Content Security Policy 1.1

This document defines a policy language used to declare a set of content restrictions for a web resource, and a mechanism for transmitting the policy from a server to a client where the policy is enforced.

Framework

This section defines the general framework for content security policies, including the delivery mechanisms and general syntax for policies. The next section contains the details of the specific directives introduced in this specification.

Policy Delivery

The server delivers the policy to the user agent via an HTTP response header.

`Content-Security-Policy` Header Field

The Content-Security-Policy header field is the preferred mechanism for delivering a policy.

"Content-Security-Policy:" 1#policy

A server MAY send more than one HTTP header field named Content-Security-Policy with a given resource representation.

A server MAY send different Content-Security-Policy header field values with different representations of the same resource or with different resources.

Upon receiving an HTTP response containing at least one Content-Security-Policy header field, the user agent MUST enforce each of the policies contained in each such header field.

`Content-Security-Policy-Report-Only` Header Field

The Content-Security-Policy-Report-Only header field lets servers experiment with policies by monitoring (rather than enforcing) a policy.

"Content-Security-Policy-Report-Only:" 1#policy

For example, a server operators might wish to develop their security policy iteratively. The operators can deploy a report-only policy based on their best estimate of how their site behaves. If their site violates this policy, instead of breaking the site, the user agent will send violation reports to a URI specified in the policy. Once a site has confidence that the policy is appropriate, they start enforcing the policy using the Content-Security-Policy header field.

A server MAY send more than one HTTP header field named Content-Security-Policy-Report-Only with a given resource representation.

A server MAY send different Content-Security-Policy-Report-Only header field values with different representations of the same resource or with different resources.

Upon receiving an HTTP response containing at least one Content-Security-Policy-Report-Only header field, the user agent MUST monitor each of the policies contained in each such header field.

HTML `meta` Element

The server MAY supply a policy in an HTML meta element with an http-equiv attribute that is a case insensitive match for either Content-Security-Policy.

Add the following entry to the pragma directives for the meta element:

Content security policy (http-equiv="content-security-policy")

If the user agent is already enforcing a policy for the document, abort these steps.
If the Document's head element is not an ancestor of the meta element, abort these steps.
If the meta element lacks a content attribute, abort these steps.
Let policy be the value of the content attribute of the meta element.
Let directive-set be the result of parsing policy.
Remove all occurances of report-uri and sandbox directives from directive-set.
Enforce each of the directives in directive-set, as defined for each directive type.

As a consequence of these requirements, a policy supplied in an HTTP header field takes precedence over policies supplied in meta elements. Similarly, the above requirements entail that the first meta element containing a policy takes precedence over policies supplied in subsequent meta elements. Authors SHOULD place the meta element as early in the document as possible to reduce the risk of content injection before a protective policy can be read and enforced.

Note that the Content-Security-Policy-Report-Only header is not supported inside a meta element.

Enforcing multiple policies.

The above sections note that when multiple policies are present, each must be enforced or reported, according to its type. An example will help clarify how that ought to work in practice. The behavior of an XMLHttpRequest might seem unclear given a site that, for whatever reason, delivered the following HTTP headers:

Content-Security-Policy: default-src 'self' http://example.com http://example.net;
                         connect-src 'none';
Content-Security-Policy: connect-src http://example.com/;
                         script-src http://example.com/

Is a connection to example.com allowed or not? The short answer is that the connection is not allowed. Enforcing both policies means that a potential connection would have to pass through both unscathed. Even though the second policy would allow this connection, the first policy contains connect-src 'none', so its enforcement blocks the connection. The impact is that adding additional policies to the list of policies to enforce can only further restrict the capabilities of the protected resource.

To demonstrate that further, consider a script tag on this page. The first policy would lock scripts down to 'self', http://example.com and http://example.net via the default-src directive. The second, however, would only allow script from http://example.com/. Script will only load if it meets both policy's criteria: in this case, the only origin that can match is http://example.com, as both policies allow it.

Syntax and Algorithms

Policies

A CSP policy consists of a U+003B SEMICOLON (;) delimited list of directives:

policy            = [ directive *( ";" [ directive ] ) ]

Each directive consists of a directive-name and (optionally) a directive-value:

directive         = *WSP [ directive-name [ WSP directive-value ] ]
directive-name    = 1*( ALPHA / DIGIT / "-" )
directive-value   = *( WSP / <VCHAR except ";" and ","> )

Parsing

To parse a policy policy, the user agent MUST use an algorithm equivalent to the following:

Let the set of directives be the empty set.
For each non-empty token returned by strictly splitting the string policy on the character U+003B SEMICOLON (;):
1. Skip whitespace.
2. Collect a sequence of characters that are not space characters. The collected characters are the directive name.
3. If there are characters remaining in token, skip ahead exactly one character (which must be a space character).
4. The remaining characters in token (if any) are the directive value.
5. If the set of directives already contains a directive whose name is a case insensitive match for directive name, ignore this instance of the directive and continue to the next token.
6. Add a directive to the set of directives with name directive name and value directive value.
Return the set of directives.

Source List

Many CSP directives use a value consisting of a source list.

Each source expression in the source list represents a location from which content of the specified type can be retrieved. For example, the source expression 'self' represents the set of URIs which are in the same origin as the protected resource and the source expression 'unsafe-inline' represents content supplied inline in the resource itself.

source-list       = *WSP [ source-expression *( 1*WSP source-expression ) *WSP ]
                  / *WSP "'none'" *WSP
source-expression = scheme-source / host-source / keyword-source / nonce-source / hash-source
scheme-source     = scheme ":"
host-source       = [ scheme "://" ] host [ port ] [ path ]
keyword-source    = "'self'" / "'unsafe-inline'" / "'unsafe-eval'"
base64-value      = 1*( ALPHA / DIGIT / "+" / "/" )*2( "=" )
nonce-value       = base64-value
hash-value        = base64-value
nonce-source      = "'nonce-" nonce-value "'"
hash-algo         = "sha256" / "sha384" / "sha512"
hash-source       = "'" hash-algo "-" base64-value "'"
scheme            = <scheme production from RFC 3986, section 3.1>
host              = "*" / [ "*." ] 1*host-char *( "." 1*host-char )
host-char         = ALPHA / DIGIT / "-"
path              = <path production from RFC 3986, section 3.3>
port              = ":" ( 1*DIGIT / "*" )

If the policy contains a nonce-source expression, the server MUST generate a fresh value for the nonce-value directive at random and independently each time it transmits a policy. This requirement ensures that the nonce-value is difficult for an attacker to predict.

