Security And Hardening

Security and Hardening

Overview

Security-first development practices for .NET applications — ASP.NET Core web apps, Blazor, Avalonia, MAUI, and library code. Treat every external input as hostile, every secret as sacred, and every authorization check as mandatory. Security isn't a phase — it's a constraint on every line of code that touches user data, authentication, or external systems.

When to Use

• Building anything that accepts user input (HTTP endpoints, message consumers, file watchers, UI text fields)
• Implementing authentication or authorization (ASP.NET Core Identity, cookie auth, JWT bearer, OAuth, policy-based authz)
• Storing or transmitting sensitive data (PII, payment info, API tokens, connection strings)
• Integrating with external APIs or services (HttpClient, gRPC, Service Bus)
• Adding file uploads, webhooks, or callbacks
• Handling payment or regulated data (GDPR, HIPAA, PCI DSS)

Process: Threat Model First

Controls bolted on without a threat model are guesses. Before hardening, spend five minutes thinking like an attacker:

1. Map the trust boundaries. Where does untrusted data cross into your system? HTTP requests (body, route, query, headers), form fields, IFormFile uploads, webhooks, third-party API responses, Service Bus / queue messages, and LLM output. Every boundary is attack surface.
2. Name the assets. What's worth stealing or breaking? Credentials, PII, payment data, admin actions, money movement.
3. Run STRIDE over each boundary — a quick lens, not a ceremony:

4. Write abuse cases next to use cases. For each feature, ask "how would I misuse this?" — then make that your first xUnit/MSTest test.

If you can't name the trust boundaries for a feature, you're not ready to secure it. This is OWASP A04: Insecure Design — most breaches begin in design, not code.

The Three-Tier Boundary System

Host-model lens. Most bullets below target ASP.NET Core server code (Web APIs, Razor Pages, Blazor Server). Cross-cutting items — secret handling, no BinaryFormatter, parameterized queries — apply to any .NET host, including Avalonia / MAUI / console apps that talk to a database or external service. Client-only items (Blazor WebAssembly localStorage, antiforgery tokens in cookie-auth apps) are labeled inline. When in doubt about whether a bullet applies to your host, check the examples.

Always Do (No Exceptions)

• Validate all external input at the system boundary (endpoints, message handlers) with FluentValidation / DataAnnotations / MediatR pipeline behaviour
• Parameterize all database queries — EF Core's LINQ does this by default; for raw SQL prefer FromSql($"...{input}") on EF Core 8+ (the FormattableString overload parameterizes automatically), or FromSqlInterpolated($"...{input}") on EF Core 7 and earlier. Never FromSqlRaw($"...{input}") — the string overload interpolates in C# before EF Core sees the query
• Encode output — Razor and Blazor auto-encode @model.Something by default; never use @Html.Raw(userInput) or MarkupString on user-sourced content
• Use HTTPS for all external communication; app.UseHsts() + app.UseHttpsRedirection() in Program.cs
• Hash passwords with ASP.NET Core Identity's PasswordHasher<TUser> (PBKDF2 with SHA-256, tuned work factor), or Argon2 via Konscious.Security.Cryptography for new systems
• Set security headers — CSP, HSTS, X-Content-Type-Options: nosniff, X-Frame-Options: DENY, Referrer-Policy: strict-origin-when-cross-origin (use NetEscapades.AspNetCore.SecurityHeaders or Microsoft.AspNetCore.Authentication.Cookies)
• Use HttpOnly, Secure, SameSite cookies for auth — ASP.NET Core cookie authentication sets these correctly by default; don't override them without a reason
• Use antiforgery tokens for state-changing requests in cookie-auth apps — [ValidateAntiForgeryToken] on MVC controllers, automatic for Razor Pages, IAntiforgery middleware for Minimal APIs
• Audit dependencies — dotnet list package --vulnerable --include-transitive before every release (add a CI step that fails on criticals)

Ask First (Requires Human Approval)

• Adding new authentication flows or changing auth logic (OIDC scheme changes, external provider integrations)
• Storing new categories of sensitive data (PII, payment info, health records)
• Adding new external service integrations
• Changing CORS configuration (app.UseCors(...))
• Adding file upload handlers
• Modifying rate limiting or throttling (AddRateLimiter(...))
• Granting elevated permissions, new [Authorize(Policy = "…")] values, or new roles
• Rotating the Data Protection key ring (will invalidate all existing auth cookies and encrypted payloads)

