memstack-security-csp-headers

🛡️ CSP Headers — Security Headers Auditor & Generator

Audit existing security headers, identify overly permissive directives, and generate a production-ready Content-Security-Policy with companion headers.

Activation

When this skill activates, output:

🛡️ CSP Headers — Auditing your security headers...

Context	Status
User says "CSP", "security headers", "Content-Security-Policy"	ACTIVE
User wants to fix unsafe-inline, unsafe-eval, or wildcard directives	ACTIVE
User mentions HSTS, X-Frame-Options, or Permissions-Policy	ACTIVE
User wants a full OWASP security audit (not just headers)	DORMANT — see owasp-top10
User wants to scan for secrets in code	DORMANT — see secrets-scanner
User wants API-level security (auth, rate limiting)	DORMANT — see api-audit

Protocol

Step 1: Gather Inputs

Ask the user for:

• Framework: Next.js, Express, Fastify, Nginx, Caddy, static hosting?
• CDN/hosting: Vercel, Netlify, Cloudflare, self-hosted?
• External resources: Which third-party scripts, fonts, images, or APIs are loaded? (analytics, payment widgets, CDNs)
• Inline scripts: Does the app use inline <script> tags or inline styles?
• Iframes: Does the app embed iframes or get embedded by others?

Step 2: Scan Existing Headers

Check for CSP and security headers in all possible locations:

Where to look:

1. Middleware — Express/Next.js middleware setting response headers
2. Config files — next.config.js (headers() function), nginx.conf, Caddyfile
3. Meta tags — <meta http-equiv="Content-Security-Policy" content="..."> in HTML
4. Hosting config — vercel.json, netlify.toml, _headers file
5. Reverse proxy — Nginx/Caddy/Apache header directives

── EXISTING HEADERS FOUND ─────────────────

Location: [file:line or "not found"]

Content-Security-Policy:
  [current policy or "MISSING"]

Strict-Transport-Security:
  [current value or "MISSING"]

X-Content-Type-Options:
  [current value or "MISSING"]

X-Frame-Options:
  [current value or "MISSING"]

Referrer-Policy:
  [current value or "MISSING"]

Permissions-Policy:
  [current value or "MISSING"]

X-XSS-Protection:
  [current value or "MISSING — deprecated but still useful for older browsers"]

Step 3: Audit CSP Directives

If a CSP exists, audit each directive for security issues:

Directive	Current Value	Issue	Severity	Fix
default-src	*	Allows loading from any origin	🔴 Critical	Restrict to known origins
script-src	'unsafe-inline'	Allows inline scripts — XSS vector	🔴 Critical	Use nonces or hashes
script-src	'unsafe-eval'	Allows dynamic code execution — injection risk	🟡 High	Remove, refactor code
style-src	'unsafe-inline'	Allows inline styles — less severe	🟠 Medium	Use nonces if feasible
img-src	*	Allows images from any origin	🟠 Medium	Restrict to known CDNs
frame-ancestors	missing	No clickjacking protection	🟡 High	Add 'self' or 'none'
connect-src	missing	No restriction on fetch/XHR targets	🟠 Medium	Restrict to API origins
base-uri	missing	Base tag injection possible	🟡 High	Add 'self'
form-action	missing	Forms can submit to any origin	🟠 Medium	Add 'self'
object-src	missing	Plugin content allowed	🟡 High	Add 'none'

Common dangerous patterns:

BAD:  default-src *                    → Defeats the entire purpose of CSP
BAD:  script-src 'unsafe-inline'       → XSS protection nullified
BAD:  script-src 'unsafe-eval'         → Dynamic code execution permitted
BAD:  script-src https:                → Any HTTPS origin can serve scripts
BAD:  script-src *.googleapis.com      → JSONP endpoints can bypass CSP
BAD:  connect-src *                    → Data exfiltration unrestricted
BAD:  frame-ancestors not set          → Clickjacking possible

Step 4: Inventory External Resources

Identify all external resources the application loads:

── RESOURCE INVENTORY ─────────────────────

Scripts (script-src):
  - https://www.googletagmanager.com    — Google Tag Manager
  - https://js.stripe.com               — Stripe.js
  - https://cdn.jsdelivr.net             — jsDelivr CDN
  - 'self'                               — Own domain scripts

Styles (style-src):
  - https://fonts.googleapis.com         — Google Fonts CSS
  - 'self'                               — Own stylesheets
  - 'unsafe-inline'                      — Needed for: [reason]

Fonts (font-src):
  - https://fonts.gstatic.com            — Google Fonts files
  - 'self'                               — Self-hosted fonts

Images (img-src):
  - https://res.cloudinary.com           — Cloudinary images
  - https://*.githubusercontent.com      — GitHub avatars
  - data:                                — Data URI images
  - 'self'                               — Own images

Connections (connect-src):
  - https://api.example.com              — Own API
  - https://api.stripe.com               — Stripe API
  - https://www.google-analytics.com     — Analytics
  - 'self'                               — Same-origin requests

Frames (frame-src):
  - https://js.stripe.com                — Stripe checkout iframe
  - https://www.youtube.com              — Embedded videos
  - 'none'                               — If no iframes needed

