rudironsoni dotnet-security-owasp
OWASP Top 10 (2021) security guidance for .NET applications. Each category includes the vulnerability description, .NET-specific risk, mitigation code examples, and common pitfalls. This skill is the canonical owner of deprecated security pattern warnings (CAS, APTCA, .NET Remoting, DCOM, BinaryFormatter).
Scope
- OWASP Top 10 (2021) vulnerability categories with .NET-specific mitigations
- Injection, broken access control, XSS, SSRF prevention patterns
- Deprecated security API warnings (CAS, APTCA, BinaryFormatter, .NET Remoting)
- Security header configuration and CORS hardening
- Rate limiting and anti-forgery middleware patterns
- NuGet package audit and dependency vulnerability scanning
Out of scope
- Authentication/authorization implementation -- see [skill:dotnet-api-security]
- Blazor auth UI -- see [skill:dotnet-blazor-auth]
- Cryptographic algorithm selection -- see [skill:dotnet-cryptography]
- Configuration binding and Options pattern -- see [skill:dotnet-csharp-configuration]
- Secrets storage and management -- see [skill:dotnet-secrets-management]
Cross-references: [skill:dotnet-secrets-management] for secrets handling, [skill:dotnet-cryptography] for cryptographic best practices, [skill:dotnet-csharp-coding-standards] for secure coding conventions.
A01: Broken Access Control
Vulnerability: Users act outside their intended permissions -- accessing other users' data, elevating privileges, or bypassing access checks.
Risk in .NET: Missing [Authorize] attributes on controllers/endpoints, insecure direct object references (IDOR)
where user IDs are taken from route parameters without ownership validation, and CORS misconfiguration allowing
unintended origins.
Mitigation
// 1. Apply authorization globally, then opt out explicitly
builder.Services.AddAuthorizationBuilder()
.SetFallbackPolicy(new AuthorizationPolicyBuilder()
.RequireAuthenticatedUser()
.Build());
var app = builder.Build();
app.MapControllers(); // All endpoints require auth by default
// 2. Resource-based authorization to prevent IDOR
public sealed class DocumentAuthorizationHandler
: AuthorizationHandler<EditRequirement, Document>
{
protected override Task HandleRequirementAsync(
AuthorizationHandlerContext context,
EditRequirement requirement,
Document resource)
{
if (resource.OwnerId == context.User.FindFirstValue(ClaimTypes.NameIdentifier))
{
context.Succeed(requirement);
}
return Task.CompletedTask;
}
}
// In the endpoint:
app.MapPut("/documents/{id}", async (
int id,
DocumentDto dto,
IAuthorizationService authService,
ClaimsPrincipal user,
AppDbContext db) =>
{
var document = await db.Documents.FindAsync(id);
if (document is null) return Results.NotFound();
var authResult = await authService.AuthorizeAsync(user, document, "Edit");
if (!authResult.Succeeded) return Results.Forbid();
document.Title = dto.Title;
await db.SaveChangesAsync();
return Results.NoContent();
});
```text
```csharp
// 3. Restrict CORS to known origins
builder.Services.AddCors(options =>
{
options.AddPolicy("Strict", policy =>
{
policy.WithOrigins("https://app.example.com")
.WithMethods("GET", "POST")
.WithHeaders("Content-Type", "Authorization");
});
});
```text
**Gotcha:** `AllowAnyOrigin()` combined with `AllowCredentials()` is rejected at runtime by ASP.NET Core, but `SetIsOriginAllowed(_ => true)` with `AllowCredentials()` silently allows all origins -- never use this pattern.
---
## A02: Cryptographic Failures
**Vulnerability:** Sensitive data exposed due to weak or missing encryption -- plaintext storage, deprecated algorithms, or improper key management.
**Risk in .NET:** Using MD5/SHA1 for hashing passwords, storing connection strings with plaintext passwords in `appsettings.json`, transmitting sensitive data over HTTP, or using `DES`/`RC2` for encryption.