Never Do

• Never commit secrets to version control (API keys, connection strings with passwords, JWT signing keys, certificate .pfx files)
• Never log sensitive data (passwords, tokens, full credit card numbers, PII beyond what's audit-required) — scrub at the ILogger boundary
• Never trust client-side validation as a security boundary (Blazor WebAssembly is client-side; validate again on the server)
• Never disable security headers for convenience
• Never use BinaryFormatter for anything, ever (officially deprecated and unsafe — the framework itself emits a compile-time error now)
• Never use MarkupString / @Html.Raw with user content without sanitization (XSS vector)
• Never store auth tokens in browser localStorage from Blazor WebAssembly — use HttpOnly cookies or sessionStorage with short lifetimes and accept the XSS exposure risk
• Never expose stack traces or internal error details to users — ASP.NET Core's production UseExceptionHandler("/Error") strips them by default, keep it that way
• Never interpolate user input into FromSqlRaw — on EF Core 8+ use FromSql($"...{input}"); on earlier versions FromSqlInterpolated($"...{input}"). Both overloads take FormattableString and parameterize each {} hole; FromSqlRaw takes a plain string and does not

OWASP Top 10 Prevention (C# / .NET)

1. Injection (SQL, NoSQL, OS Command)

Server-side (EF Core raw-SQL APIs) — FromSqlRaw, FromSqlInterpolated, and FromSql look similar at the call site but have opposite safety semantics:

// BAD: SQL injection — FromSqlRaw takes a `string`; the interpolation happens in C#
//      and the user-controlled value lands directly inside the query text.
var user = await db.Users
    .FromSqlRaw($"SELECT * FROM Users WHERE Id = '{userId}'")
    .FirstOrDefaultAsync(cancellationToken);

// GOOD: LINQ — EF Core parameterizes automatically. Prefer this whenever the
//       query shape can be expressed in LINQ.
var user = await db.Users
    .SingleOrDefaultAsync(u => u.Id == userId, cancellationToken);

// GOOD (EF Core 8+, the canonical form): FromSql — the `FormattableString`
//        overload sees each `{}` hole and parameterizes it, even though the
//        syntax looks like C# interpolation.
var user = await db.Users
    .FromSql($"SELECT * FROM Users WHERE Id = {userId}")
    .FirstOrDefaultAsync(cancellationToken);

// GOOD (EF Core 7 and earlier): FromSqlInterpolated — identical safety to the
// EF Core 8+ FromSql above. Still works on EF Core 8+, but FromSql is the
// recommended name going forward.
var user = await db.Users
    .FromSqlInterpolated($"SELECT * FROM Users WHERE Id = {userId}")
    .FirstOrDefaultAsync(cancellationToken);

// GOOD: Dapper — anonymous-object parameters, never string concatenation
var user = await connection.QuerySingleOrDefaultAsync<User>(
    "SELECT * FROM Users WHERE Id = @Id",
    new { Id = userId });

The trap: FromSqlRaw($"...{userId}...") and FromSql($"...{userId}...") look nearly identical in source, but the method overload resolution picks opposite paths — FromSqlRaw takes a string (C# formats it before EF Core sees anything) while FromSql and FromSqlInterpolated take a FormattableString (EF Core sees the holes and parameterizes them). When in doubt, call FromSql explicitly — the compiler will reject a plain concatenated string.

Server-side (OS commands) — for Process.Start, never concatenate user input into ProcessStartInfo.Arguments (single string, prone to quoting bugs). Use ProcessStartInfo.ArgumentList instead — each item is escaped individually by the runtime.

2. Broken Authentication

// ASP.NET Core Identity — hashes with PBKDF2 by default
// Program.cs
builder.Services
    .AddIdentity<ApplicationUser, IdentityRole>(options =>
    {
        options.Password.RequiredLength = 12;
        options.Password.RequireNonAlphanumeric = true;
        options.Lockout.MaxFailedAccessAttempts = 5;
        options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(15);
    })
    .AddEntityFrameworkStores<AppDbContext>()
    .AddDefaultTokenProviders();

// Cookie auth hardening
builder.Services.ConfigureApplicationCookie(options =>
{
    options.Cookie.HttpOnly = true;
    options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
    options.Cookie.SameSite = SameSiteMode.Lax;
    options.ExpireTimeSpan = TimeSpan.FromHours(24);
    options.SlidingExpiration = true;
});

For JWT bearer auth, validate issuer + audience + signing key + lifetime + clock skew explicitly:

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"]!)),
            ClockSkew = TimeSpan.FromMinutes(1),
        };
    });