Parsing

To parse a source list source list, the user agent MUST use an algorithm equivalent to the following:

If source list (with leading and trailing whitespace stripped) is a case insensitive match for the string 'none' (including the quotation marks), return the empty set.
Let the set of source expressions be the empty set.
For each token returned by splitting source list on spaces, if the token matches the grammar for source-expression, add the token to the set of source expressions.
Return the set of source expressions.

Note that characters like U+003B SEMICOLON (;) and U+002C COMMA (,) cannot appear in source expressions directly: if you'd like to include these characters in a source expression, they must be percent encoded as %3B and %2C respectively.

Matching

To check whether a URI matches a source expression, the user agent MUST use an algorithm equivalent to the following:

Normalize the URI according to RFC 3986, section 6.
If the source expression a consists of a single U+002A ASTERISK character (*), then return does match.
If the source expression matches the grammar for scheme-source:
1. If the URI's scheme is a case-insensitive match for the source expression's scheme, return does match.
2. Otherwise, return does not match.
If the source expression matches the grammar for host-source:
1. If the URI does not contain a host, then return does not match.
2. Let uri-scheme, uri-host, and uri-port be the scheme, host, and port of the URI, respectively. If the URI does not have a port, then let uri-port be the default port for uri-scheme. Let uri-path be the path of the URI after decoding percent-encoded characters. If the URI does not have a path, then let uri-path be the U+002F SOLIDUS character (/).
3. If the source expression has a scheme that is not a case insensitive match for uri-scheme, then return does not match.
4. If the source expression does not have a scheme, return does not match if
  1. the scheme of the protected resource's URI is a case insensitive match for HTTP, and uri-scheme is not a case insensitive match for either HTTP or HTTPS
  2. the scheme of the protected resource's URI is not a case insensitive match for HTTP, and uri-scheme is not a case insensitive match for the scheme of the protected resource's URI.
5. If the first character of the source expression's host is an U+002A ASTERISK character (*) and the remaining characters, including the leading U+002E FULL STOP character (.), are not a case insensitive match for the rightmost characters of uri-host, then return does not match.
6. If the first character of the source expression's host is not an U+002A ASTERISK character (*) and uri-host is not a case insensitive match for the source expression's host, then return does not match.
7. If the source expression does not contain a port and uri-port is not the default port for uri-scheme, then return does not match.
8. If the source expression does contain a port, then return does not match if
  1. port does not contain an U+002A ASTERISK character (*), and
  2. port does not represent the same number as uri-port.
9. If the source expression contains a non-empty path, then:
  1. Let decoded-path be the result of decoding path's percent-encoded characters.
  2. If the final character of decoded-path is the U+002F SOLIDUS character (/), and decoded-path is not a prefix of uri-path, then return does not match.
  3. If the final character of decoded-path is not the the U+002F SOLIDUS character (/), and decoded-path is not an exact match for uri-path then return does not match.
10. Otherwise, return does match.
If the source expression is a case insensitive match for 'self' (including the quotation marks), then:
1. Return does match if the URI has the same scheme, host, and port as the protected resource's URI (using the default port for the appropriate scheme if either or both URIs are missing ports).
Otherwise, return does not match.

A URI matches a source list, if, and only if, the URI matches at least one source expression in the set of source expressions obtained by parsing the source list. Notice that no URIs match an empty set of source expressions, such as the set obtained by parsing the source list 'none'.

Path Matching

The rules for matching source expressions that contain paths are simpler than they look: paths that end with the '/' character match all files in a directory and its subdirectories. Paths that do not end with the '/' character match only one specific file. A few examples should make this clear:

The source expression example.com has no path, and therefore matches any file served from that host.
The source expression example.com/scripts/ matches any file in the scripts directory of example.com, and any of its subdirectories. For example, both https://example.com/scripts/file.js and https://example.com/scripts/js/file.js would match.
The source expression example.com/scripts/file.js matches only the file named file.js in the scripts directory of example.com.
Likewise, the source expression example.com/js matches only the file named js. In particular, note that it would not match files inside a directory named js. Files like example.com/js/file.js would be matched only if the source expression ended with a trailing "/", as in example.com/js/.

Note that query strings have no impact on matching: the source expression example.com/file?key=value matches all of https://example.com/file, https://example.com/file?key=value, https://example.com/file?key=notvalue, and https://example.com/file?notkey=notvalue.

Valid Nonces

An element has a valid nonce for a set of source expressions if the value of the nonce attribute of the element after stripping leading and trailing whitespace is a case-sensitive match for the nonce-value component of at least one nonce-source expression in the set of source expressions.

Valid Hashes

An element's contents are the script block's source for script elements, or the value of the element's textContent IDL attribute for non-script elements such as style.

The algorithm digest of an element's contents is the result of applying algorithm to the element's contents.

To determine whether element has a valid hash for a set of source expressions, execute the following steps:

Let hashes be a list of all hash-source expressions in the set of source expressions.
For each hash in hashes:
1. Let algorithm be:
  - SHA-256 if the hash-algo component of hash is a case-insensitive match for the string "sha256"
  - SHA-384 if the hash-algo component of hash is a case-insensitive match for the string "sha384"
  - SHA-512 if the hash-algo component of hash is a case-insensitive match for the string "sha512"
  (SHA-256, SHA-384, and SHA-512 are the digest algorithms defined by the NIST)
2. Let expected be the hash-value component of hash.
3. Let actual be the base64 encoding of the binary algorithm digest of element's contents.
4. If actual is a case-sensitive match for expected, return true and abort these steps.
Return false.

Note that if an element has an invalid hash, it would be helpful if the user agent reported the failure to the author by adding a warning message containing the actual hash value.

Media Type List

The experimental plugin-types directive uses a value consisting of a media type list.

Each media type in the media type list represents a specific type of resource that can be retrieved and used to instantiate a plugin in the protected resource.

media-type-list   = media-type *( 1*WSP media-type )
media-type        = <type from RFC 2045> "/" <subtype from RFC 2045>

Parsing

To parse a media type list media type list, the user agent MUST use an algorithm equivalent to the following:

Let the set of media types be the empty set.
For each token returned by splitting media type list on spaces, if the token matches the grammar for media-type, add the token to the set of media types. Otherwise ignore the token.
Return the set of media types.

Matching

A media type matches a media type list if, and only if, the media type is a case-insensitive match for at least one token in the set of media types obtained by parsing the media type list.

