Allowlist Over Blocklist

Allowlist Over Blocklist

Category: Security Time Saved: Prevents security incidents Battle-tested: Yes — fundamental security principle

The Problem

You need to validate user input — URLs, file paths, commands, or any constrained values. You think "I'll block the dangerous ones" and create a blocklist. Later, an attacker finds a format you didn't think of.

Why Blocklists Fail

Blocklists are incomplete by definition. They only block what you anticipated. Attackers find edge cases, encodings, and variants you didn't consider.

Example: URL validation blocklist

// ❌ BLOCKLIST APPROACH
const blocked = ['javascript:', 'data:', 'vbscript:'];
if (blocked.some(b => url.toLowerCase().startsWith(b))) {
  reject();
}
// Bypassed by: JAVASCRIPT:, java\nscript:, data\x00:text/html

The Rule

Enumerate what's permitted. Reject everything else.

// ✅ ALLOWLIST APPROACH
const allowed = ['https:', 'http:'];
const protocol = new URL(url).protocol;
if (!allowed.includes(protocol)) {
  reject();
}
// Only https: and http: pass. Everything else fails.

When to Apply

Implementation Patterns

URL Validation

const ALLOWED_PROTOCOLS = new Set(['https:', 'http:']);

function isValidUrl(input) {
  try {
    const url = new URL(input);
    return ALLOWED_PROTOCOLS.has(url.protocol);
  } catch {
    return false;
  }
}

File Extension Validation

const ALLOWED_EXTENSIONS = new Set(['.jpg', '.jpeg', '.png', '.gif', '.pdf']);

function isAllowedFile(filename) {
  const ext = path.extname(filename).toLowerCase();
  return ALLOWED_EXTENSIONS.has(ext);
}

Command Execution

const ALLOWED_COMMANDS = new Set(['git', 'npm', 'node', 'tsc']);

function safeExec(command, args) {
  if (!ALLOWED_COMMANDS.has(command)) {
    throw new Error(`Command not allowed: ${command}`);
  }
  return execFileSync(command, args);
}

API Method Validation

const ALLOWED_METHODS = new Set(['GET', 'POST', 'PUT']);

function validateMethod(method) {
  if (!ALLOWED_METHODS.has(method.toUpperCase())) {
    throw new Error(`Method not allowed: ${method}`);
  }
}

Combining with Other Validations

Allowlists are your first gate, not your only gate:

function validateUpload(file) {
  // Gate 1: Allowlist extension
  if (!ALLOWED_EXTENSIONS.has(path.extname(file.name).toLowerCase())) {
    throw new Error('File type not allowed');
  }
  
  // Gate 2: Check MIME type
  if (!ALLOWED_MIMES.has(file.mimetype)) {
    throw new Error('MIME type not allowed');
  }
  
  // Gate 3: Check magic bytes
  if (!verifyMagicBytes(file.buffer)) {
    throw new Error('File content mismatch');
  }
}

Common Mistakes

Verification Checklist

• All validation uses allowlist approach
• Allowlists are const or frozen
• Case normalization applied before check
• Default action is reject (not allow)
• No blocklist fallbacks

Related Skills

• shell-injection-prevention — Command validation
• path-traversal-prevention — File path validation