3. Cross-Site Scripting (XSS)

For Razor Pages / MVC / Blazor, auto-encoding is the default:

@* GOOD: Razor auto-encodes by default *@
<p>@Model.UserInput</p>

@* BAD: Raw HTML rendering of user input *@
<p>@Html.Raw(Model.UserInput)</p>

@* Blazor equivalent — auto-encoded *@
<p>@userInput</p>

@* BAD in Blazor: MarkupString bypasses encoding *@
<p>@((MarkupString)userInput)</p>

If you must render user HTML (rich-text editors, markdown), sanitize with HtmlSanitizer:

var sanitizer = new HtmlSanitizer();
var clean = sanitizer.Sanitize(userHtml);

For Avalonia/MAUI apps, XSS doesn't apply (no HTML renderer) but the same principle holds: anything you push through a WebView or format-string sink needs encoding.

4. Broken Access Control

Always check authorization, not just authentication. Prefer policy-based authz over role-checking strings scattered in code:

// Program.cs — define policies centrally
builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("TaskOwner", policy =>
        policy.Requirements.Add(new ResourceOwnerRequirement()));
});

builder.Services.AddScoped<IAuthorizationHandler, ResourceOwnerHandler>();

// Endpoint
app.MapPatch("/api/tasks/{id}", async (
    string id,
    UpdateTaskInput input,
    ITaskService service,
    IAuthorizationService authz,
    ClaimsPrincipal user,
    CancellationToken cancellationToken) =>
{
    var task = await service.FindByIdAsync(id, cancellationToken);
    if (task is null) return Results.NotFound();

    var authzResult = await authz.AuthorizeAsync(user, task, "TaskOwner");
    if (!authzResult.Succeeded)
    {
        return Results.Problem(
            statusCode: StatusCodes.Status403Forbidden,
            title: "Forbidden",
            detail: "Not authorized to modify this task");
    }

    var updated = await service.UpdateAsync(id, input, cancellationToken);
    return Results.Ok(updated);
})
.RequireAuthorization();

Authentication answers "are you logged in"; authorization answers "can you do this to this resource". Both must pass.

5. Security Misconfiguration

// Program.cs — security headers
app.Use(async (context, next) =>
{
    context.Response.Headers["X-Content-Type-Options"] = "nosniff";
    context.Response.Headers["X-Frame-Options"] = "DENY";
    context.Response.Headers["Referrer-Policy"] = "strict-origin-when-cross-origin";
    context.Response.Headers["Permissions-Policy"] = "camera=(), microphone=(), geolocation=()";
    await next();
});

// Content Security Policy — prefer the NetEscapades.AspNetCore.SecurityHeaders package
app.UseSecurityHeaders(policies =>
    policies
        .AddContentSecurityPolicy(builder =>
        {
            builder.AddDefaultSrc().Self();
            builder.AddScriptSrc().Self();
            builder.AddStyleSrc().Self().UnsafeInline(); // tighten if possible
            builder.AddImgSrc().Self().From("data:").From("https:");
            builder.AddConnectSrc().Self();
        }));

// HSTS + HTTPS redirect
if (!app.Environment.IsDevelopment())
{
    app.UseHsts();
}
app.UseHttpsRedirection();

// CORS — restrict to known origins (NEVER "*" with credentials)
builder.Services.AddCors(options =>
{
    options.AddDefaultPolicy(policy => policy
        .WithOrigins(builder.Configuration.GetSection("AllowedOrigins").Get<string[]>() ?? [])
        .AllowAnyHeader()
        .AllowAnyMethod()
        .AllowCredentials());
});

6. Sensitive Data Exposure

Never return sensitive fields in API responses — use DTOs that exclude them:

// Don't serialize the entity directly. Project to a DTO.
public sealed record PublicUserDto(string Id, string Email, string DisplayName);

public static PublicUserDto ToPublicDto(this ApplicationUser user) =>
    new(user.Id, user.Email!, user.DisplayName);

// Don't put secrets in source
// BAD
public const string ApiKey = "sk_live_…";

// GOOD: read from IConfiguration (dotnet user-secrets / env / Key Vault)
public sealed class StripeOptions
{
    public required string ApiKey { get; init; }
}

builder.Services
    .AddOptions<StripeOptions>()
    .Bind(builder.Configuration.GetSection("Stripe"))
    .ValidateDataAnnotations()
    .ValidateOnStart();

ASP.NET Core Data Protection handles session cookies and anti-forgery tokens for you, but for encrypting arbitrary data at rest (PII fields in a database, for example), use IDataProtectionProvider with a named purpose string and configure a shared key ring on Azure Key Vault / Redis / file share when running multiple instances.