Reporting

To strip uri for reporting, the user agent MUST use an algorithm equivalent to the following:

If the origin of uri is a globally unique identifier (for example, uri has a scheme of data, blob, or file), then abort these steps, and return the ASCII serialization of uri's scheme.
If the origin of uri is not the same as the origin of the protected resource, then abort these steps, and return the ASCII serialization of uri's origin.
Return uri, with any <fragment> component removed.

To generate a violation report object, the user agent MUST use an algorithm equivalent to the following:

Prepare a JSON object violation with the following keys and values:

blocked-uri

The URI of the resource that was prevented from loading, stripped for reporting, or the empty string if the resource has no URI (inline script and inline style, for example).

document-uri

The address of the protected resource, stripped for reporting.

effective-directive

The name of the policy directive that was violated. This will contain the directive whose enforcement triggered the violation, even if that directive does not explicitly appear in the policy, but is implicitly activated via the default-src directive.

original-policy

The original policy, as received by the user agent.

referrer

The referrer attribute of the protected resource, or the empty string if the protected resource has no referrer.

status-code

The status-code of the HTTP response that contained the protected resource, if the protected resource was obtained over HTTP. Otherwise, the number 0.

violated-directive

The policy directive that was violated, as it appears in the policy. This will contain the default-src directive in the case of violations caused by falling back to the default sources when enforcing a directive.
If a specific line or a specific file can be identified as the cause of the violation (for example, script execution that violates the script-src directive), the user agent MAY add the following keys and values to violation:

source-file

The URI of the resource where the violation occurred, stripped for reporting.

line-number

The line number in source-file on which the violation occurred.

column-number

The column number in source-file on which the violation occurred.
Return violation.

To send violation reports, the user agent MUST use an algorithm equivalent to the following:

Prepare a JSON object report object with a single key, csp-report, whose value is the result of generating a violation report object.
Let report body be the JSON stringification of report object.
For each report URI in the set of report URIs:
1. If the user agent has already sent a violation report for the protected resource to report URI, and that report contained an entity body that exactly matches report body, the user agent MAY abort these steps and continue to the next report URI.
2. Queue a task to fetch report URI from the origin of the protected resource, with the synchronous flag not set, using HTTP method POST, with a Content-Type header field of application/csp-report, and an entity body consisting of report body. If the origin of report URI is not the same as the origin of the protected resource, the block cookies flag MUST also be set. The user agent MUST NOT follow redirects when fetching this resource. (Note: The user agent ignores the fetched resource.) The task source for this task is the Content Security Policy task source.

To report a violation, the user agent MUST:

Fire a violation event at the protected resource's Document, and
If the set of report URIs is non-empty, send violation reports to each.

Processing Model

To enforce a policy, the user agent MUST parse the policy and enforce each of the directives contained in the policy, where the specific requirements for enforcing each directive are defined separately for each directive (See Directives, below).

Generally speaking, enforcing a directive prevents the protected resource from performing certain actions, such as loading scripts from URIs other than those indicated in a source list. These restrictions make it more difficult for an attacker to abuse an injection vulnerability in the resource because the attacker will be unable to usurp the resource's privileges that have been restricted in this way.

Enforcing a policy SHOULD NOT interfere with the operation of user-supplied scripts such as third-party user-agent add-ons and JavaScript bookmarklets.

To monitor a policy, the user agent MUST parse the policy and monitor each of the directives contained in the policy.

Monitoring a directive does not prevent the protected resource from undertaking any actions. Instead, any actions that would have been prevented by the directives are instead reported to the developer of the web application. Monitoring a policy is useful for testing whether enforcing the policy will cause the web application to malfunction.

A server MAY cause user agents to monitor one policy while enforcing another policy by returning both Content-Security-Policy and Content-Security-Policy-Report-Only header fields. For example, if a server operator is using one policy but wishes to experiment with a stricter policy, the server operator can monitor the stricter policy while enforcing the original policy. Once the server operator is satisfied that the stricter policy does not break the web application, the server operator can start enforcing the stricter policy.

If the user agent monitors or enforces a policy that does not contain any directives, the user agent SHOULD report a warning message in the developer console.

If the user agent monitors or enforces a policy that contains an unrecognized directive, the user agent SHOULD report a warning message in the developer console indicating the name of the unrecognized directive.

Whenever a user agent runs a worker: [[!WEBWORKERS]]

If the worker's script's origin is a globally unique identifier (for example, the worker's script's URL has a scheme of data, blob, or file), then:
- If the user agent is enforcing a CSP policy for the owner document, the user agent MUST enforce the CSP policy for the worker.
- If the user agent is monitoring a CSP policy for the owner document, the user agent MUST monitor the CSP policy for the worker.
Otherwise:
- If the worker's script is delivered with a Content-Security-Policy HTTP header containing the value policy, the user agent MUST enforce policy for the worker.
- If the worker's script is delivered with a Content-Security-Policy-Report-Only HTTP header containing the value policy, the user agent MUST monitor policy for the worker.

Whenever a user agent creates a an iframe srcdoc document in a browsing context nested in the protected resource, if the user agent is enforcing any CSP policies for the protected resource, the user agent MUST enforce those CSP policies on the iframe srcdoc document as well.

Whenever a user agent creates a an iframe srcdoc document in a browsing context nested in the protected resource, if the user agent is monitoring any CSP policies for the protected resource, the user agent MUST monitor those CSP policies on the iframe srcdoc document as well.

Script Interfaces

`SecurityPolicyViolationEvent` Events

readonly attribute DOMString documentURI: Refer to the document-uri property of violation reports for a description of this property.
readonly attribute DOMString referrer: Refer to the referrer property of violation reports for a description of this property.
readonly attribute DOMString blockedURI: Refer to the blocked-uri property of violation reports for a description of this property.
readonly attribute DOMString violatedDirective: Refer to the violated-directive property of violation reports for a description of this property.
readonly attribute DOMString effectiveDirective: Refer to the effective-directive property of violation reports for a description of this property.
readonly attribute DOMString originalPolicy: Refer to the original-policy property of violation reports for a description of this property.
readonly attribute DOMString sourceFile: Refer to the source-file property of violation reports for a description of this property.
readonly attribute long lineNumber: Refer to the line-number property of violation reports for a description of this property.
readonly attribute long columnNumber: Refer to the column-number property of violation reports for a description of this property.

