Alexander Garcia

Building OAuth Guardian: Part 2

Part 2: Architecture & Design. How TypeScript's type system enabled a security-first architecture that's both extensible and maintainable.

Read time is about 13 minutes

Building OAuth Guardian: Part 2 - Architecture & Design

How TypeScript's type system enabled a security-first architecture that's both extensible and maintainable

In Part 1, I explained why OAuth Guardian needed to exist: OAuth misconfigurations are everywhere, and I learned this firsthand building authentication for 200M+ users at VA.gov. Now let's dive into how I architected the tool to be both powerful and developer-friendly.

Design Principles

Before writing any code, I established core principles based on my VA.gov experience:

1. Security First, Always

Every architectural decision prioritizes security:

Type safety prevents runtime errors in security checks
Immutable data structures prevent state manipulation
No eval() or code execution from configuration
Strict input validation with Zod schemas
No external dependencies for core crypto operations

2. Extensibility Without Complexity

OAuth Guardian ships with built-in checks, but teams have unique requirements. The architecture enables:

Custom security checks extending BaseCheck
Plugin system for third-party checks
Configuration-driven check filtering
Severity customization per environment

3. Developer Experience Matters

I learned at VA.gov that security tools nobody uses don't improve security. OAuth Guardian prioritizes DX:

Beautiful terminal output with progress indicators
Clear error messages with remediation guidance
Multiple report formats (terminal, JSON, HTML)
Zero-config defaults that just work
Comprehensive TypeScript types for IDE autocomplete

4. CI/CD Native

Security checks must run automatically:

JSON output for programmatic consumption
Exit codes for pipeline control
Fast execution (< 10 seconds for most audits)
Configurable failure thresholds

The Core Architecture

OAuth Guardian uses a layered architecture that separates concerns:

┌─────────────────────────────────────────┐
│           CLI Interface                 │  (commander, chalk, ora)
│        src/cli.ts                       │
└───────────────┬─────────────────────────┘
                │
┌───────────────▼─────────────────────────┐
│      Configuration System               │  (js-yaml, zod)
│  src/config/loader.ts                   │
│  src/config/schema.ts                   │
│  src/config/defaults.ts                 │
└───────────────┬─────────────────────────┘
                │
┌───────────────▼─────────────────────────┐
│        Audit Engine                     │
│    src/auditor/engine.ts                │  ← Orchestrates everything
│    src/auditor/http-client.ts           │
└───────┬───────────────────────┬─────────┘
        │                       │
┌───────▼───────┐      ┌────────▼──────────┐
│  Check System │      │  Report System    │
│  src/checks/  │      │  src/reporters/   │
│   - oauth/    │      │   - json          │
│   - nist/     │      │   - html          │
│   - owasp/    │      │   - terminal      │
│   - custom/   │      │   - markdown      │
└───────────────┘      └───────────────────┘

Let's explore each layer.

Layer 1: Type System Foundation

Everything starts with TypeScript types in src/types/. This was crucial at VA.gov where type safety prevented security bugs.

Check Result Type

Every security check returns a structured result:

export interface CheckResult {
  id: string; // Unique check identifier
  name: string; // Human-readable name
  category: CheckCategory; // OAUTH | NIST | OWASP | CUSTOM
  status: CheckStatus; // PASS | FAIL | WARNING | SKIPPED | ERROR
  severity?: Severity; // CRITICAL | HIGH | MEDIUM | LOW | INFO
  description: string; // What this check validates
  message?: string; // Details about the finding
  remediation?: string; // How to fix it
  references?: string[]; // RFC specs, OWASP guides
  metadata?: Record<string, unknown>; // Additional context
  timestamp: Date; // When check ran
  executionTime?: number; // Performance tracking
}

Why this structure?

Consistent reporting: Every check speaks the same language
Type-safe aggregation: The engine can process results without runtime errors
Clear separation: Status (what happened) vs Severity (how bad is it)
Actionable guidance: Remediation isn't optional it's first-class
Audit trail: Timestamps and execution time for compliance

Check Context Type

Checks need access to shared resources:

export interface CheckContext {
  targetUrl: string; // OAuth server being audited
  config?: Record<string, unknown>; // User configuration
  httpClient?: HttpClient; // Shared HTTP client
  logger?: {
    // Optional logging
    debug: (message: string, ...args: unknown[]) => void;
    info: (message: string, ...args: unknown[]) => void;
    warn: (message: string, ...args: unknown[]) => void;
    error: (message: string, ...args: unknown[]) => void;
  };
}

This dependency injection pattern enables:

Testability: Mock httpClient in tests
Shared state: One HTTP client for all checks (connection pooling)
Consistent logging: All checks use the same logger
Configuration access: Checks can customize behavior

Configuration Type

OAuth Guardian is highly configurable, and Zod validates everything:

export interface AuditorConfig {
  target: string; // OAuth server URL
  oauth?: OAuthCheckConfig; // OAuth-specific settings
  nist?: NISTCheckConfig; // NIST compliance settings
  owasp?: OWASPCheckConfig; // OWASP settings
  checks?: CheckFilterConfig; // Which checks to run
  reporting?: ReportingConfig; // Output format
  timeout?: number; // HTTP timeout
  verbose?: boolean; // Detailed logging
  userAgent?: string; // Custom User-Agent
  headers?: Record<string, string>; // Additional headers
  pluginsDir?: string; // Custom check plugins
}

Layer 2: The Check System

The check system is OAuth Guardian's heart. I designed it based on my VA.gov experience writing dozens of security checks.