7. Server-Side Request Forgery (SSRF)

Any time the server fetches a URL the user influenced — webhooks, "import from URL", image proxies, link previews — an attacker can aim it at internal services (cloud metadata, localhost, private IPs). The #1 target is the cloud metadata endpoint 169.254.169.254 (Azure IMDS, AWS/GCP metadata), which can hand out managed-identity tokens.

// BAD: fetch whatever the user gives you
var content = await httpClient.GetStringAsync(input.WebhookUrl, ct);

// GOOD: allowlist scheme + host, and pin the connection to a validated public IP
static readonly HashSet<string> AllowedHosts = new(StringComparer.OrdinalIgnoreCase) { "hooks.example.com" };

static Uri AssertSafeUrl(string raw)
{
    if (!Uri.TryCreate(raw, UriKind.Absolute, out var uri) || uri.Scheme != Uri.UriSchemeHttps)
        throw new ValidationException("https URL required");
    if (!AllowedHosts.Contains(uri.Host))
        throw new ValidationException("host not allowed");
    return uri;
}

// Pin DNS at connect time so a short-TTL record can't rebind to an internal IP
// between validation and connection (the TOCTOU gap).
var handler = new SocketsHttpHandler
{
    AllowAutoRedirect = false,                 // a 302 to http://169.254.169.254 must not be followed
    ConnectCallback = async (ctx, ct) =>
    {
        var entries = await Dns.GetHostAddressesAsync(ctx.DnsEndPoint.Host, ct);
        foreach (var ip in entries)
            if (IsPrivateOrReserved(ip))        // loopback, link-local (169.254/16, fe80::/10), private, ULA
                throw new ValidationException("resolves to a private/reserved address");
        var socket = new Socket(SocketType.Stream, ProtocolType.Tcp) { NoDelay = true };
        await socket.ConnectAsync(entries, ctx.DnsEndPoint.Port, ct);
        return new NetworkStream(socket, ownsSocket: true);
    }
};

IsPrivateOrReserved should reject loopback, link-local (169.254.0.0/16, fe80::/10), private (10/8, 172.16/12, 192.168/16), and unique-local (fc00::/7) ranges across IPv4 and IPv6 — IPAddress.IsLoopback plus explicit range checks. Disabling redirects is load-bearing: without it, an allowlisted host can 302 you straight to the metadata endpoint.

Input Validation Patterns

Schema Validation at Boundaries (FluentValidation)

public sealed record CreateTaskInput(
    string Title,
    string? Description,
    TaskPriority Priority = TaskPriority.Medium,
    DateTimeOffset? DueDate = null);

public sealed class CreateTaskValidator : AbstractValidator<CreateTaskInput>
{
    public CreateTaskValidator()
    {
        RuleFor(x => x.Title).NotEmpty().MaximumLength(200);
        RuleFor(x => x.Description).MaximumLength(2000);
        RuleFor(x => x.Priority).IsInEnum();
        RuleFor(x => x.DueDate).GreaterThan(DateTimeOffset.UtcNow).When(x => x.DueDate is not null);
    }
}

// Endpoint
app.MapPost("/api/tasks", async (
    CreateTaskInput input,
    IValidator<CreateTaskInput> validator,
    ITaskService service,
    CancellationToken cancellationToken) =>
{
    var result = await validator.ValidateAsync(input, cancellationToken);
    if (!result.IsValid)
    {
        return Results.ValidationProblem(result.ToDictionary());
    }

    var task = await service.CreateAsync(input, cancellationToken);
    return Results.Created($"/api/tasks/{task.Id}", task);
});

File Upload Safety

private static readonly HashSet<string> AllowedContentTypes = new(StringComparer.OrdinalIgnoreCase)
{
    "image/jpeg", "image/png", "image/webp"
};

private static readonly Dictionary<string, byte[]> MagicBytes = new()
{
    ["image/jpeg"] = [0xFF, 0xD8, 0xFF],
    ["image/png"]  = [0x89, 0x50, 0x4E, 0x47],
    ["image/webp"] = [0x52, 0x49, 0x46, 0x46], // 'RIFF', plus WEBP at offset 8
};

private const long MaxSizeBytes = 5 * 1024 * 1024; // 5 MB

public async Task ValidateUploadAsync(IFormFile file, CancellationToken cancellationToken)
{
    if (file.Length == 0 || file.Length > MaxSizeBytes)
    {
        throw new ValidationException("File is empty or exceeds the 5 MB limit.");
    }

    if (!AllowedContentTypes.Contains(file.ContentType))
    {
        throw new ValidationException("Content type not allowed.");
    }