DOMString documentURI: Refer to the document-uri property of violation reports for a description of this property.
DOMString referrer: Refer to the referrer property of violation reports for a description of this property.
DOMString blockedURI: Refer to the blocked-uri property of violation reports for a description of this property.
DOMString violatedDirective: Refer to the violated-directive property of violation reports for a description of this property.
DOMString effectiveDirective: Refer to the effective-directive property of violation reports for a description of this property.
DOMString originalPolicy: Refer to the original-policy property of violation reports for a description of this property.
DOMString sourceFile: Refer to the source-file property of violation reports for a description of this property.
long lineNumber: Refer to the line-number property of violation reports for a description of this property.
long columnNumber: Refer to the line-number property of violation reports for a description of this property.

Firing events using the `SecurityPolicyViolationEvent` interface

To fire a violation event, the user agent MUST use an algorithm equivalent to the following:

Let report object be the result of generating a violation report object.
Queue a task to fire an event named securitypolicyviolation using the SecurityPolicyViolationEvent interface with the following initializations:
- blockedURI MUST be initialized to the value of report object's blocked-uri key.
- documentURI MUST be initialized to the value of report object's document-uri key.
- effectiveDirective MUST be initialized to the value of report object's effective-directive key.
- originalPolicy MUST be initialized to the value of report object's original-policy key.
- referrer MUST be initialized to the value of report object's referrer key.
- violatedDirective MUST be initialized to the value of report object's violated-directive key.
- sourceFile MUST be initialized to the value of report object's source-file key.
- lineNumber MUST be initialized to the value of report object's line-number key.
- columnNumber MUST be initialized to the value of report object's column-number key.

The task source for this task is the Content Security Policy task source.

Directives

This section describes the content security policy directives introduced in this specification. Note that directive names are case insensitive.

In order to protect against Cross-Site Scripting (XSS), web application authors SHOULD include

both the script-src and object-src directives, or
include a default-src directive, which covers both scripts and plugins.

In either case, authors SHOULD NOT include either 'unsafe-inline' or data: as valid sources in their policies. Both enable XSS attacks by allowing code to be included directly in the document itself; they are best avoided completely.

`base-uri`

The base-uri directive restricts the URIs that can be used to specify a document's base URL. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "base-uri"
directive-value   = source-list

The term allowed base URIs refers to the result of parsing the base-uri directive's value as a source list.

Step 4 of the algorithm defined in HTML5 to obtain a document's base URL MUST be changed to:

If the previous step was not successful, or the result of the previous step does not match the allowed base URIs, then the document base URL is fallback base URL. Otherwise, it is the result of the previous step.

`child-src`

The child-src governs the creation of nested and auxiliary browsing contexts, as well as Worker execution contexts. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "child-src"
directive-value   = source-list

The term allowed child sources refers to the result of parsing the child-src directive's value as a source list if a child-src directive is explicitly specified, and otherwise to the default sources.

Nested Browsing Contexts

To enforce the child-src directive the user agent MUST enforce the following directives:

frame-src

Auxiliary Browsing Contexts

Whenever the user agent fetches a URL (including when following redirects) in the course of one of the following activities, if the URL does not match the allowed popup sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting data for display in an auxiliary browsing context whose opener browsing context is the protected resource (for example, a popup window created by a call to window.open).
Navigating such an auxiliary browsing context.

Workers

Whenever the user agent fetches a URL (including when following redirects) while processing the Worker or SharedWorker constructors [[!WEBWORKERS]], the user agent MUST act as if it had received an empty HTTP 400 response and report a violation if the URI does not match the allowed worker sources.

`connect-src`

The connect-src directive restricts which URIs the protected resource can load using script interfaces. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "connect-src"
directive-value   = source-list

The term allowed connection targets refers to the result of parsing the connect-src directive's value as a source list if the policy contains an explicit connect-src, or otherwise to the default sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed connection targets, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Processing the send() method of an XMLHttpRequest object.
Processing the WebSocket constructor.
Processing the EventSource constructor.

Usage

JavaScript offers a few mechanisms that directly connect to an external server to send or receive information. EventSource maintains an open HTTP connection to a server in order to receive push notifications, WebSockets open a bidirectional communication channel between your browser and a server, and XMLHttpRequest makes arbitrary HTTP requests on your behalf. These are powerful APIs that enable useful functionality, but also provide tempting avenues for data exfiltration.

The connect-src directive allows you to ensure that these sorts of connections are only opened to origins you trust. Sending a policy that defines a list of source expressions for this directive is straightforward. For example, to limit connections to only example.com, send the following header:

Content-Security-Policy: connect-src example.com

All of the following will fail with the preceeding directive in place:

new WebSocket("wss://evil.com/");
(new XMLHttpRequest()).open("GET", "https://evil.com/", true);
new EventSource("https://evil.com");

`default-src`

The default-src directive sets a default source list for a number of directives. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "default-src"
directive-value   = source-list

Let the default sources be the result of parsing the default-src directive's value as a source list if a default-src directive is explicitly specified, and otherwise the U+002A ASTERISK character (*).

To enforce the default-src directive, the user agent MUST enforce the following directives:

child-src
connect-src
font-src
img-src
media-src
object-src
script-src
style-src

If not specified explicitly in the policy, the directives listed above will use the default sources.

Usage

default-src, as the name implies, serves as a default source list which the other source list-style directives will use as a fallback if they're not otherwise explicitly set. That is, consider the following policy declaration:

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

Under this policy, fonts, frames, images, media, objects, scripts, and styles will all only load from the same origin as the protected resource, and connections will only be made to the same origin. Adding a more specific declaration to the policy would completely override the default source list for that resource type.

Content-Security-Policy: default-src 'self'; script-src example.com

Under this new policy, fonts, frames, and etc. continue to be load from the same origin, but scripts will only load from example.com. There's no inheritance; the script-src directive sets the allowed sources of script, and the default list is not used for that resource type.

Given this behavior, one good way of building a policy for a site would be to begin with a default-src of 'none', and to build up a policy from there that contains only those resource types which are actually in use for the page you'd like to protect. If you don't use webfonts, for instance, there's no reason to specify a source list for font-src; specifying only those resource types a page uses ensures that the possible attack surface for that page remains as small as possible.

`font-src`

The font-src directive restricts from where the protected resource can load fonts. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "font-src"
directive-value   = source-list

The term allowed font sources refers to the result of parsing the font-src directive's value as a source list if the policy contains an explicit font-src, or otherwise to the default sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed font sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting data for display in a font, such as when processing the @font-face Cascading Style Sheets (CSS) rule.

`form-action`

The form-action restricts which URIs can be used as the action of HTML form elements. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "form-action"
directive-value   = source-list

The term allowed form actions refers to the result of parsing the form-action directive's value as a source list.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed form actions, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Processing an HTML form element.