### Mitigation
```csharp
// Enforce HTTPS and HSTS
builder.Services.AddHttpsRedirection(options =>
{
options.HttpsPort = 443;
});
var app = builder.Build();
app.UseHsts(); // Strict-Transport-Security header
app.UseHttpsRedirection();
// Never store secrets in appsettings.json -- use user secrets or env vars
// See [skill:dotnet-secrets-management] for proper secrets handling
```json
```csharp
// Use Data Protection API for symmetric encryption of application data
public sealed class TokenProtector(IDataProtectionProvider provider)
{
private readonly IDataProtector _protector =
provider.CreateProtector("Tokens.V1");
public string Protect(string plaintext) => _protector.Protect(plaintext);
public string Unprotect(string ciphertext) => _protector.Unprotect(ciphertext);
}
```text
See [skill:dotnet-cryptography] for algorithm selection (AES-GCM, RSA, ECDSA) and key derivation.
---
## A03: Injection
**Vulnerability:** Untrusted data sent to an interpreter as part of a command or query -- SQL injection, command injection, LDAP injection, and cross-site scripting (XSS).
**Risk in .NET:** String concatenation in SQL queries, `Process.Start` with unsanitized input, rendering user input as raw HTML in Razor pages.
### Mitigation
```csharp
// SQL injection prevention: always use parameterized queries
// EF Core is parameterized by default via LINQ
var orders = await db.Orders
.Where(o => o.CustomerId == customerId)
.ToListAsync();
// When raw SQL is needed, use parameterized interpolation
var results = await db.Orders
.FromSqlInterpolated($"SELECT * FROM Orders WHERE Status = {status}")
.ToListAsync();
// NEVER concatenate user input into SQL:
// var bad = db.Orders.FromSqlRaw("SELECT * FROM Orders WHERE Status = '" + status + "'");
```text
```csharp
// XSS prevention: Razor encodes output by default.
// Use @Html.Raw() ONLY for trusted, pre-sanitized HTML.
// In Minimal APIs, return typed results -- not raw strings:
app.MapGet("/greeting", (string name) =>
Results.Content($"<p>Hello, {HtmlEncoder.Default.Encode(name)}</p>",
"text/html"));
// Command injection prevention: avoid Process.Start with user input.
// If unavoidable, validate against an allowlist:
public static bool IsAllowedTool(string toolName) =>
toolName is "dotnet" or "git" or "nuget";
```bash
**Gotcha:** `FromSqlRaw` with string concatenation bypasses parameterization. Always use `FromSqlInterpolated` or pass `SqlParameter` objects to `FromSqlRaw`.
---
## A04: Insecure Design
**Vulnerability:** Flaws in design patterns that cannot be fixed by implementation alone -- missing rate limiting, lack of defense in depth, unrestricted resource consumption.
**Risk in .NET:** APIs without rate limiting, unbounded file uploads, missing anti-forgery tokens on state-changing operations.
### Mitigation
```csharp
// Rate limiting with built-in middleware (.NET 7+)
builder.Services.AddRateLimiter(options =>
{
options.AddFixedWindowLimiter("api", limiterOptions =>
{
limiterOptions.PermitLimit = 100;
limiterOptions.Window = TimeSpan.FromMinutes(1);
limiterOptions.QueueLimit = 0;
});
options.RejectionStatusCode = StatusCodes.Status429TooManyRequests;
});
var app = builder.Build();
app.UseRateLimiter();
app.MapGet("/api/data", () => Results.Ok("data"))
.RequireRateLimiting("api");
```text
```csharp
// Anti-forgery for Minimal APIs (.NET 8+)
builder.Services.AddAntiforgery();
var app = builder.Build();
app.UseAntiforgery();
// Form-bound endpoint: antiforgery validated automatically
app.MapPost("/orders", async ([FromForm] string productId, AppDbContext db) =>
{
var order = new Order { ProductId = productId };
db.Orders.Add(order);
await db.SaveChangesAsync();
return Results.Created($"/orders/{order.Id}", order);
});
// JSON endpoint: opt in explicitly with RequireAntiforgery()
app.MapPost("/api/orders", async (CreateOrderDto dto, AppDbContext db) =>
{
var order = new Order { ProductId = dto.ProductId };
db.Orders.Add(order);
await db.SaveChangesAsync();
return Results.Created($"/api/orders/{order.Id}", order);
}).RequireAntiforgery();
```text
**Gotcha:** `UseRateLimiter()` must be called after `UseRouting()` and before `MapControllers()`/`MapGet()` to apply correctly.