    // Don't trust the file extension or the declared ContentType — sniff the magic bytes.
    await using var stream = file.OpenReadStream();
    Span<byte> header = stackalloc byte[8];
    var read = await stream.ReadAsync(header.ToArray(), cancellationToken);

    var matches = MagicBytes
        .Where(kv => file.ContentType.Equals(kv.Key, StringComparison.OrdinalIgnoreCase))
        .Any(kv => header[..kv.Value.Length].SequenceEqual(kv.Value));

    if (!matches)
    {
        throw new ValidationException("File contents do not match the declared content type.");
    }
}

Store uploads outside the web root, scan with an AV pipeline (ClamAV, Defender) if accepting from untrusted users, and serve through a controller that sets Content-Disposition: attachment with a sanitized filename.

Triaging dotnet list package --vulnerable Results

Not all audit findings require immediate action. Use this decision tree:

dotnet list package --vulnerable --include-transitive reports a vulnerability
├── Severity: Critical or High
│   ├── Is the vulnerable code reachable in your app?
│   │   ├── YES → Fix immediately (update NuGet version, replace the package, or patch)
│   │   └── NO (dev-only dep, unused code path, netstandard2.0 polyfill that's never hit)
│   │       → Fix soon, but not a release-blocker
│   └── Is a fix available?
│       ├── YES → Bump in Directory.Packages.props, verify via dotnet build + dotnet test
│       └── NO → Consider replacing the package, apply a manual workaround, or add to
│                an allowlist with a review date
├── Severity: Moderate
│   ├── Reachable in production? → Fix in the next release cycle
│   └── Dev-only / Test-only? → Fix when convenient, track in backlog
└── Severity: Low
    └── Track and fix during regular dependency updates (Renovate / Dependabot PRs)

Key questions:

• Is the vulnerable type/method actually called in your code path? dotnet list package --vulnerable --include-transitive shows transitive dependencies — not all transitive criticals are reachable.
• Is the dependency in a test-only project or a build-time tool? Test-only criticals still need tracking but don't block production.
• Is the vulnerability exploitable given your deployment context (e.g., a server-side RCE in a package you only use client-side)?

When you defer a fix, document the reason and set a review date. Consider wiring Renovate or Dependabot to create PRs for security updates automatically so the backlog doesn't rot.

Supply-Chain Hygiene

dotnet list package --vulnerable catches known CVEs; it won't catch a malicious or typosquatted package. Also:

• Lock and restore reproducibly. Commit packages.lock.json (enable with <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>) and restore with dotnet restore --locked-mode in CI — no silent transitive version drift between machines.
• Pin package sources. Configure <packageSourceMapping> in nuget.config so each package can only come from the expected feed; a public-feed package can't silently shadow a private one (dependency-confusion defense).
• Review new dependencies before adding them — maintenance, download counts, and whether they truly earn their place. Every dependency is attack surface (OWASP A06: Vulnerable Components, LLM03: Supply Chain).
• Be wary of build-time code execution — NuGet packages can ship MSBuild .targets/.props and (legacy) install scripts that run during restore/build. Treat an unfamiliar package's build hooks like any untrusted code.
• Prefer signed packages and consider <trustedSigners> in nuget.config for high-assurance environments.
• Watch for typosquats — Newtonsoft.Json vs Newtonsoft.Jsons, Serilog vs Seri1og.

Rate Limiting

.NET 7+ ships built-in rate limiting via Microsoft.AspNetCore.RateLimiting:

builder.Services.AddRateLimiter(options =>
{
    options.AddFixedWindowLimiter("general", opt =>
    {
        opt.PermitLimit = 100;
        opt.Window = TimeSpan.FromMinutes(15);
        opt.QueueProcessingOrder = QueueProcessingOrder.OldestFirst;
        opt.QueueLimit = 0;
    });

    options.AddFixedWindowLimiter("auth", opt =>
    {
        opt.PermitLimit = 10;
        opt.Window = TimeSpan.FromMinutes(15);
    });

    options.RejectionStatusCode = StatusCodes.Status429TooManyRequests;
});

app.UseRateLimiter();

app.MapGroup("/api").RequireRateLimiting("general");
app.MapGroup("/api/auth").RequireRateLimiting("auth");

For distributed scenarios (multiple instances), back the limiter with a shared store (Redis via RedisRateLimiting) or accept that per-instance limits are approximations.

