services.AddRateLimiter(options =>
{
    options.AddFixedWindowLimiter("api", opt =>
    {
        opt.PermitLimit = 100;
        opt.Window = TimeSpan.FromMinutes(1);
    });

    options.AddFixedWindowLimiter("expensive", opt =>
    {
        opt.PermitLimit = 10;
        opt.Window = TimeSpan.FromMinutes(1);
    });
});

app.UseRateLimiter();
app.MapGet("/api/search", Search).RequireRateLimiting("expensive");

Checklist:

• Rate limiting on all API endpoints
• Stricter limits on expensive operations (search, AI calls, file uploads)
• 429 Too Many Requests response with Retry-After header

8. Sensitive Data

// WRONG: logging secrets
_logger.LogInformation("User login: {Email} {Password}", email, password);

// CORRECT: redact sensitive fields
_logger.LogInformation("User login: {Email}", email);

// WRONG: exposing internals
catch (Exception ex)
{
    return Problem(detail: ex.ToString()); // Stack trace to client!
}

// CORRECT: generic error
catch (Exception ex)
{
    _logger.LogError(ex, "Order creation failed");
    return Problem("An error occurred. Please try again.");
}

Checklist:

• No passwords, tokens, or connection strings in logs
• Error messages generic for clients, detailed in server logs only
• No stack traces exposed to users
• PII handled per GDPR/compliance requirements

9. Problem Details (RFC 7807)

// Configure Problem Details
builder.Services.AddProblemDetails(options =>
{
    options.CustomizeProblemDetails = context =>
    {
        context.ProblemDetails.Extensions["traceId"] =
            Activity.Current?.Id ?? context.HttpContext.TraceIdentifier;
    };
});

// Custom exception handler
public class DomainExceptionHandler : IExceptionHandler
{
    public async ValueTask<bool> TryHandleAsync(HttpContext context, Exception exception, CancellationToken ct)
    {
        var (statusCode, title) = exception switch
        {
            ArgumentException => (400, "Bad Request"),
            KeyNotFoundException => (404, "Not Found"),
            InvalidOperationException => (409, "Conflict"),
            UnauthorizedAccessException => (403, "Forbidden"),
            _ => (500, "Internal Server Error")
        };

        context.Response.StatusCode = statusCode;
        await context.Response.WriteAsJsonAsync(new ProblemDetails
        {
            Status = statusCode,
            Title = title,
            Detail = exception is not InvalidOperationException ? null : exception.Message
        }, ct);
        return true;
    }
}

Checklist:

• AddProblemDetails() configured with trace ID extension
• IExceptionHandler implemented for domain exceptions
• Error responses use application/problem+json content type
• No internal details leaked in Problem Details responses

10. Event Sourcing Security

When using Event Sourcing (e.g., EventSourcing with Azure Blob Storage), additional security considerations apply.

Event Stream Isolation

• Event streams must be scoped per tenant/user — never allow cross-read between tenants
• Validate stream IDs to prevent path traversal in blob storage keys
• Use container-level access policies to enforce tenant boundaries

Projection Security

• Projection rebuilds should require admin authorization
• Log all projection rebuild requests with actor identity and timestamp
• Consider read-only replicas for projections to limit blast radius

PII in Events

• Events are immutable — plan for GDPR before storing PII
• Use crypto-shredding: encrypt PII fields per user, delete the key on erasure request
• Mark events containing PII with metadata for auditing
• Consider separating PII into a linked, deletable store

Checklist:

• Event streams scoped per tenant/user
• Blob storage keys validated and sanitized (no path traversal)
• Projection rebuilds require admin authorization
• PII handling strategy documented (crypto-shredding or separation)
• Event metadata tracks PII presence for GDPR compliance

11. Supply Chain Security (OWASP A03:2025)

Software supply chain attacks target NuGet packages, build pipelines, and transitive dependencies.

Checklist:

• dotnet list package --vulnerable and --deprecated clean
• Package sources restricted to trusted feeds (nuget.org, private feed)
• Lock file (packages.lock.json) committed when using RestorePackagesWithLockFile
• No wildcard version ranges in .csproj (pin exact or minor range)
• CI/CD pipeline dependencies pinned (no latest tags for tools/images)
• Review transitive dependencies for known-compromised packages

12. Exceptional Condition Handling (OWASP A10:2025)

Mishandled errors, edge cases, and unexpected states can expose data or cause cascading failures.

Checklist:

• All async methods have proper try/catch — no unhandled task exceptions
• Global exception handler (IExceptionHandler) catches everything
• Empty catch blocks either log or have explicit justification
• Null/empty collection edge cases handled (no FirstOrDefault() without null check)
• Cancellation tokens propagated through async chains
• Timeout policies configured for external calls (Polly, HttpClient.Timeout)
• Circuit breaker patterns for downstream service failures

Pre-Deployment Checklist

OWASP Top 10:2025 Coverage

• A01 Broken Access Control: All endpoints require auth, role/policy checks on sensitive ops
• A02 Security Misconfiguration: Security headers set, HTTPS enforced, debug disabled in prod
• A03 Supply Chain: Dependencies audited, lock files committed, no vulnerable packages
• A04 Cryptographic Failures: Secrets in Key Vault, no hardcoded credentials, TLS enforced
• A05 Injection: All queries parameterized, no string concatenation in SQL/commands
• A06 Insecure Design: Rate limiting on all endpoints, stricter on expensive operations
• A07 Authentication Failures: HttpOnly cookies, SameSite=Strict, session timeout configured
• A08 Integrity Failures: Anti-forgery tokens, CI/CD pipeline integrity
• A09 Logging & Alerting: No secrets in logs, structured logging with alerts on auth failures
• A10 Exceptional Conditions: Global error handler, no unhandled exceptions, Problem Details (RFC 7807)

Stack-Specific

• Event Sourcing: Stream isolation, projection auth, PII strategy (if applicable)
• Dependencies: dotnet list package --vulnerable clean

Flags

Aspire Runtime Security Checks (--scan / --full)

If the Aspire AppHost is running, the --scan and --full flags include runtime security checks:

Step 1 — Check for leaked error details:

mcp__aspire__list_structured_logs  resourceName: "api"

Scan for responses that leak stack traces, internal paths, or sensitive data in error messages. Production APIs should return Problem Details (RFC 7807) without internal details.

Step 2 — Check for auth failures in logs: Filter structured logs for 401/403 responses. A high volume may indicate misconfigured auth, missing policies, or endpoints that should require auth but don't.

Step 3 — Check console for security warnings:

mcp__aspire__list_console_logs  resourceName: "api"

Look for:

• CORS policy warnings
• Certificate validation failures
• Auth middleware errors
• Rate limiting rejections (confirm they're working)

Skip if Aspire is not running.