Step 5: Generate Recommended CSP

Build a CSP policy based on actual resource usage:

── RECOMMENDED CSP ────────────────────────

Content-Security-Policy:
  default-src 'self';
  script-src 'self' 'nonce-{SERVER_GENERATED}' https://www.googletagmanager.com https://js.stripe.com;
  style-src 'self' 'nonce-{SERVER_GENERATED}' https://fonts.googleapis.com;
  font-src 'self' https://fonts.gstatic.com;
  img-src 'self' https://res.cloudinary.com data:;
  connect-src 'self' https://api.stripe.com https://www.google-analytics.com;
  frame-src https://js.stripe.com https://www.youtube.com;
  frame-ancestors 'self';
  base-uri 'self';
  form-action 'self';
  object-src 'none';
  upgrade-insecure-requests;

Nonce-based script loading (recommended over 'unsafe-inline'):

// Express/Next.js middleware example
const crypto = require('crypto');

function cspMiddleware(req, res, next) {
  const nonce = crypto.randomBytes(16).toString('base64');
  res.locals.cspNonce = nonce;

  res.setHeader('Content-Security-Policy', [
    "default-src 'self'",
    `script-src 'self' 'nonce-${nonce}'`,
    `style-src 'self' 'nonce-${nonce}'`,
    "font-src 'self' https://fonts.gstatic.com",
    "img-src 'self' data:",
    "connect-src 'self'",
    "frame-ancestors 'self'",
    "base-uri 'self'",
    "form-action 'self'",
    "object-src 'none'",
    "upgrade-insecure-requests",
  ].join('; '));

  next();
}

<!-- Use nonce in script/style tags -->
<script nonce="<%= cspNonce %>">
  // Inline script allowed by nonce
</script>

Hash-based alternative (for static inline scripts):

# Generate hash for a known inline script
echo -n "console.log('hello')" | openssl dgst -sha256 -binary | openssl enc -base64
# Result: 'sha256-xxxxxxxxx'
# Add to CSP: script-src 'sha256-xxxxxxxxx'

Step 6: Generate Companion Security Headers

CSP alone isn't enough. Generate the full security headers suite:

── COMPLETE SECURITY HEADERS ──────────────

# HSTS — Force HTTPS for all future visits
Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

# Prevent MIME type sniffing attacks
X-Content-Type-Options: nosniff

# Clickjacking protection (legacy — frame-ancestors in CSP is preferred)
X-Frame-Options: SAMEORIGIN

# Control referrer information leakage
Referrer-Policy: strict-origin-when-cross-origin

# Restrict browser features
Permissions-Policy: camera=(), microphone=(), geolocation=(), payment=self

# Legacy XSS filter (deprecated but harmless)
X-XSS-Protection: 0

Header explanations:

Header	Purpose	Recommended Value
Strict-Transport-Security	Force HTTPS, prevent SSL stripping	max-age=63072000; includeSubDomains; preload
X-Content-Type-Options	Prevent MIME sniffing (serving JS as image)	nosniff
X-Frame-Options	Prevent clickjacking (legacy, CSP preferred)	SAMEORIGIN or DENY
Referrer-Policy	Control referer header leakage	strict-origin-when-cross-origin
Permissions-Policy	Disable unused browser APIs	Disable camera, mic, geo unless needed
X-XSS-Protection	Legacy XSS filter	0 (disable — can cause issues, CSP is better)

Step 7: Report-Only Mode

Always deploy CSP in report-only mode first to avoid breaking production:

── DEPLOYMENT STRATEGY ────────────────────

Phase 1: REPORT-ONLY (1-2 weeks)
  Header: Content-Security-Policy-Report-Only
  Purpose: Collect violations without blocking anything
  Report endpoint: /api/csp-report

Phase 2: ENFORCE (after zero violations)
  Header: Content-Security-Policy
  Purpose: Block unauthorized resources
  Keep report-uri for ongoing monitoring

Report-only header:

Content-Security-Policy-Report-Only:
  [same policy as above];
  report-uri /api/csp-report;
  report-to csp-endpoint

Violation report handler:

// POST /api/csp-report
app.post('/api/csp-report', express.json({ type: 'application/csp-report' }), (req, res) => {
  const report = req.body['csp-report'];
  console.warn('CSP Violation:', {
    blockedUri: report['blocked-uri'],
    directive: report['violated-directive'],
    documentUri: report['document-uri'],
    sourceFile: report['source-file'],
    lineNumber: report['line-number'],
  });
  res.status(204).end();
});

Common violations to expect:

• Browser extensions injecting scripts (ignore — not your code)
• Inline styles from third-party libraries (add nonce or hash)
• Data URIs for images (add data: to img-src if needed)
• Dynamic code execution in legacy libraries (refactor or whitelist as last resort)

