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Everything You Need to Know About Content Security Policy (CSP)

by Mozilla ContributorsAugust 3rd, 2021

Too Long; Didn't Read

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. CSP is designed to be fully backward compatible (except CSP version 2 where there are some inconsistencies in backward compatibility; more details here section 1.1). Browsers that don't support CSP still work with servers that implement it, and vice-versa: browsers simply ignore it. If the site doesn't offer the CSP header, browsers likewise use the standard same-origin policy.

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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 the distribution of malware.

CSP is designed to be fully backward compatible (except CSP version 2 where there are some explicitly-mentioned inconsistencies in backward compatibility; more details here section 1.1). Browsers that don't support it still work with servers that implement it, and vice-versa: browsers that don't support CSP simply ignore it, functioning as usual, defaulting to the standard same-origin policy for web content. If the site doesn't offer the CSP header, browsers likewise use the standard same-origin policy.

To enable CSP, you need to configure your web server to return the

Content-Security-Policy

HTTP header. (Sometimes you may see mentions of the

X-Content-Security-Policy

header, but that's an older version and you don't need to specify it anymore.)

Alternatively, the

<meta>

element can be used to configure a policy, for example:

<meta http-equiv="Content-Security-Policy" content="default-src 'self'; img-src https://*; child-src 'none';">

Threats

Mitigating cross site scripting

A primary goal of CSP is to mitigate and report XSS attacks. XSS attacks exploit the browser's trust of the content received from the server. Malicious scripts are executed by the victim's browser because the browser trusts the source of the content, even when it's not coming from where it seems to be coming from.

CSP makes it possible for server administrators to reduce or eliminate the vectors by which XSS can occur by specifying the domains that the browser should consider to be valid sources of executable scripts. A CSP compatible browser will then only execute scripts loaded in source files received from those allowlisted domains, ignoring all other script (including inline scripts and event-handling HTML attributes).

As an ultimate form of protection, sites that want to never allow scripts to be executed can opt to globally disallow script execution.

Mitigating packet sniffing attacks

In addition to restricting the domains from which content can be loaded, the server can specify which protocols are allowed to be used; for example (and ideally, from a security standpoint), a server can specify that all content must be loaded using HTTPS. A complete data transmission security strategy includes not only enforcing HTTPS for data transfer, but also marking all cookies with the secure attribute and providing automatic redirects from HTTP pages to their HTTPS counterparts. Sites may also use the

Strict-Transport-Security

HTTP header to ensure that browsers connect to them only over an encrypted channel.

Using CSP

Configuring Content Security Policy involves adding the

Content-Security-Policy

HTTP header to a web page and giving it values to control what resources the user agent is allowed to load for that page. For example, a page that uploads and displays images could allow images from anywhere, but restrict a form action to a specific endpoint. A properly designed Content Security Policy helps protect a page against a cross site scripting attack. This article explains how to construct such headers properly, and provides examples.

Specifying your policy

You can use the

Content-Security-Policy

HTTP header to specify your policy, like this:

Content-Security-Policy: policy

The policy is a string containing the policy directives describing your Content Security Policy.

Writing a policy

A policy is described using a series of policy directives, each of which describes the policy for a certain resource type or policy area. Your policy should include a

default-src

policy directive, which is a fallback for other resource types when they don't have policies of their own (for a complete list, see the description of the

default-src

directive). A policy needs to include a

default-src

script-src

directive to prevent inline scripts from running, as well as blocking the use of eval(). A policy needs to include a

default-src

style-src

directive to restrict inline styles from being applied from a

<style>

element or a style attribute. There are specific directives for a wide variety of types of items, so that each type can have its own policy, including fonts, frames, images, audio and video media, scripts, and workers.

Examples: Common use cases

This section provides examples of some common security policy scenarios.

Example 1

A web site administrator wants all content to come from the site's own origin (this excludes subdomains.)

Content-Security-Policy: default-src 'self'

Example 2

A web site administrator wants to allow content from a trusted domain and all its subdomains (it doesn't have to be the same domain that the CSP is set on.)

Content-Security-Policy: default-src 'self' *.trusted.com

Example 3

A web site administrator wants to allow users of a web application to include images from any origin in their own content, but to restrict audio or video media to trusted providers, and all scripts only to a specific server that hosts trusted code.

Content-Security-Policy: default-src 'self'; img-src *; media-src media1.com media2.com; script-src userscripts.example.com

Here, by default, content is only permitted from the document's origin, with the following exceptions:

Images may load from anywhere (note the "*" wildcard).
Media is only allowed from media1.com and media2.com (and not from subdomains of those sites).
Executable script is only allowed from userscripts.example.com.

