defense-in-depth-validation — Claude Code skill: validate every layer

---
name: defense-in-depth-validation
description: Validate at every layer data passes through to make bugs impossible. Use when invalid data causes failures deep in execution, requiring validation at multiple system layers.
metadata:
  version: "1.1.0"
license: MIT
---

# 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

```typescript
function createProject(name: string, workingDirectory: string) {
  if (!workingDirectory || workingDirectory.trim() === '') {
    throw new Error('workingDirectory cannot be empty');
  }
  if (!existsSync(workingDirectory)) {
    throw new Error(`workingDirectory does not exist: ${workingDirectory}`);
  }
  if (!statSync(workingDirectory).isDirectory()) {
    throw new Error(`workingDirectory is not a directory: ${workingDirectory}`);
  }
  // ... proceed
}
```

### Layer 2: Business Logic Validation
**Purpose:** Ensure data makes sense for this operation

```typescript
function initializeWorkspace(projectDir: string, sessionId: string) {
  if (!projectDir) {
    throw new Error('projectDir required for workspace initialization');
  }
  // ... proceed
}
```

### Layer 3: Environment Guards
**Purpose:** Prevent dangerous operations in specific contexts

```typescript
async function gitInit(directory: string) {
  // In tests, refuse git init outside temp directories
  if (process.env.NODE_ENV === 'test') {
    const normalized = normalize(resolve(directory));
    const tmpDir = normalize(resolve(tmpdir()));

    if (!normalized.startsWith(tmpDir)) {
      throw new Error(
        `Refusing git init outside temp dir during tests: ${directory}`
      );
    }
  }
  // ... proceed
}
```

### Layer 4: Debug Instrumentation
**Purpose:** Capture context for forensics

```typescript
async function gitInit(directory: string) {
  const stack = new Error().stack;
  logger.debug('About to git init', {
    directory,
    cwd: process.cwd(),
    stack,
  });
  // ... 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.

## Security defense-in-depth

The same layered approach is the standard pattern for security controls: trust
is never granted at a single point, so that a bug or bypass in any one layer
cannot by itself compromise the system. Each request is checked independently
at the edge, in the handler, and at the data layer.

- **Layer 1 (edge)**: rate-limit and filter at the edge (CDN/WAF such as
  Cloudflare or AWS WAF) so abusive traffic never reaches the origin.
- **Layer 2 (handler)**: re-derive authorization server-side from the session
  — never trust a client-supplied claim like a `userId` in the body.
- **Layer 3 (data)**: enforce the ownership invariant in the database query so
  that even a handler bug cannot cross tenant boundaries.

```javascript
// Layer 2 — re-derive authz server-side, never trust client claims
app.put('/docs/:id', async (req, res) => {
  const user = await verifySession(req.cookies.session); // throws if invalid
  // Layer 3 — DB clause re-checks ownership; a handler bug still can't cross tenants
  const updated = await db.query(
    'UPDATE docs SET title = $1 WHERE id = $2 AND owner_id = $3 RETURNING *',
    [req.body.title, req.params.id, user.id]
  );
  if (!updated.rowCount) return res.status(403).send('Not allowed');
  res.json(updated.rows[0]);
});
```

Defense in depth means no single layer's failure is sufficient to compromise
the system.