Note that form-action does not fall back to the default source list when the directive is not defined. That is, a policy that defines default-src 'none' but not form-action will still allow form submissions to any target.

`frame-ancestors`

The frame-ancestors directive indicates whether the user agent should allow embedding the resource using a frame, iframe, object, embed or applet tag, or equivalent functionality in non-HTML resources. Resources can use this directive to avoid many UI Redressing [[UIREDRESS]] attacks by avoiding being embedded into potentially hostile contexts.

The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name  = "frame-ancestors"
directive-value = source-list

The term allowed frame ancestors refers to the result of parsing the frame-ancestors directive's value as a source list. Note that this directive is not influenced by the default sources. If a frame-ancestors directive is not explicitly included in the policy, the allowed frame ancestors is "*".

To enforce the frame-ancestors directive, whenever the user agent would load the protected resource into a nested browsing context, the user agent MUST perform the following steps:

Let nestedContext be the nested browsing context into which the protected resource is being loaded.
Let ancestorList be the list of all ancestor browsing contexts of nestedContext.
For each ancestorContext in ancestorList:
1. Let document be ancestorContext's active document.
2. If document's URL does not match the allowed frame ancestors, the user agent MUST:
  1. Abort loading the protected resource.
  2. Act as if it received an empty HTTP 200 response.
  3. Parse the sandboxing directive using the empty string as the input and the newly created document's forced sandboxing flag set as the output.

Steps 2.2 and 2.3 ensure that the blocked frame appears to be a normal cross-origin document's load. If these steps are ignored, leakage of a document's policy state is possible. The user agent MAY implement these steps by instead redirecting the user to friendly error page in a unique origin which provides the option of opening the blocked page in a new top-level browsing context.

Note that the frame-ancestors directive will be ignored when monitoring a policy, and when a contained in a policy defined via a meta element.

Note that this directive is similar to the non-standard X-Frame-Options header that several user agents have implemented. The 'none' source expression is roughly equivilant to that header's DENY, 'self' to SAMEORIGIN, and so on. The major difference is that many user agents implement SAMEORIGIN such that it only matches against the top-level document's location. This directive checks each ancestor. If any ancestor doesn't match, the load is cancelled.

When generating a violation report for a frame-ancestors violation, the user agent MUST NOT include the value of the embedding ancestor as a blocked-uri value unless it is same-origin with the protected resource, as disclosing the value of cross-origin ancestors is a violation of the Same-Origin Policy.

Multiple Host Source Values

Multiple source-list espressions are allowed in a single policy (in contrast to X-Frame-Options, which allows only one) to enable scenarios involving embedded application compoments that are multiple levels below the top-level browsing context.

Many common scenarios for permissioned embedding (e.g. embeddable payment, sharing or social apps) involve potentially many hundreds or thousands of valid source-list expressions, but it is strongly recommended against accomodating such scenarios with a static frame-ancestors directive listing mulitple values. In such cases it is beneficial to generate this value dynamically, based on an HTTP Referer header or an explicitly passed-in value, to allow only the source(s) necessary for each given embedding of the resource.

Consider a service providing a payments application at https://payments/makeEmbedded. The service allows this resource to be embedded by both merchant Alice and merchant Bob, who compete with each other. Sending:

Content-Security-Policy: frame-options https://alice https://bob

would allow Bob to re-frame Alice's resource and create fradulent clicks, perhaps discrediting Alice with her customers or the payments service. If the payments service used additional information (e.g. as part of a URL like https://payments/makeEmbedded?merchant=alice) to send individually-tailored headers listing only the source-list expressions needed by each merchant, this attack would be eliminated.

`frame-src`

The frame-src directive is deprecated. Authors who wish to govern nested browsing contexts SHOULD use the child-src directive instead.

The frame-src directive restricts from where the protected resource can embed frames. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "frame-src"
directive-value   = source-list

The term allowed frame sources refers to the result of parsing the frame-src directive's value as a source list if the policy contains an explicit frame-src, or otherwise to the list of allowed child sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed frame sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting data for display in a nested browsing context in the protected resource created by an iframe or a frame element.
Navigating such a nested browsing context.

`img-src`

The img-src directive restricts from where the protected resource can load images. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "img-src"
directive-value   = source-list

The term allowed image sources refers to the result of parsing the img-src directive's value as a source list if the policy contains an explicit img-src, or otherwise to the default sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed image sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting data for an image, such as when processing the src or srcset attributes of an img element, the src attribute of an input element with a type of image, the poster attribute of a video element, the url(), image() or image-set() values on any Cascading Style Sheets (CSS) property that is capable of loading an image [CSS3-Images], or the href attribute of a link element with an image-related rel attribute, such as icon.

`media-src`

The media-src directive restricts from where the protected resource can load video, audio, and associated text tracks. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "media-src"
directive-value   = source-list

The term allowed media sources refers to the result of parsing the media-src directive's value as a source list if the policy contains an explicit media-src, or otherwise to the default sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed media sources, the user agent MUST act as if it had received an empty HTTP 400 response:

Requesting data for a video or audio clip, such as when processing the src attribute of a video, audio, source, or track elements.

`object-src`

The object-src directive restricts from where the protected resource can load plugins. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "object-src"
directive-value   = source-list

The term allowed object sources refers to the result of parsing the object-src directive's value as a source list if the policy contains an explicit object-src, or otherwise to the default sources.

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed object sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting data for a plugin, such as when processing the data attribute of an object element, the src attribute of an embed elements, or the code or archive attributes of an applet element.
Requesting data for display in a nested browsing context in the protected resource created by an object or an embed element.
Navigating such a nested browsing context.

It is not required that the consumer of the element's data be a plugin in order for the object-src directive to be enforced. Data for any object, embed, or applet element MUST match the allowed object sources in order to be fetched. This is true even when the element data is semantically equivalent to content which would otherwise be restricted by one of the other directives, such as an object element with a text/html MIME type.

Whenever the user agent would load a plugin without an associated URI (e.g., because the object element lacked a data attribute), if the protected resource's URI does not match the allowed object sources, the user agent MUST NOT load the plugin.