Secrets Management

Development:
  ├── dotnet user-secrets init --project src/MyApp
  ├── dotnet user-secrets set "Stripe:ApiKey" "sk_test_..." --project src/MyApp
  └── (stored outside the repo at %APPDATA%\Microsoft\UserSecrets\<id>\secrets.json)

Production:
  ├── Azure Key Vault + Managed Identity (AddAzureKeyVault in Program.cs)
  ├── AWS Secrets Manager / Parameter Store
  ├── Environment variables (last resort — appears in process listings, logs)
  └── NEVER appsettings.json committed to git

.gitignore must include:
  appsettings.*.local.json
  *.pfx
  *.key
  *.pem
  secrets.json
  .env

Always check before committing:

# Check for accidentally staged secrets
git diff --cached | grep -iE "password|secret|api.?key|connectionstring|bearer|-----BEGIN"

Wire up a pre-commit hook (Husky.Net) that runs this and fails the commit on a match.

If a secret is ever committed, rotate it. Deleting the line or rewriting history is not enough — assume it's compromised the moment it reaches a remote (forks, CI logs, and mirrors may already have it). Revoke and reissue the key first (rotate the Key Vault secret / regenerate the API key / cycle the connection string), then purge it from history.

Securing AI / LLM Features

If your app calls an LLM — chatbots, summarizers, agents, RAG, anything through Microsoft.Extensions.AI, Semantic Kernel, or the Azure OpenAI SDK — it inherits a new attack surface. Map it to the OWASP Top 10 for LLM Applications (2025):

• Treat all model output as untrusted input (LLM05: Improper Output Handling). Never pass model output straight into FromSqlRaw, Process.Start, a MarkupString / @Html.Raw sink, a file path, or a reflection/Type.GetType call. Validate and encode it exactly as you would raw user input.
• Assume prompts can be hijacked (LLM01: Prompt Injection). Untrusted text in the context window — a user message, a fetched web page, a PDF — can carry instructions. The system prompt is not a security boundary; enforce permissions in code (policy-based [Authorize], per-resource checks), not in the prompt.
• Keep secrets and other users' data out of prompts (LLM02 / LLM07). Anything in the context can be echoed back. Don't put API keys, connection strings, cross-tenant data, or the full system prompt where the model can repeat it.
• Constrain tool and agent permissions (LLM06: Excessive Agency). Scope Semantic Kernel functions / tool callbacks to the minimum, require confirmation for destructive or irreversible actions, and validate every tool argument before executing.
• Bound consumption (LLM10: Unbounded Consumption). Cap MaxOutputTokens, request rate (AddRateLimiter), and loop/recursion depth so a crafted input can't run up cost or hang the system. Always thread a CancellationToken.
• Isolate retrieval data (LLM08: Vector and Embedding Weaknesses). In RAG, treat the vector store as a trust boundary: partition embeddings per tenant so one user can't retrieve another's data, and validate documents before indexing so poisoned content can't steer answers.

// BAD: trusting model output as a command or as markup
var sql = await chat.CompleteAsync($"Write SQL for: {userQuestion}", ct);
await db.Database.ExecuteSqlRawAsync(sql.Text, ct);              // arbitrary query execution
var html = (MarkupString)(await chat.CompleteAsync(userMessage, ct)).Text;  // stored XSS, via the model

// GOOD: model output is data — parse defensively, then validate, then act through an allowlist
CommandIntent intent;
try
{
    var json = (await chat.CompleteAsync(userMessage, ct)).Text;
    intent = JsonSerializer.Deserialize<CommandIntent>(json) ?? throw new ValidationException("null intent");
    await _validator.ValidateAndThrowAsync(intent, ct);          // FluentValidation
}
catch (Exception ex) when (ex is JsonException or ValidationException)
{
    throw new ValidationException("unexpected model output");
}
await RunAllowlistedActionAsync(intent.Action, intent.Params, ct);

Security Review Checklist

### Authentication
- [ ] Passwords hashed with PasswordHasher<TUser> or Argon2 (Konscious)
- [ ] Cookies are HttpOnly + Secure + SameSite (defaults in ASP.NET Core)
- [ ] Login has rate limiting (AddFixedWindowLimiter on /api/auth)
- [ ] Password reset tokens expire (ASP.NET Core Identity default: 1 day)
- [ ] JWT bearer validates issuer + audience + lifetime + signing key + clock skew
- [ ] MFA available for admin accounts