Example 4

A web site administrator for an online banking site wants to ensure that all its content is loaded using TLS, in order to prevent attackers from eavesdropping on requests.

Content-Security-Policy: default-src https://onlinebanking.jumbobank.com

The server permits access only to documents being loaded specifically over HTTPS through the single origin onlinebanking.jumbobank.com.

Example 5

A web site administrator of a web mail site wants to allow HTML in email, as well as images loaded from anywhere, but not JavaScript or other potentially dangerous content.

Content-Security-Policy: default-src 'self' *.mailsite.com; img-src *

Note that this example doesn't specify a

script-src

; with the example CSP, this site uses the setting specified by the

default-src

directive, which means that scripts can be loaded only from the originating server.

Testing your policy

To ease deployment, CSP can be deployed in report-only mode. The policy is not enforced, but any violations are reported to a provided URI. Additionally, a report-only header can be used to test a future revision to a policy without actually deploying it.

You can use the

Content-Security-Policy-Report-Only

HTTP header to specify your policy, like this:

Content-Security-Policy-Report-Only: policy

If both a

Content-Security-Policy-Report-Only

header and a

Content-Security-Policy

header are present in the same response, both policies are honored. The policy specified in

Content-Security-Policy

headers is enforced while the

Content-Security-Policy-Report-Only

policy generates reports but is not enforced.

Enabling reporting

By default, violation reports aren't sent. To enable violation reporting, you need to specify the

report-uri

policy directive, providing at least one URI to which to deliver the reports:

Content-Security-Policy: default-src 'self'; report-uri http://reportcollector.example.com/collector.cgi

Then you need to set up your server to receive the reports; it can store or process them in whatever manner you determine is appropriate.

Violation report syntax

The report JSON object contains the following data:

blocked-uri

The URI of the resource that was blocked from loading by the Content Security Policy. If the blocked URI is from a different origin than the

document-uri

, then the blocked URI is truncated to contain just the scheme, host, and port.

disposition

Either

"enforce"

"report"

depending on whether the

Content-Security-Policy-Report-Only

header or the

Content-Security-Policy

header is used.

document-uri

The URI of the document in which the violation occurred.

effective-directive

The directive whose enforcement caused the violation.

original-policy

The original policy as specified by the

Content-Security-Policy

HTTP header.

referrer

The referrer of the document in which the violation occurred.

script-sample

The first 40 characters of the inline script, event handler, or style that caused the violation.

status-code

The HTTP status code of the resource on which the global object was instantiated.

violated-directive

The name of the policy section that was violated.

Sample violation report

Let's consider a page located at

http://example.com/signup.html

. It uses the following policy, disallowing everything but stylesheets from

cdn.example.com

Content-Security-Policy: default-src 'none'; style-src cdn.example.com; report-uri /_/csp-reports

The HTML of

signup.html

looks like this:

<!DOCTYPE html>
<html>
  <head>
    <title>Sign Up</title>
    <link rel="stylesheet" href="css/style.css">
  </head>
  <body>
    ... Content ...
  </body>
</html>

Can you spot the mistake? Stylesheets are allowed to be loaded only from

cdn.example.com

, yet the website tries to load one from its own origin (

http://example.com

). A browser capable of enforcing CSP would send the following violation report as a POST request to

http://example.com/_/csp-reports

, when the document is visited:

{
  "csp-report": {
    "document-uri": "http://example.com/signup.html",
    "referrer": "",
    "blocked-uri": "http://example.com/css/style.css",
    "violated-directive": "style-src cdn.example.com",
    "original-policy": "default-src 'none'; style-src cdn.example.com; report-uri /_/csp-reports"
  }
}

As you can see, the report includes the full path to the violating resource in blocked-uri. This is not always the case. For example, if the signup.html attempted to load CSS from

http://anothercdn.example.com/stylesheet.css

, the browser would not include the full path, but only the origin (

http://anothercdn.example.com

). The CSP specification gives an explanation of this odd behaviour. In summary, this is done to prevent leaking sensitive information about cross-origin resources.

Browser compatibility

A specific incompatibility exists in some versions of the Safari web browser, whereby if a Content Security Policy header is set, but not a Same Origin header, the browser will block self-hosted content and off-site content, and incorrectly report that this is due to a the Content Security Policy not allowing the content.

Source: https://developer.mozilla.org/en-US/docs/Web/HTTP/CS
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