Dotnet Aspnetcore Security

ASP.NET Core Security Audit

Audit ASP.NET Core applications (.NET 6, 7, 8, 9).

When this skill applies

• Reviewing ASP.NET Core middleware pipeline
• Auditing controllers / Minimal API endpoints
• Reviewing EF Core for SQL injection
• Checking authentication / authorization setup
• Auditing appsettings.json for secret handling

Workflow

Follow ../_shared/audit-workflow.md.

Phase 1: Stack detection

find . -name '*.csproj' -not -path '*/bin/*' -not -path '*/obj/*'
find . -name 'Program.cs' -not -path '*/bin/*' -not -path '*/obj/*'
dotnet --version 2>/dev/null

Phase 2: Inventory

# Middleware pipeline
grep -rn 'app\.Use\|app\.Map\|builder\.Services' Program.cs Startup.cs 2>/dev/null

# Authorize attributes
grep -rn '\[Authorize\|\[AllowAnonymous' . --include='*.cs'

# EF queries
grep -rn 'FromSqlRaw\|ExecuteSqlRaw\|FromSqlInterpolated' . --include='*.cs'

# Configuration
ls appsettings*.json 2>/dev/null

Phase 3: Detection — the checks

Middleware pipeline order

// Program.cs (.NET 6+ minimal hosting)
var app = builder.Build();

app.UseHttpsRedirection();
app.UseHsts();              // HSTS
app.UseStaticFiles();       // Static files before auth (intentional)
app.UseRouting();
app.UseCors(policyName);    // After routing, before auth
app.UseAuthentication();    // Authentication
app.UseAuthorization();     // Authorization (after authentication)
app.UseAntiforgery();       // .NET 8+ explicit
app.MapControllers();
app.Run();

• DNC-MW-1 UseAuthentication before UseAuthorization. Reverse = authorization runs before identity is set.
• DNC-MW-2 UseCors between UseRouting and UseAuthorization.
• DNC-MW-3 UseHsts enabled in production (typically inside if (!app.Environment.IsDevelopment())).
• DNC-MW-4 UseHttpsRedirection so HTTP → HTTPS.

Authentication

• DNC-AUTH-1 builder.Services.AddAuthentication(...) configured; scheme matches what controllers expect.
• DNC-AUTH-2 JWT: AddJwtBearer configured with TokenValidationParameters:

``csharp options.TokenValidationParameters = new TokenValidationParameters { ValidateIssuer = true, ValidateAudience = true, ValidateLifetime = true, ValidateIssuerSigningKey = true, ValidIssuer = config["Jwt:Issuer"], ValidAudience = config["Jwt:Audience"], IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(config["Jwt:Key"])), ClockSkew = TimeSpan.Zero, // optional, tighter }; ``

• DNC-AUTH-3 ValidateLifetime, ValidateIssuer, ValidateAudience, ValidateIssuerSigningKey ALL true. Any false = bypass.
• DNC-AUTH-4 Identity uses Argon2/PBKDF2 (default PBKDF2 acceptable; verify iteration count).
• DNC-AUTH-5 Cookie authentication: SecurePolicy = CookieSecurePolicy.Always, HttpOnly = true, SameSite = SameSiteMode.Lax.

Authorization

• DNC-AZ-1 [Authorize] on controllers requiring auth. Or global filter:

``csharp builder.Services.AddControllers(opts => { var policy = new AuthorizationPolicyBuilder().RequireAuthenticatedUser().Build(); opts.Filters.Add(new AuthorizeFilter(policy)); }); ` Public actions then need [AllowAnonymous]`.

• DNC-AZ-2 Policy-based authz ([Authorize(Policy = "AdminOnly")]) with handlers checking specific claims.
• DNC-AZ-3 Resource-based authz via IAuthorizationService.AuthorizeAsync(user, resource, policy) for per-instance checks.
• DNC-AZ-4 No [Authorize] missing from sensitive endpoints — common bug class.

Antiforgery (CSRF)

• DNC-CSRF-1 [ValidateAntiForgeryToken] on Razor Pages POST handlers, or global filter for MVC.
• DNC-CSRF-2 API endpoints using cookie auth: CSRF protection still required. Use [ValidateAntiForgeryToken] or send via header.
• DNC-CSRF-3 Bearer token APIs (no cookie auth): CSRF not needed.