BaseCheck Abstract Class

All checks extend this base:

export abstract class BaseCheck {
  // Check metadata (subclass defines these)
  abstract readonly id: string;
  abstract readonly name: string;
  abstract readonly category: CheckCategory;
  abstract readonly defaultSeverity: Severity;
  abstract readonly description: string;
  protected references: string[] = [];

  // Main execution method (subclass implements this)
  abstract execute(context: CheckContext): Promise<CheckResult>;

  // Public run method with error handling & timing
  async run(context: CheckContext): Promise<CheckResult> {
    const startTime = Date.now();

    try {
      const result = await this.execute(context);
      return {
        ...result,
        executionTime: Date.now() - startTime,
      };
    } catch (error) {
      return this.error(
        "Check execution failed",
        error instanceof Error ? error : undefined
      );
    }
  }

  // Helper methods for creating results
  protected pass(
    message?: string,
    metadata?: Record<string, unknown>
  ): CheckResult {
    return {
      id: this.id,
      name: this.name,
      category: this.category,
      status: CheckStatus.PASS,
      description: this.description,
      message,
      references: this.references,
      metadata,
      timestamp: new Date(),
    };
  }

  protected fail(
    message: string,
    severity?: Severity,
    remediation?: string,
    metadata?: Record<string, unknown>
  ): CheckResult {
    return {
      id: this.id,
      name: this.name,
      category: this.category,
      status: CheckStatus.FAIL,
      severity: severity ?? this.defaultSeverity,
      description: this.description,
      message,
      remediation,
      references: this.references,
      metadata,
      timestamp: new Date(),
    };
  }

  // Similar helpers for warning(), skip(), error()...
}

Why This Design?

Template Method Pattern: run() provides structure, execute() contains logic. This ensures:

Consistent error handling across all checks
Automatic timing for performance tracking
Type-safe result construction

Helper Methods: pass(), fail(), warning() make checks readable:

// Compare this:
return {
  id: this.id,
  name: this.name,
  category: this.category,
  status: CheckStatus.FAIL,
  severity: Severity.CRITICAL,
  description: this.description,
  message: "PKCE not supported",
  remediation: "...",
  references: this.references,
  metadata: {},
  timestamp: new Date(),
};

// To this:
return this.fail(
  "PKCE not supported",
  Severity.CRITICAL,
  "Implement PKCE (RFC 7636)..."
);

Extensibility: Creating a new check is straightforward:

export class PKCECheck extends BaseCheck {
  readonly id = "oauth-pkce";
  readonly name = "PKCE Implementation Check";
  readonly category = CheckCategory.OAUTH;
  readonly defaultSeverity = Severity.HIGH;
  readonly description = "Validates PKCE (RFC 7636) implementation";

  protected references = ["https://datatracker.ietf.org/doc/html/rfc7636"];

  async execute(context: CheckContext): Promise<CheckResult> {
    const httpClient = context.httpClient as HttpClient;

    // 1. Discover OAuth metadata
    const metadata = await httpClient.discoverOAuthMetadata(context.targetUrl);

    if (!metadata.success) {
      return this.warning(
        "Unable to discover OAuth metadata",
        "Implement OAuth 2.0 Authorization Server Metadata (RFC 8414)"
      );
    }

    // 2. Check PKCE support
    const methods = metadata.data?.code_challenge_methods_supported;

    if (!methods || !methods.includes("S256")) {
      return this.fail(
        "PKCE not supported or S256 method missing",
        Severity.CRITICAL,
        "Enable PKCE with S256 code challenge method...\n\nExample:\n..."
      );
    }

    return this.pass("PKCE properly supported with S256 method");
  }
}

Layer 3: The Audit Engine

The AuditEngine orchestrates check execution:

export class AuditEngine {
  private config: AuditorConfig;
  private httpClient: HttpClient;
  private checks: BaseCheck[] = [];
  private logger?: Logger;

  constructor(config: AuditorConfig) {
    this.config = config;
    this.httpClient = new HttpClient({
      timeout: config.timeout,
      userAgent: config.userAgent,
      headers: config.headers,
      verbose: config.verbose,
    });
    if (config.verbose) {
      this.setupLogger();
    }
  }

  registerCheck(check: BaseCheck): void {
    this.checks.push(check);
  }

  registerChecks(checks: BaseCheck[]): void {
    this.checks.push(...checks);
  }

  async run(): Promise<Report> {
    const startTime = new Date();