### Authorization
- [ ] Every endpoint has [Authorize] or RequireAuthorization() unless explicitly public
- [ ] Users can only access their own resources (resource-based policy with IAuthorizationHandler)
- [ ] Admin actions require a dedicated policy, not just a role claim check
- [ ] Antiforgery tokens on state-changing requests in cookie-auth apps

### Input
- [ ] All user input validated at the boundary (FluentValidation / DataAnnotations)
- [ ] EF Core LINQ everywhere; any FromSqlRaw use is justified in a code comment
- [ ] Razor/Blazor auto-encoding preserved; no MarkupString / @Html.Raw on user content
- [ ] File uploads validate magic bytes, size, and content type; stored outside web root

### Data
- [ ] No secrets in code, appsettings.json, or git history
- [ ] Sensitive fields excluded from DTOs (never serialize Identity entities directly)
- [ ] PII encrypted at rest via IDataProtectionProvider (key ring shared across instances)

### Infrastructure
- [ ] HSTS + HTTPS redirect enabled in non-Development
- [ ] Security headers configured (CSP, X-Content-Type-Options, X-Frame-Options)
- [ ] CORS restricted to known origins (never "*" with AllowCredentials)
- [ ] dotnet list package --vulnerable clean or documented allowlist with review date
- [ ] Production error page (UseExceptionHandler) strips internal details
- [ ] Server-side fetches of user-supplied URLs are allowlisted and pinned (no SSRF to 169.254.169.254 / internal services)

### Supply Chain
- [ ] packages.lock.json committed; CI restores with --locked-mode
- [ ] nuget.config uses packageSourceMapping (dependency-confusion defense)
- [ ] New dependencies reviewed (maintenance, downloads, build .targets/.props hooks)

### AI / LLM (if used)
- [ ] Model output treated as untrusted (no FromSqlRaw / Process.Start / MarkupString / @Html.Raw / reflection sinks)
- [ ] Secrets and other users' data kept out of prompts; system prompt not echoable
- [ ] Tool/agent permissions scoped; destructive actions require confirmation; tokens/rate/recursion bounded

See Also

• Upstream checklist (generic, pre-dates this adaptation): https://github.com/addyosmani/agent-skills/blob/44dac80216da709913fb410f632a65547866346f/references/security-checklist.md
• ASP.NET Core security docs: https://learn.microsoft.com/aspnet/core/security/
• EF Core raw-SQL safety: https://learn.microsoft.com/ef/core/querying/sql-queries
• Data Protection: https://learn.microsoft.com/aspnet/core/security/data-protection/

Common Rationalizations

• User input passed directly to FromSqlRaw, Process.Start(string), or @Html.Raw
• Secrets in appsettings.json checked into git, or dotnet user-secrets not set up for the project
• HTTP endpoints without [Authorize], RequireAuthorization(), or an explicit "public" annotation
• Missing CORS configuration, or AllowAnyOrigin() combined with AllowCredentials()
• No rate limiting on auth endpoints
• Stack traces or ex.ToString() returned to clients in production
• NuGet dependencies with known Critical or High vulnerabilities and no allowlist entry
• BinaryFormatter anywhere in new code
• MarkupString / @Html.Raw applied to user-sourced HTML
• JWT validation missing any of: issuer, audience, lifetime, signing key, clock skew
• Auth tokens stored in Blazor WebAssembly localStorage
• Server fetches a user-supplied URL with a bare HttpClient (no host allowlist / IP pinning / redirect block) — SSRF
• LLM/model output passed into FromSqlRaw, Process.Start, MarkupString / @Html.Raw, or a reflection sink
• Secrets, PII, or the full system prompt placed inside an LLM context window

Verification

After implementing security-relevant code:

• dotnet list package --vulnerable --include-transitive shows no unreviewed Critical or High vulnerabilities
• No secrets in source code or git history (git log -p | grep -iE "password|apikey" returns nothing)
• All user input validated at system boundaries
• Authentication and authorization checked on every protected endpoint (resource-based authz for user-owned resources)
• Security headers present in response (check with browser DevTools or curl -I)
• Production error responses don't expose internal details
• Rate limiting active on auth endpoints
• File upload handlers validate magic bytes, not just declared content type
• Data Protection key ring is persisted and shared across instances for multi-node deployments
• A threat model (trust boundaries + STRIDE) was sketched for new attack surface before hardening
• Server-side URL fetches are allowlisted and IP-pinned with redirects disabled (no SSRF)
• LLM/model output is parsed defensively, validated, and encoded before use (if AI features present)

