Defense In Depth

Defense-in-Depth Validation

Overview

When you fix a bug caused by invalid data, adding validation at one place feels sufficient. But that single check can be bypassed by different code paths, refactoring, or mocks.

Core principle: Validate at EVERY layer data passes through. Make the bug structurally impossible.

Why Multiple Layers

Single validation: "We fixed the bug" Multiple layers: "We made the bug impossible"

Different layers catch different cases:

• Entry validation catches most bugs
• Business logic catches edge cases
• Environment guards prevent context-specific dangers
• Debug logging helps when other layers fail

The Four Layers

Layer 1: Entry Point Validation

Purpose: Reject obviously invalid input at API boundary

public static async Task CreateProjectAsync(string name, string workingDirectory)
{
    if (string.IsNullOrWhiteSpace(workingDirectory))
    {
        throw new ArgumentException("Working directory cannot be empty or whitespace", nameof(workingDirectory));
    }
    
    if (!Directory.Exists(workingDirectory))
    {
        throw new DirectoryNotFoundException($"Working directory does not exist: {workingDirectory}");
    }
    
    var dirInfo = new DirectoryInfo(workingDirectory);
    if (!dirInfo.Exists)
    {
        throw new ArgumentException($"Working directory is not accessible: {workingDirectory}", nameof(workingDirectory));
    }
    
    // ... proceed
}

Layer 2: Business Logic Validation

Purpose: Ensure data makes sense for this operation

public static async Task InitializeWorkspaceAsync(string projectDir, string sessionId)
{
    if (string.IsNullOrEmpty(projectDir))
    {
        throw new InvalidOperationException("projectDir required for workspace initialization");
    }
    
    // ... proceed
}

Layer 3: Environment Guards

Purpose: Prevent dangerous operations in specific contexts

public static async Task GitInitAsync(string directory)
{
    // In tests, refuse git init outside temp directories
    var environment = Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT");
    if (environment == "Test")
    {
        var normalized = Path.GetFullPath(directory);
        var tempPath = Path.GetFullPath(Path.GetTempPath());

        if (!normalized.StartsWith(tempPath, StringComparison.OrdinalIgnoreCase))
        {
            throw new InvalidOperationException(
                $"Refusing git init outside temp dir during tests: {directory}");
        }
    }
    
    // ... proceed
}

Layer 4: Debug Instrumentation

Purpose: Capture context for forensics

public static async Task GitInitAsync(string directory)
{
    var stackTrace = Environment.StackTrace;
    
    _logger.LogDebug("About to git init: {Directory}, CWD: {CurrentDirectory}, Stack: {StackTrace}", 
        directory, 
        Environment.CurrentDirectory, 
        stackTrace);
        
    // ... proceed
}

Applying the Pattern

When you find a bug:

1. Trace the data flow - Where does bad value originate? Where used?
2. Map all checkpoints - List every point data passes through
3. Add validation at each layer - Entry, business, environment, debug
4. Test each layer - Try to bypass layer 1, verify layer 2 catches it

Example from Session

Bug: Empty projectDir caused git init in source code

Data flow:

1. Test setup → empty string
2. Project.create(name, '')
3. WorkspaceManager.createWorkspace('')
4. git init runs in process.cwd()

Four layers added:

• Layer 1: Project.create() validates not empty/exists/writable
• Layer 2: WorkspaceManager validates projectDir not empty
• Layer 3: WorktreeManager refuses git init outside tmpdir in tests
• Layer 4: Stack trace logging before git init

Result: All 1847 tests passed, bug impossible to reproduce

Key Insight

All four layers were necessary. During testing, each layer caught bugs the others missed:

• Different code paths bypassed entry validation
• Mocks bypassed business logic checks
• Edge cases on different platforms needed environment guards
• Debug logging identified structural misuse

Don't stop at one validation point. Add checks at every layer.

Source & license

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

• Author: alexsandrocruz
• Source: alexsandrocruz/ZenPowers
• License: MIT

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