HTTP Headers are a great booster for web security with easy implementation. Proper HTTP headers can prevent security vulnerabilities like Cross-Site Scripting, Click-jacking, Packet sniffing and, information disclosure.

In this article, we'll take a quick look at all security-related HTTP headers and the recommended configurations. Below are the main sections of this document.

• Security headers list
• Implementation of HTTP headers in Nginx, Apache, PHP, etc.
• Testing of HTTP headers in your website
• References

The source for this document is available on GitHub. Your contributions are most welcome to complete it and keep it updated 👐

Security Headers

X-Frame-Options

The X-Frame-Options HTTP response header can be used to indicate whether or not a browser should be allowed to render a page in a <frame>, <iframe>, <embed> or <object>. Sites can use this to avoid click-jacking attacks, by ensuring that their content is not embedded into other sites.

Recommendation

Do not allow displaying of the page in a frame.

X-Frame-Options: DENY

X-XSS-Protection

The HTTP X-XSS-Protection response header is a feature of Internet Explorer, Chrome, and Safari that stops pages from loading when they detect reflected cross-site scripting (XSS) attacks.

Recommendation

Enable XSS filtering and prevent browsers from rendering pages if an attack is detected.

X-XSS-Protection: 1; mode=block

X-Content-Type-Options

The X-Content-Type-Options response HTTP header is used by the server to prevent browsers from guessing the media type ( MIME type).
This is known as MIME sniffing in which the browser guesses the correct MIME type by looking at the contents of the resource.
The absence of this header might cause browsers to transform non-executable content into executable content.

Recommendation

X-Content-Type-Options: nosniff

Referrer-Policy

The Referrer-Policy HTTP header controls how much referrer information (sent via the Referer header) should be included with requests.

Recommendation

Send everything to the same site but only the origin for other sites.

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

• NOTE: This is the default in modern browsers

Content-Type

The Content-Type representation header is used to indicate the original media type of the resource (before any content encoding is applied for sending).

Recommendation

Content-Type: text/html; charset=UTF-8

• NOTE: the charset attribute is necessary to prevent XSS in HTML pages
• NOTE: the text/html can be any of the possible MIME types

Set-Cookie

The Set-Cookie HTTP response header is used to send a cookie from the server to the user agent, so the user agent can send it back to the server later. To send multiple cookies, multiple Set-Cookie headers should be sent in the same response.

Recommendation

Set-Cookie: name=value; Secure; HttpOnly; SameSite=Strict

• NOTE: The Domain attribute has been removed intentionally

Strict-Transport-Security

The HTTP Strict-Transport-Security response header (often abbreviated as HSTS) lets a website tell browsers that it should only be accessed using HTTPS, instead of using HTTP.

Recommendation

Enable HTTPS-only access for the site and sub domains.

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

Expect-CT

The Expect-CT header lets sites opt-in to reporting and/or enforcement of Certificate Transparency requirements, to prevent the use of misissued certificates for that site from going unnoticed.

Recommendation

Enforce Certificate Transparency for 24 hours.

Expect-CT: max-age=86400

Content-Security-Policy

Content Security Policy (CSP) is an added layer of security that helps to detect and mitigate certain types of attacks, including Cross-Site Scripting (XSS) and data injection attacks. These attacks are used for everything from data theft to site defacement to distribution of malware.

Recommendation

Restrict most of the resource types to the same site and subdomains of yourdoamin.com

Content-Security-Policy: default-src 'self' *.yourdomain.com; block-all-mixed-content; font-src 'self' https: data:; img-src 'self' data: blob:; object-src 'none'; script-src-attr 'none'; style-src 'self' https: 'unsafe-inline'; upgrade-insecure-requests;

• WARNING: Inline script elements and inline script event handlers like onload will stop working with the above header. This is required to neutralize XSS attacks.

Access-Control-Allow-Origin

The Access-Control-Allow-Origin response header indicates whether the response can be shared with requesting code from the given origin.

Recommendation

Use * or specific domain names.

Access-Control-Allow-Origin: *

Cross-Origin-Opener-Policy

The HTTP Cross-Origin-Opener-Policy (COOP) response header allows you to ensure a top-level document does not share a browsing context group with cross-origin documents.

Recommendation

Isolates the browsing context exclusively to same-origin documents.

HTTP Cross-Origin-Opener-Policy: same-origin

Cross-Origin-Resource-Policy

The Cross-Origin-Resource-Policy (CORP) header allows you to control the set of origins that are empowered to include a resource. It is a robust defense against attacks like Spectre, as it allows browsers to block a given response before it enters an attacker's process.

