Content Security Policy (CSP)
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 malware distribution.
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 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:
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<meta
http-equiv="Content-Security-Policy"
content="default-src 'self'; img-src https://*; child-src 'none';" />Note: Some features, such as sending CSP violation reports, are only available when using the HTTP headers.
A primary goal of CSP is to mitigate and report XSS attacks. XSS attacks exploit the browser's trust in 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 allowed domains, ignoring all other scripts (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.
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.
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.
You can use the Content-Security-Policy HTTP header to specify your policy, like this:
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The policy is a string containing the policy directives describing your Content Security 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 or 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 or 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.
For a complete list of policy directives, see the reference page for the Content-Security-Policy header.
This section provides examples of some common security policy scenarios.
A website administrator wants all content to come from the site's own origin (this excludes subdomains.)
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A website 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.)
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A website 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.
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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 example.org and example.net (and not from subdomains of those sites).
Executable script is only allowed from userscripts.example.com.
A website 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.
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The server permits access only to documents being loaded specifically over HTTPS through the single origin onlinebanking.example.com.
A website 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.
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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.
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:
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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.
By default, violation reports aren't sent. To enable violation reporting, you need to specify the report-to policy directive, providing at least one URI to which to deliver the reports:
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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.
The report JSON object is sent with an application/csp-report Content-Type and contains the following data:
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.
Either "enforce" or "report" depending on whether the Content-Security-Policy-Report-Only header or the Content-Security-Policy header is used.
The URI of the document in which the violation occurred.
The directive whose enforcement caused the violation. Some browsers may provide different values, such as Chrome providing style-src-elem/style-src-attr, even when the actually enforced directive was style-src.
The original policy as specified by the Content-Security-Policy HTTP header.
referrer Deprecated Non-standard
The referrer of the document in which the violation occurred.
The first 40 characters of the inline script, event handler, or style that caused the violation. Only applicable to script-src* and style-src* violations, when they contain the 'report-sample'
The HTTP status code of the resource on which the global object was instantiated.
violated-directive Deprecated
The directive whose enforcement caused the violation. The violated-directive is a historic name for the effective-directive field and contains the same value.
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.
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The HTML of signup.html looks like this:
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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:
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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 behavior. In summary, this is done to prevent leaking sensitive information about cross-origin resources.
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