`plugin-types`

The plugin-types restricts the set of plugins that can be invoked by the protected resource by limiting the types of resources that can be embedded. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "plugin-types"
directive-value   = media-type-list

The term allowed plugin media types refers to the result of parsing the plugin-types directive's value as a media type list

Whenever the user agent would instantiate a plugin to handle resource while enforcing the plugin-types directive, the user agent MUST instead act as though the plugin reported an error and report a violation if any of the following conditions hold:

The plugin is embedded into the protected resource via an object or embed element that does not explicitly declare a MIME type with a type attribute.
resource's media type does not match the list of allowed plugin media types.
The plugin is embedded into the protected resource via an object or embed element, and the media type declared in the element's type attribute is not a case-insensitive match for the resource's media type.
The plugin is embedded into the protected resource via an applet element, and resource's media type is not a case-insensitive match for application/x-java-applet.

Note that in any of these cases, acting as though the plugin reported an error will cause the user agent to display the fallback content.

Whenever the user agent creates a plugin document in a browsing context nested in the protected resource, if the user agent is enforcing any plugin-types directives for the protected resource, the user agent MUST enforce those plugin-types directives on the plugin document as well.

Whenever the user agent creates a plugin document in a browsing context nested in the protected resource, if the user agent is monitoring any plugin-types directives for the protected resource, the user agent MUST monitor those plugin-types directives on the plugin document as well.

Usage

The plugin-types directive whitelists a certain set of MIME types that can be embedded in a protected resource. For example, a site might want to ensure that PDF content loads, but that no other plugins can be instantiated. The following directive would satisfy that requirement:

Content-Security-Policy: plugin-types application/pdf;

Resources embedded via an embed or object element delivered with an application/pdf content type would be rendered in the appropriate plugin; resources delivered with some other content type would be blocked. Multiple types can be specified, in any order. If the site decided to additionally allow Flash at some point in the future, it could do so with the following directive:

Content-Security-Policy: plugin-types application/pdf application/x-shockwave-flash;

Note that wildcards are not accepted in the plugin-types directive. Only the resource types explicitly listed in the directive will be allowed.

Predeclaration of expected media types

Enforcing the plugin-types directive requires that object and embed elements declare the expected media type of the resource they include via the type attribute. If an author expects to load a PDF, she could specify this as follows:

<object data="resource" type="application/pdf"></object>

If resource isn't actually a PDF file, it won't load. This prevents certain types of attacks that rely on serving content that unexpectedly invokes a plugin other than that which the author intended.

Note that resource will not load in this scenario even if its media type is otherwise whitelisted: resources will only load when their media type is whitelisted and matches the declared type in their containing element.

`referrer`

The referrer directive specifies a referrer policy that the user agent applies when determining what refererrer information should be included with requests made, and browsing contexts created from the context of the protected resource. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "referrer"
directive-value   = "never" / "default" / "origin" / "always"

Note that the directive name does not share the HTTP header's misspelling.

The term referrer policy refers to the value of the referrer directive, or to the empty string if no referrer directive is present in the protected resource's policy.

Replace step 7 of the "Determine referrer" algorithm defined in Fetch with the following:

If referrer is null, or if the referrer policy is never, return null.
If the referrer policy is origin, return the ASCII serialization of referrer.
If the referrer policy is always, return referrer. Note that this might cause HTTPS referrer information to be sent over the network as part of unencrypted HTTP requests.
Return null if the scheme component of referrer represents a protocol that uses transport-layer security and the scheme component of the resource being fetched does not.
Return referrer.

Note: This specification should not be interpreted as limiting user agents' ability to apply other restrictions to limit referrer leakage.

Processing multiple referrer policies

If a referrer policy is specified in multiple places (e.g. both a meta element and a referrer directive), resolve the conflict as follows:

If any referrer policy is never, the protected resource's referrer policy is never.
Otherwise, if any referrer policy is origin, the protected resource's referrer policy is origin.
Otherwise, if any referrer policy is default, the protected resource's referrer policy is default.
Otherwise, if any referrer policy is always, the protected resource's referrer policy is always.
Otherwise, the protected resource's referrer policy is default.

Usage

A protected resource can prevent referrer leakage by specifying never as the value of its policy's referrer directive:

Content-Security-Policy: referrer never;

This will cause all requests made from the protected resource's context to have an empty Referer [sic] header.

`reflected-xss`

The reflected-xss directive instructs a user agent to active or disactivate any heuristics used to filter or block reflected cross-site scripting attacks. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "reflected-xss"
directive-value   = "allow" / "block" / "filter"

A user agent with support for XSS protection MUST enforce this directive as follows:

If the value of the directive is allow, the user agent MUST disable its active protections against reflected cross-site scripting attacks for the protected resource.
If the value of the directive is filter, the user agent MUST enable its active protections against reflected cross-site scripting attacks for the protected resource. This might result in filtering script that is believed to be reflected being filtered or selectively blocking script execution.
If the value of the directive is block, the user agent MUST stop rendering the protected resource upon detection of reflected script, and instead act as though it received an empty HTTP 400 response for the protected resource itself.

If the user agent's active protections against reflected cross-site scripting attacks detect or prevent script execution, the user agent MUST report a violation.

Relationship to `X-XSS-Protection`

This directive is meant to subsume the functionality provided by the propriatary X-XSS-Protection HTTP header which is supported by a number of user agents. Roughly speaking:

reflected-xss allow is equivalent to X-XSS-Protection: 0
reflected-xss filter is equivalent to X-XSS-Protection: 1
reflected-xss block is equivalent to X-XSS-Protection: 1; mode=block

`report-uri`

The report-uri directive specifies a URI to which the user agent sends reports about policy violation. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "report-uri"
directive-value   = uri-reference *( 1*WSP uri-reference )
uri-reference     = <URI-reference from RFC 3986>

The set of report URIs is the value of the report-uri directive, each resolved relative to the protected resource's URI.

The process of sending violation reports to the URIs specified in this directive's value is defined in this document's Reporting section.

Note that the report-uri directive will be ignored if contained within a meta element.

`sandbox`

The sandbox directive specifies an HTML sandbox policy that the user agent applies to the protected resource. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "sandbox"
directive-value   = token *( 1*WSP token )
token             = <token from RFC 2616>

When enforcing the sandbox directive, the user agent MUST parse the sandboxing directive using the directive-value as the input and protected resource's forced sandboxing flag set as the output. [[!HTML5]]

Note that the sandbox directive will be ignored when monitoring a policy, and when a contained in a policy defined via a meta element.

Usage

HTML5 defines a sandbox attribute for iframe elements, intended to allow web authors to reduce the risk of including potentially untrusted content by imposing restrictions on that content's abilities. When the attribute is set, the content is forced into a unique origin, prevented from submitting forms, running script, creating or navigating other browsing contexts, and prevented from running plugins. These restrictions can be loosened by setting certain flags as the attribute's value.