Recommendation

Limit current resource loading to the site and sub-domains only.

Cross-Origin-Resource-Policy: same-site

Cross-Origin-Embedder-Policy

The HTTP Cross-Origin-Embedder-Policy (COEP) response header prevents a document from loading any cross-origin resources that don't explicitly grant the document permission (using CORP or CORS).

Recommendation

A document can only load resources from the same origin, or resources explicitly marked as loadable from another origin.

Cross-Origin-Embedder-Policy: require-corp

• NOTE: you can bypass it by adding the crossorigin attribute like below:
• <img src="https://thirdparty.com/img.png" crossorigin>

Server

The Server header describes the software used by the origin server that handled the request — that is, the server that generated the response.

Recommendation

Remove this header or set non-informative values.

Server: webserver

X-Powered-By

The X-Powered-By header describes the technologies used by the webserver. This information exposes the server to attackers. Using the information in this header, attackers can find vulnerabilities easier.

Recommendation

Remove all X-Powered-By headers.

X-AspNet-Version

Provides information about the .NET version.

Recommendation

Disable sending this header. Review the ASP.NET Version Disclosure issue for details.

X-AspNetMvc-Version

Provides information about the .NET version.

Recommendation

Disable sending this header. Review the ASP.NET Version Disclosure issue for details.

X-DNS-Prefetch-Control

The X-DNS-Prefetch-Control HTTP response header controls DNS prefetching, a feature by which browsers proactively perform domain name resolution on both links that the user may choose to follow as well as URLs for items referenced by the document, including images, CSS, JavaScript, and so forth.

Recommendation

The default behavior of browsers is to perform DNS caching which is good for most websites.
If you do not control links on your website, you might want to set off as a value to disable DNS prefetch to avoid leaking information to those domains.

Public-Key-Pins ❌

The HTTP Public-Key-Pins response header is used to associate a specific cryptographic public key with a certain web server to decrease the risk of MITM attacks with forged certificates.

Recommendation

This header is deprecated. Use Expect-CT instead.

Origin-Agent-Cluster

X-Permitted-Cross-Domain-Policies

Adobe only, default safe

X-Download-Options

IE Only

Adding Http Headers in Different Technologies

PHP

Below sample code sets the X-XSS-Protection header in PHP.

header("X-XSS-Protection: 1; mode=block");

Apache

Below .htaccess sample configuration sets the X-XSS-Protection header in Apache.

<IfModule mod_headers.c>
Header set X-XSS-Protection "1; mode=block"
</IfModule>

IIS

Add below configurations to your Web.config in ISS to send the X-XSS-Protection header

<system.webServer>
...
 <httpProtocol>
   <customHeaders>
     <add name="X-XSS-Protection" value="1; mode=block" />
   </customHeaders>
 </httpProtocol>
...
</system.webServer>

HAProxy

Add the below line to your font-end, listen, or backend configurations to send the X-XSS-Protection header

http-response set-header X-XSS-Protection 1; mode=block

Nginx

Below sample configuration, sets the X-XSS-Protection header in Nginx.

add_header "X-XSS-Protection" "1; mode=block";

Express

You can use helmet to setup HTTP headers in Express. Below code is sample for adding the X-Frame-Options header.

const helmet = require('helmet');
const app = express();
// Sets "X-Frame-Options: SAMEORIGIN"
app.use(
 helmet.frameguard({
   action: "sameorigin",
 })
);

Testing Proper Implementation of Security Headers

Mozilla Observatory

The Mozilla Observatory is an online tool that you can check your website's header status.

SmartScanner

SmartScanner has a dedicated test profile for testing security of HTTP headers.
Online tools usually test the homepage of the given address. But SmartScanner scans the whole website. So, you can make sure all of your web pages have the right HTTP Headers in place.

References

• Mozilla: X-Frame-Options
• Mozilla: X-XSS-Protection
• hstspreload.org
• Mozilla: Strict-Transport-Security
• Mozilla: Content-Type
• Mozilla: Expect-CT
• Mozilla: Referrer-Policy
• Mozilla: Set-Cookie
• content-security-policy.com
• Mozilla: Access-Control-Allow-Origin
• Mozilla: Cross-Origin-Opener-Policy
• resourcepolicy.fyi
• Mozilla: Cross-Origin-Resource-Policy
• Mozilla: Cross-Origin-Embedder-Policy
• Mozilla: Server Header