Step 8: Framework-Specific Implementation

Next.js (next.config.js):

const securityHeaders = [
  { key: 'Content-Security-Policy', value: "default-src 'self'; ..." },
  { key: 'Strict-Transport-Security', value: 'max-age=63072000; includeSubDomains; preload' },
  { key: 'X-Content-Type-Options', value: 'nosniff' },
  { key: 'X-Frame-Options', value: 'SAMEORIGIN' },
  { key: 'Referrer-Policy', value: 'strict-origin-when-cross-origin' },
  { key: 'Permissions-Policy', value: 'camera=(), microphone=(), geolocation=()' },
];

module.exports = {
  async headers() {
    return [{ source: '/(.*)', headers: securityHeaders }];
  },
};

Express middleware:

app.use((req, res, next) => {
  res.setHeader('Content-Security-Policy', "default-src 'self'; ...");
  res.setHeader('Strict-Transport-Security', 'max-age=63072000; includeSubDomains; preload');
  res.setHeader('X-Content-Type-Options', 'nosniff');
  res.setHeader('X-Frame-Options', 'SAMEORIGIN');
  res.setHeader('Referrer-Policy', 'strict-origin-when-cross-origin');
  res.setHeader('Permissions-Policy', 'camera=(), microphone=(), geolocation=()');
  next();
});

Nginx:

add_header Content-Security-Policy "default-src 'self'; ..." always;
add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;
add_header X-Content-Type-Options "nosniff" always;
add_header X-Frame-Options "SAMEORIGIN" always;
add_header Referrer-Policy "strict-origin-when-cross-origin" always;
add_header Permissions-Policy "camera=(), microphone=(), geolocation=()" always;

Caddy:

header {
    Content-Security-Policy "default-src 'self'; ..."
    Strict-Transport-Security "max-age=63072000; includeSubDomains; preload"
    X-Content-Type-Options "nosniff"
    X-Frame-Options "SAMEORIGIN"
    Referrer-Policy "strict-origin-when-cross-origin"
    Permissions-Policy "camera=(), microphone=(), geolocation=()"
}

Vercel (vercel.json):

{
  "headers": [
    {
      "source": "/(.*)",
      "headers": [
        { "key": "Content-Security-Policy", "value": "default-src 'self'; ..." },
        { "key": "Strict-Transport-Security", "value": "max-age=63072000; includeSubDomains; preload" },
        { "key": "X-Content-Type-Options", "value": "nosniff" },
        { "key": "X-Frame-Options", "value": "SAMEORIGIN" },
        { "key": "Referrer-Policy", "value": "strict-origin-when-cross-origin" },
        { "key": "Permissions-Policy", "value": "camera=(), microphone=(), geolocation=()" }
      ]
    }
  ]
}

Step 9: Output

Present the complete security headers audit and configuration:

━━━ SECURITY HEADERS REPORT ━━━━━━━━━━━━━━
Project: [name]
Framework: [framework]
Hosting: [platform]

── AUDIT RESULTS ──────────────────────────
Headers found: [X] of 6
Issues found: [count]
  🔴 Critical: [count]
  🟡 High:     [count]
  🟠 Medium:   [count]

── ISSUES ─────────────────────────────────
[directive-level audit table]

── RESOURCE INVENTORY ─────────────────────
[categorized external resources]

── RECOMMENDED CSP ────────────────────────
[complete CSP policy]

── COMPANION HEADERS ──────────────────────
[HSTS, X-Content-Type-Options, etc.]

── IMPLEMENTATION ─────────────────────────
[framework-specific config, copy-paste ready]

── DEPLOYMENT PLAN ────────────────────────
Phase 1: Report-only ([duration])
Phase 2: Enforce (after zero violations)
Report endpoint: [handler code]

── SECURITY SCORE ─────────────────────────
Score: [A+ / A / B / C / D / F]
  CSP:              [pass/fail]
  HSTS:             [pass/fail]
  X-Content-Type:   [pass/fail]
  X-Frame-Options:  [pass/fail]
  Referrer-Policy:  [pass/fail]
  Permissions:      [pass/fail]

Grading:

• A+: All headers present, CSP with nonces, no unsafe directives
• A: All headers present, minor CSP weaknesses
• B: Most headers present, some unsafe directives
• C: CSP present but overly permissive
• D: Missing critical headers
• F: No security headers at all

Inputs

• Framework / hosting platform
• External resource list (scripts, fonts, APIs, CDNs)
• Inline script/style usage
• Iframe embedding requirements
• Existing CSP or security headers (auto-scanned)

Outputs

• Existing headers audit with directive-level issues
• External resource inventory categorized by CSP directive
• Generated CSP policy based on actual resource usage
• Nonce-based inline script/style implementation
• Complete companion security headers suite
• Framework-specific implementation code (Next.js, Express, Nginx, Caddy, Vercel)
• Report-only deployment plan with violation handler
• Security headers grade (A+ through F)

Level History

• Lv.1 — Base: CSP directive audit with severity classification, external resource inventory, nonce/hash-based inline script handling, companion headers suite (HSTS, X-Content-Type-Options, X-Frame-Options, Referrer-Policy, Permissions-Policy), framework-specific implementations (Next.js, Express, Nginx, Caddy, Vercel), report-only deployment strategy, security grade. (Origin: MemStack v3.2, Mar 2026)