---
## A05: Security Misconfiguration
**Vulnerability:** Insecure default configurations, incomplete configurations, open cloud storage, unnecessary features enabled, verbose error messages.
**Risk in .NET:** Detailed exception pages in production (`UseDeveloperExceptionPage`), default Kestrel settings exposing server headers, debug endpoints left enabled, or missing security headers.
### Mitigation
```csharp
// Remove server identity headers (configure BEFORE Build)
builder.WebHost.ConfigureKestrel(options =>
{
options.AddServerHeader = false;
});
var app = builder.Build();
if (app.Environment.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
// Generic error handler in production -- no stack traces
app.UseExceptionHandler("/error");
app.UseHsts();
}
// Add security headers via middleware
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("Referrer-Policy", "strict-origin-when-cross-origin");
context.Response.Headers.Append(
"Content-Security-Policy",
"default-src 'self'; script-src 'self'; style-src 'self'");
await next();
});
```text
```csharp
// Constrain request body size to prevent resource exhaustion (configure BEFORE Build)
builder.WebHost.ConfigureKestrel(options =>
{
options.Limits.MaxRequestBodySize = 10 * 1024 * 1024; // 10 MB
options.Limits.MaxRequestHeadersTotalSize = 32 * 1024; // 32 KB
options.Limits.RequestHeadersTimeout = TimeSpan.FromSeconds(30);
});
// Then: var app = builder.Build();
```text
**Gotcha:** `UseDeveloperExceptionPage()` leaks source code paths and stack traces. Ensure it is gated behind `IsDevelopment()` and never enabled in production or staging.
---
## A06: Vulnerable and Outdated Components
**Vulnerability:** Using components with known vulnerabilities, unsupported frameworks, or unpatched dependencies.
**Risk in .NET:** Running on out-of-support .NET versions, NuGet packages with known CVEs, transitive dependency vulnerabilities not audited.
### Mitigation
```xml
<!-- Enable NuGet audit in Directory.Build.props or csproj -->
<PropertyGroup>
<NuGetAudit>true</NuGetAudit>
<NuGetAuditLevel>low</NuGetAuditLevel>
<NuGetAuditMode>all</NuGetAuditMode> <!-- Audit direct + transitive -->
</PropertyGroup>
```text
```bash
# Audit NuGet packages for known vulnerabilities
dotnet list package --vulnerable --include-transitive
# Keep packages up to date
dotnet outdated # requires dotnet-outdated-tool
# Check .NET SDK/runtime support status
dotnet --info
```text
**Gotcha:** `NuGetAuditMode` defaults to `direct` -- transitive vulnerabilities are hidden unless you set `all`. Always use `all` in CI to catch deep dependency issues.
---
## A07: Identification and Authentication Failures
**Vulnerability:** Weak authentication mechanisms, credential stuffing, session fixation, missing multi-factor authentication.
**Risk in .NET:** Default Identity password policies that are too weak, session cookies without `Secure`/`SameSite` attributes, missing account lockout configuration.
### Mitigation
```csharp
// Configure strong Identity password and lockout policies
## Detailed Examples
See [references/detailed-examples.md](references/detailed-examples.md) for complete code samples and advanced patterns.