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Content Security Policy

 * Introduction
 * Why CSP
 * Strict CSP
 * Adopting CSP
 * FAQ
 * Resources




INTRODUCTION

Content Security Policy is a mechanism designed to make applications more secure
against common web vulnerabilities, particularly cross-site scripting. It is
enabled by setting the Content-Security-Policy HTTP response header.

The core functionality of CSP can be divided into three areas:

 * Requiring that all scripts are safe and trusted by the application owner
   (ideally by making sure they match an unpredictable identifier specified in
   the policy called the CSP nonce),

 * Ensuring that page resources, such as images, stylesheets, or frames, are
   loaded from trusted sources,

 * Miscellaneous other security features: preventing the application from being
   framed by untrusted domains, transparently upgrading all resource requests to
   HTTPS, and others.

By adopting a strict policy, which prevents the loading of untrusted scripts or
plugins, an application can add an important defense-in-depth layer against
markup injection attacks. This documentation focuses on the XSS mitigation
aspect of CSP because XSS is one of the most common and dangerous web
vulnerabilities.

An application can define a policy by setting the following header:

Content-Security-Policy: default-src https:; script-src 'nonce-{random}'; object-src 'none'


This policy will require all resources to be loaded over HTTPS, allow only
<script> elements with the correct nonce attribute, and prevent loading any
plugins.

Note: Real policies are a bit more complicated for compatibility and security
reasons; see this example.

CSP support is available in several popular template systems and frameworks (for
example, Closure Templates can automatically add CSP nonces). Several helper
tools can assist you in building a secure policy, identifying any necessary
markup changes, and monitoring the effects of the policy after deployment.


WHAT SHOULD I DO NOW?

Adopting a safe CSP policy can be an important security improvement for many
applications. To see if it's right for your app, and learn what you need to do
to enable CSP, take a look at the following pages:

 * Why CSP is a short explainer document discussing the benefits of using CSP,
   and examining when a policy is most useful for an application.

 * Strict CSP describes the proposed approach for using CSP to defend against
   XSS. Adopting CSP shows the recommended way to make an application compatible
   with CSP, including a production-ready policy, example code, and overview of
   tools which help you deploy a secure policy.

 * The FAQ discusses common adoption and security issues, including a comparison
   of the strict CSP approach to traditional policies.

 * The Resources page contains links to useful tools, code examples, and
   additional documentation.




SITE CONTENTS

 * Introduction
 * Why CSP
 * Strict CSP
 * Adopting CSP
 * FAQ
 * Resources