everything-claude-code/rules/csharp/security.md

---
paths:
  - "**/*.cs"
  - "**/*.csx"
  - "**/*.csproj"
  - "**/appsettings*.json"
---
# C# Security

> This file extends [common/security.md](../common/security.md) with C#-specific content.

## Secret Management

- Never hardcode API keys, tokens, or connection strings in source code
- Use environment variables, user secrets for local development, and a secret manager in production
- Keep `appsettings.*.json` free of real credentials

```csharp
// BAD
const string ApiKey = "sk-live-123";

// GOOD
var apiKey = builder.Configuration["OpenAI:ApiKey"]
    ?? throw new InvalidOperationException("OpenAI:ApiKey is not configured.");
```

## SQL Injection Prevention

- Always use parameterized queries with ADO.NET, Dapper, or EF Core
- Never concatenate user input into SQL strings
- Validate sort fields and filter operators before using dynamic query composition

```csharp
const string sql = "SELECT * FROM Orders WHERE CustomerId = @customerId";
await connection.QueryAsync<Order>(sql, new { customerId });
```

## Input Validation

- Validate DTOs at the application boundary
- Use data annotations, FluentValidation, or explicit guard clauses
- Reject invalid model state before running business logic

## Authentication and Authorization

- Prefer framework auth handlers instead of custom token parsing
- Enforce authorization policies at endpoint or handler boundaries
- Never log raw tokens, passwords, or PII

## Error Handling

- Return safe client-facing messages
- Log detailed exceptions with structured context server-side
- Do not expose stack traces, SQL text, or filesystem paths in API responses

## References

See skill: `security-review` for broader application security review checklists.