Source & Modifications

• Upstream: https://github.com/addyosmani/agent-skills/blob/3a6fc6392823e31e2362091bd4e3cddf5b77af14/skills/security-and-hardening/SKILL.md
• Pinned commit: 3a6fc6392823e31e2362091bd4e3cddf5b77af14 (synced 2026-06-14; body originally ported at 44dac80, unchanged upstream through 1f66d57, expanded by upstream PR #219 at 3a6fc63)
• Status: modified (heavy — nearly every code example and tooling reference has been retargeted; upstream structure, OWASP numbering, three-tier boundary frame, and rationalization-table schema preserved)
• Changes:
  • Three-tier Always/Ask-First/Never lists rewritten for the .NET ecosystem: FluentValidation, EF Core parameterization, Razor/Blazor auto-encoding, ASP.NET Core Identity PasswordHasher<TUser> / Argon2, NetEscapades.AspNetCore.SecurityHeaders, ASP.NET Core cookie defaults, antiforgery (IAntiforgery / [ValidateAntiForgeryToken]), dotnet list package --vulnerable, BinaryFormatter ban, MarkupString / @Html.Raw ban, FromSqlRaw vs FromSqlInterpolated, Data Protection key ring rotation
  • OWASP #1 Injection rewritten with EF Core LINQ vs FromSqlRaw vs FromSqlInterpolated, Dapper parameterized queries, ProcessStartInfo.ArgumentList for command injection
  • OWASP #2 Broken Authentication replaced bcrypt + express-session with ASP.NET Core Identity + ConfigureApplicationCookie + JWT bearer TokenValidationParameters (issuer/audience/lifetime/signing-key/clock-skew)
  • OWASP #3 XSS rewritten for Razor/Blazor auto-encoding and MarkupString foot-gun, pointing at HtmlSanitizer; Avalonia/MAUI called out as non-applicable except through WebView/format sinks
  • OWASP #4 Broken Access Control rewritten as policy-based authz with IAuthorizationService, ResourceOwnerRequirement, ClaimsPrincipal, Results.Problem(403)
  • OWASP #5 Misconfiguration replaced helmet / cors with ASP.NET Core header middleware, NetEscapades.AspNetCore.SecurityHeaders CSP builder, UseHsts/UseHttpsRedirection, AddCors
  • OWASP #6 Sensitive Data Exposure rewritten to project to DTOs (don't serialize Identity entities); IOptions binding with ValidateDataAnnotations() + ValidateOnStart(); IDataProtectionProvider for encrypted PII at rest with shared key ring guidance
  • Input validation rewritten as FluentValidation + Results.ValidationProblem
  • File upload safety rewritten around IFormFile with magic-byte sniffing, AllowedContentTypes, MagicBytes, AV scanning note
  • "Triaging npm audit Results" replaced with "Triaging dotnet list package --vulnerable Results" — decision tree retargeted (netstandard2.0 polyfill examples, Directory.Packages.props bumps, Renovate/Dependabot)
  • Rate limiting section replaced express-rate-limit with .NET 7+ AddRateLimiter + AddFixedWindowLimiter + Microsoft.AspNetCore.RateLimiting, with distributed-limiter note
  • Secrets management rewritten around dotnet user-secrets init, Azure Key Vault + Managed Identity, .gitignore covers .pfx/.key/.pem/secrets.json/appsettings.*.local.json
  • Pre-commit secret check grep extended with connectionstring, bearer, -----BEGIN (PEM private key prefix)
  • Security Review Checklist fully rewritten with .NET-specific bullets (PasswordHasher, JWT validation parameters, MFA, [Authorize], FluentValidation, magic-byte uploads, Data Protection key ring, HSTS)
  • Added "See Also" links to Microsoft Learn pages for ASP.NET Core security, EF Core raw-SQL safety, and Data Protection
  • Rationalizations table adds Blazor WebAssembly client-side-trust rationalization
  • Red-flag list rewritten with .NET-specific vectors (FromSqlRaw, @Html.Raw, MarkupString, missing [Authorize], AllowAnyOrigin() + AllowCredentials(), BinaryFormatter, JWT missing validations, Blazor WebAssembly localStorage)
  • Verification checklist retargeted (dotnet list package --vulnerable, git log -p | grep secret scan, multi-node Data Protection key-ring check, magic-byte upload verification)
  • Preserved: three-tier Always/Ask-First/Never framework, OWASP Top 10 numbering (#1 Injection, #

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