    // 1. Filter checks based on config
    const checksToRun = this.filterChecks();

    // 2. Create shared context
    const context: CheckContext = {
      targetUrl: this.config.target,
      config: this.config as unknown as Record<string, unknown>,
      httpClient: this.httpClient,
      logger: this.logger,
    };

    // 3. Run all checks
    const results: CheckResult[] = [];
    for (const check of checksToRun) {
      this.logger?.info(`Running check: ${check.name}`);
      const result = await check.run(context);
      results.push(result);
    }

    const endTime = new Date();

    // 4. Generate comprehensive report
    return this.generateReport(results, startTime, endTime);
  }

  private generateReport(
    results: CheckResult[],
    startTime: Date,
    endTime: Date
  ): Report {
    return {
      metadata: {
        targetUrl: this.config.target,
        startTime,
        endTime,
        executionTime: endTime.getTime() - startTime.getTime(),
        version: "0.1.0",
      },
      summary: this.generateSummary(results),
      results,
      findings: this.generateFindings(results),
      compliance: this.generateComplianceScorecard(results),
    };
  }

  // Risk scoring, compliance calculation, etc...
}

Key Design Decisions

Sequential Execution: Checks run sequentially (not parallel) to:

Avoid HTTP connection pool exhaustion
Provide predictable progress indicators
Simplify error handling

Shared HTTP Client: One client instance for all checks:

Connection reuse
Consistent timeout handling
Centralized request logging

Rich Report Generation: The engine calculates:

Pass/fail/warning counts
Risk score (weighted by severity)
Compliance percentage
Per-category scorecards

Layer 4: Configuration System

OAuth Guardian uses YAML configuration validated by Zod schemas.

Why YAML + Zod?

YAML: Human-friendly, comment-supported, widely adopted Zod: Runtime type validation, great error messages, TypeScript integration

import { z } from "zod";

const OAuthCheckConfigSchema = z
  .object({
    pkce: z.union([z.boolean(), z.enum(["error", "warning"])]).optional(),
    state: z.union([z.boolean(), z.enum(["error", "warning"])]).optional(),
    // ...
  })
  .optional();

export const AuditorConfigSchema = z.object({
  target: z.string().url(),
  oauth: OAuthCheckConfigSchema,
  nist: NISTCheckConfigSchema,
  owasp: OWASPCheckConfigSchema,
  // ...
});

export type ValidatedConfig = z.infer<typeof AuditorConfigSchema>;

Benefits:

Compile-time types for IDE autocomplete
Runtime validation catches config errors
Clear error messages: oauth.pkce: Expected boolean, received string

Configuration Auto-Discovery

OAuth Guardian searches for config files automatically:

const CONFIG_FILE_NAMES = [
  "oauth-guardian.config.yml",
  "oauth-guardian.config.yaml",
  ".oauth-guardian.yml",
  ".oauth-guardian.yaml",
];

export async function discoverAndLoadConfig(): Promise<AuditorConfig | null> {
  for (const fileName of CONFIG_FILE_NAMES) {
    try {
      const filePath = resolve(process.cwd(), fileName);
      return await loadConfigFromFile(filePath);
    } catch {
      continue;
    }
  }
  return null;
}

This enables zero-config operation while supporting advanced customization.

Layer 5: Report System

OAuth Guardian supports multiple output formats, all generated from the same Report type.

Terminal Reporter

Beautiful CLI output with colored formatting:

export class TerminalReporter {
  generate(report: Report): string {
    const lines: string[] = [];

    // Summary with colors
    lines.push(chalk.bold("📊 Audit Results"));
    lines.push(`  ${chalk.green("✓")} Passed: ${report.summary.passed}`);
    lines.push(`  ${chalk.red("✗")} Failed: ${report.summary.failed}`);

    // Check results with icons
    for (const result of report.results) {
      const icon = this.getStatusIcon(result.status);
      lines.push(`${icon} ${chalk.bold(result.name)}`);
      lines.push(`  ${chalk.gray(result.description)}`);
    }

    return lines.join("\n");
  }
}

HTML Reporter

Professional reports with Handlebars templates:

export class HTMLReporter {
  async generate(report: Report): Promise<string> {
    const template = Handlebars.compile(htmlTemplate);

    return template({
      metadata: report.metadata,
      summary: report.summary,
      results: report.results,
      compliance: report.compliance,
    });
  }
}

JSON Reporter

Machine-readable for CI/CD:

export class JSONReporter {
  generate(report: Report): string {
    return JSON.stringify(report, null, 2);
  }
}

What's Next?

In Part 3, I'll walk through the implementation journey:

Building the first OAuth check (PKCE detection)
Implementing HTML reports with Handlebars
Adding configuration validation with Zod
Testing strategies for security tools
Lessons learned shipping an open-source project

This is Part 2 of a 3-part series. ← Read Part 1: The Problem | Read Part 3: Implementation →