The sandbox directive allows any resource, framed or not, to ask for the same sorts of restrictions to be applied to itself.

For example, a message board or email system might provide downloads of arbitrary attachments provided by other users. Attacks that rely on tricking a client into rendering one of these attachments could be mitigated by requesting that resources only be rendered in a very restrictive sandbox. Sending the sandbox directive with an empty value establishes such an environment:

Content-Security-Policy: sandbox

More trusted resources might be allowed to run in an environment with fewer restrictions by adding allow-* flags to the directive's value. For example, you can allow a page that you trust to run script, while ensuring that it isn't treated as same-origin with the rest of your site. This can be accomplished by sending the sandbox directive with the allow-scripts flag:

Content-Security-Policy: sandbox allow-scripts

The set of flags available to the CSP directive should match those available to the iframe attribute. Currently, those include:

allow-forms
allow-pointer-lock
allow-popups
allow-same-origin
allow-scripts, and
allow-top-navigation

Note as well that, like the rest of Content Security Policy, the sandbox directive is meant as a defense-in-depth. Web authors would be well-served to use it in addition to standard sniffing-mitigation and privilege-reduction techniques.

`script-src`

The script-src directive restricts which scripts the protected resource can execute. The directive also controls other resources, such as XSLT style sheets [[!XSLT]], which can cause the user agent to execute script. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "script-src"
directive-value   = source-list

The term allowed script sources refers to the result of parsing the script-src directive's value as a source list if the policy contains an explicit script-src, or otherwise to the default sources.

If 'unsafe-inline' is not in the list of allowed script sources, or if at least one nonce-source or hash-source is present in the list of allowed script sources:

Whenever the user agent would execute an inline script from a script element that lacks a valid nonce and lacks a valid hash for the allowed script sources, instead the user agent MUST NOT execute script, and MUST report a violation.
Whenever the user agent would execute an inline script from an inline event handler, instead the user agent MUST NOT execute script, and MUST report a violation.
Whenever the user agent would execute script contained in a javascript URI, instead the user agent MUST NOT execute the script, and MUST report a violation. (The user agent SHOULD ignore this step when processing script contained in "bookmarklets").

If 'unsafe-eval' is not in allowed script sources:

Instead of evaluating their arguments, both operator eval and function eval [[!ECMA-262]] MUST throw a SecurityError exception. [[!DOM4]]
When called as a constructor, the function Function [[!ECMA-262]] MUST throw a SecurityError exception. [[!DOM4]]
When called with a first argument that is non-callable (e.g., not a function), the setTimeout function MUST return zero without creating a timer.
When called with a first argument that is non-callable (e.g., not a function), the setInterval function MUST return zero without creating a timer.

The term callable refers to an object whose interface has one or more callers as defined in the Web IDL specification [[!WEBIDL]].

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed script sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting a script while processing the src attribute of a script element that lacks a valid nonce for the allowed script sources.
Requesting a script while processing the Worker or SharedWorker constructors. [[!WEBWORKERS]]
Requesting a script while invoking the importScripts method on a WorkerGlobalScope object. [[!WEBWORKERS]]
Requesting an HTML component, such as when processing the href attribute of a link element with a rel attribute containing the token import. [[!IMPORTS]]
Requesting an Extensible Stylesheet Language Transformations (XSLT) [[!XSLT]], such as when processing the <?xml-stylesheet?> processing directive in an XML document [[!XML11]], the href attributes on <xsl:include> element, or the href attributes on <xsl:import> element.

Nonce usage for `script` elements

The script-src directive lets developers specify exactly which script elements on a page were intentionally included for execution. Ideally, developers would avoid inline script entirely and whitelist scripts by URL. However, in some cases, removing inline scripts can be difficult or impossible. For those cases, developers can whitelist scripts using a randomly generated nonce.

Usage is straightforward. For each request, the server generates a unique value at random, and includes it in the Content-Security-Policy header:

Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com 'nonce-$RANDOM'

This same value is then applied as a nonce attribute to each script element that ought to be executed. For example, if the server generated the random value Nc3n83cnSAd3wc3Sasdfn939hc3, the server would send the following policy:

Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com 'nonce-Nc3n83cnSAd3wc3Sasdfn939hc3'

Script elements can then execute either because their src URLs are whitelisted or because they have an appropriate nonce:

<script>
alert("Blocked because the policy doesn't have 'unsafe-inline'.")
</script>

<script nonce="EDNnf03nceIOfn39fn3e9h3sdfa">
alert("Still blocked because nonce is wrong.")
</script>

<script nonce="Nc3n83cnSAd3wc3Sasdfn939hc3">
alert("Allowed because nonce is valid.")
</script>

<script src="https://example.com/allowed-because-of-src.js"></script>

<script nonce="EDNnf03nceIOfn39fn3e9h3sdfa"
        src="https://elsewhere.com/blocked-because-nonce-is-wrong.js"></script>

<script nonce="Nc3n83cnSAd3wc3Sasdfn939hc3"
        src="https://elsewhere.com/allowed-because-nonce-is-valid.js"></script>

Note that the nonce's value is not a hash or signature that verifies the contents of the script resources. It's quite simply a random string that informs the user agent which scripts were intentionally included in the page.

Script elements with the proper nonce execute, regardless of whether they're inline or external. Script elements without the proper nonce don't execute unless their URLs are whitelisted. Even if an attacker is able to inject markup into the protected resource, the attack will be blocked by the attacker's inability to guess the random value.

Hash usage for `script` elements

The script-src directive lets developers whitelist a particular inline script by specifying its hash as an allowed source of script.

Usage is straightforward. The server computes the hash of a particular script block's contents, and includes the base64 encoding of that value in the Content-Security-Policy header:

Content-Security-Policy: default-src 'self'; script-src 'self' https://example.com 'sha256-base64 encoded hash'

Each inline script block's contents are hashed, and compared against the whitelisted value. If there's a match, the script is executed. For example, the SHA-256 digest of alert('Hello, world.'); is YWIzOWNiNzJjNDRlYzc4MTgwMDhmZDlkOWI0NTAyMjgyY2MyMWJlMWUyNjc1ODJlYWJhNjU5MGU4NmZmNGU3OAo=. If the server sent the following header:

Content-Security-Policy: script-src 'sha256-YWIzOWNiNzJjNDRlYzc4MTgwMDhmZDlkOWI0NTAyMjgyY2MyMWJlMWUyNjc1ODJlYWJhNjU5MGU4NmZmNGU3OAo='

Then the following script tag would result in script execution:

<script>alert('Hello, world.');</script>

Note that whitespace is significant. The following scripts blocks would not hash to the same value, and would therefore not execute:

<script> alert('Hello, world.');</script>
<script>alert('Hello, world.'); </script>
<script> alert('Hello, world.'); </script>
<script>
  alert('Hello, world.');
</script>

Note also that the hash applies only to inline script. An externalized script containing the value alert('Hello, world.'); would not execute if its origin was not whitelisted as a valid source of script.