Model binding — overposting

• DNC-MB-1 Action methods accept dedicated DTOs/ViewModels, NOT entity classes:

```csharp // BAD — User has IsAdmin property, attacker sets it public IActionResult Create([FromBody] User user) { ... }

// GOOD public IActionResult Create([FromBody] CreateUserDto dto) { ... } ```

• DNC-MB-2 [Bind("Name,Email")] attribute used to limit binding when entity must be used.
• DNC-MB-3 Validation attributes ([Required], [StringLength], [RegularExpression]) on DTOs.

SQL injection (EF Core)

• DNC-SQL-1 FromSqlRaw($"SELECT * FROM Users WHERE Id = {id}") is injection. Use:

``csharp // GOOD context.Users.FromSqlInterpolated($"SELECT * FROM Users WHERE Id = {id}"); // OR context.Users.FromSqlRaw("SELECT * FROM Users WHERE Id = {0}", id); ``

• DNC-SQL-2 ExecuteSqlRaw similarly — interpolated or parameterized only.
• DNC-SQL-3 EF Core LINQ queries parameterize automatically — safe.

CORS

• DNC-COR-1 CORS policy defines specific origins, not AllowAnyOrigin for credentialed APIs.
• DNC-COR-2 AllowCredentials() only with specific origins (combined with AllowAnyOrigin is rejected by spec).

Configuration / secrets

• DNC-CFG-1 appsettings.json and appsettings.Development.json don't contain real production secrets.
• DNC-CFG-2 Production secrets via environment variables, Azure Key Vault, AWS Secrets Manager — accessed through IConfiguration.
• DNC-CFG-3 User Secrets used for local dev (dotnet user-secrets set ...) — never in production.
• DNC-CFG-4 Connection strings without password baked in (use integrated auth or env vars).

Data Protection

ASP.NET Core's Data Protection provides keys for cookies, antiforgery tokens, etc.

• DNC-DP-1 Data Protection keys persisted to durable storage (Azure Blob, Redis, filesystem) — not in-memory if you have multiple instances.
• DNC-DP-2 Keys encrypted at rest if filesystem-based.
• DNC-DP-3 SetApplicationName set if multiple apps share the key ring.

File uploads

• DNC-UP-1 IFormFile size limited via RequestSizeLimit attribute or Kestrel.Limits.MaxRequestBodySize.
• DNC-UP-2 Content type validated via byte sniffing (use a library like SixLabors.ImageSharp for images), not trusted from header.
• DNC-UP-3 Filenames sanitized; use UUIDs.

Logging

• DNC-LOG-1 Sensitive parameter logging disabled; EF Core EnableSensitiveDataLogging NEVER true in production.
• DNC-LOG-2 [LogProperties] on sensitive DTOs excludes password/secret fields.

Exception handling

• DNC-EX-1 Production uses app.UseExceptionHandler("/error") (not UseDeveloperExceptionPage).
• DNC-EX-2 Custom error response doesn't include stack traces.
• DNC-EX-3 app.UseStatusCodePages configured if custom 404/500 pages needed.

Headers

• DNC-HDR-1 app.UseSecurityHeaders(...) (NWebsec or similar) OR explicit middleware setting CSP, X-Content-Type-Options, X-Frame-Options.
• DNC-HDR-2 Server header removed (Kestrel: AddServerHeader = false).

Minimal API specifics

• DNC-MA-1 Minimal API endpoints use .RequireAuthorization() for protected routes.
• DNC-MA-2 Endpoint filters for cross-cutting validation.

Razor Pages / MVC views

• DNC-RAZ-1 Razor auto-encodes @Model.Foo. @Html.Raw(...) and Html.Raw(Model.Foo) skip encoding — review usages.
• DNC-RAZ-2 No @(Model.Bar) patterns rendering unencoded HTML from user input.

Dependencies

• DNC-DEP-1 Target framework current (.NET 8 LTS or .NET 9 STS).
• DNC-DEP-2 dotnet list package --vulnerable clean.
• DNC-DEP-3 Old Newtonsoft.Json JSON serializer with TypeNameHandling.Auto / All on untrusted input = RCE. Use System.Text.Json or restrict TypeNameHandling.

Phase 4: Triage

Critical: [Authorize] missing on admin endpoints; JWT validation with any of the 4 validates false; FromSqlRaw with string interpolation; ASP.NET Core version with known CVE.

Phase 5: Report

Use ../_shared/findings-schema.md. Prefix IDs with DNC-.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hlsitechio
• Source: hlsitechio/claude-skills-security
• License: MIT

Install and usage instructions live in the source repository linked above.