`style-src`

The style-src directive restricts which styles the user applies to the protected resource. The syntax for the name and value of the directive are described by the following ABNF grammar:

directive-name    = "style-src"
directive-value   = source-list

The term allowed style sources refers to the result of parsing the style-src directive's value as a source list if the policy contains an explicit style-src, or otherwise to the default sources.

If 'unsafe-inline' is not in the list of allowed style sources, or if at least one nonce-source or hash-source is present in the list of allowed style sources:

Whenever the user agent would apply style from a style element that lacks a valid nonce and lacks a valid hash for the allowed style sources, instead the user agent MUST ignore the style, and MUST report a violation.
Whenever the user agent would apply style from a style attribute, instead the user agent MUST ignore the style, and MUST report a violation.

Note: These restrictions on inline do not prevent the user agent from applying style from an external stylesheet (e.g., found via <link rel="stylesheet">).

If 'unsafe-eval' is not in allowed style sources, then:

Whenever the user agent would invoke the Cascading Style Sheets Object Model algorithms insert a CSS rule, parse a CSS rule, or parse a CSS declaration block, instead the user agent MUST throw a SecurityError exception and terminate the algorithm. This would include, for example, all invocations of CSSOM's various cssText setters and insertRule methods. [[!CSSOM]] [[!DOM4]]

Whenever the user agent fetches a URI (including when following redirects) in the course of one of the following activities, if the URI does not match the allowed style sources, the user agent MUST act as if it had received an empty HTTP 400 response and report a violation:

Requesting external style sheets, such as when processing the href attribute of a link element with a rel attribute containing the token stylesheet or when processing the @import directive in a stylesheet.

Note: The style-src directive does not restrict the use of XSLT. XSLT is restricted by the script-src directive because the security consequences of including an untrusted XSLT stylesheet are similar to those incurred by including an untrusted script.

Nonce usage for `style` elements

See the script-src nonce usage information for detail; the application of nonces to style elements is similar enough to avoid repetition here.

Hash usage for `style` elements

See the script-src hash usage information for detail; the application of hashes to style elements is similar enough to avoid repetition here.

Examples

Sample Policy Definitions

This section provides some sample use cases and accompanying security policies.

Example 1: A server wishes to load resources only form its own origin:

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

Example 2: An auction site wishes to load images from any URI, plugin content from a list of trusted media providers (including a content distribution network), and scripts only from a server under its control hosting sanitized ECMAScript:

Content-Security-Policy: default-src 'self'; img-src *;
                         object-src media1.example.com media2.example.com *.cdn.example.com;
                         script-src trustedscripts.example.com

Example 3: Online banking site wishes to ensure that all of the content in its pages is loaded over TLS to prevent attackers from eavesdropping on insecure content requests:

Content-Security-Policy: default-src https: 'unsafe-inline' 'unsafe-eval'

This policy allows inline content (such as inline script elements), use of eval, and loading resources over https. Note: This policy does not provide any protection from cross-site scripting vulnerabilities.

Example 4: A website that relies on inline script elements wishes to ensure that script is only executed from its own origin, and those elements it intentionally inserted inline:

Content-Security-Policy: script-src 'self' 'nonce-$RANDOM';

The inline script elements would then only execute if they contained a matching nonce attribute:

<script nonce="$RANDOM">...</script>

Sample Violation Report

This section contains an example violation report the user agent might sent to a server when the protected resource violations a sample policy.

In the following example, the user agent rendered a representation of the resource http://example.org/page.html with the following policy:

default-src 'self'; report-uri http://example.org/csp-report.cgi

The protected resource loaded an image from http://evil.example.com/image.png, violating the policy.

{
  "csp-report": {
    "document-uri": "http://example.org/page.html",
    "referrer": "http://evil.example.com/haxor.html",
    "blocked-uri": "http://evil.example.com/image.png",
    "violated-directive": "default-src 'self'",
    "effective-directive": "img-src",
    "original-policy": "default-src 'self'; report-uri http://example.org/csp-report.cgi"
  }
}

Security Considerations

Cascading Style Sheet (CSS) Parsing

The style-src directive restricts the locations from which the protected resource can load styles. However, if the user agent uses a lax CSS parsing algorithm, an attacker might be able to trick the user agent into accepting malicious "style sheets" hosted by an otherwise trustworthy origin.

These attacks are similar to the CSS cross-origin data leakage attack described by Chris Evans in 2009. User agents SHOULD defend against both attacks using the same mechanism: stricter CSS parsing rules for style sheets with improper MIME types.

Violation Reports

The violation reporting mechanism in this document has been designed to mitigate the risk that a malicious web site could use violation reports to probe the behavior of other servers. For example, consider a malicious web site that white lists https://example.com as a source of images. If the malicious site attempts to load https://example.com/login as an image, and the example.com server redirects to an identity provider (e.g., idenityprovider.example.net), CSP will block the request. If violation reports contained the full blocked URI, the violation report might contain sensitive information contained in the redirected URI, such as session identifiers or purported identities. For this reason, the user agent includes only the origin of the blocked URI.

Implementation Considerations

The Content-Security-Policy header is an end-to-end header. It is processed and enforced at the client and, therefore, SHOULD NOT be modified or removed by proxies or other intermediaries not in the same administrative domain as the resource.

The originating administrative domain for a resource might wish to apply a Content-Security-Policy header outside of the immediate context of an application. For example, a large organization might have many resources and applications managed by different individuals or teams but all subject to a uniform organizational standard. In such situations, a Content-Security-Policy header might be added or combined with an existing one at a network-edge security gateway device or web application firewall. To enforce multiple policies, the administrator SHOULD combine the policy into a single header. An administrator might wish to use different combination algorithms depending on his or her intended semantics.

One sensible policy combination algorithm is to start by allowing a default set of sources and then letting individual upstream resource owners expand the set of allowed sources by including additional origins. In this approach, the resultant policy is the union of all allowed origins in the input policies.

Another sensible policy combination algorithm is to intersect the given policies. This approach enforces that content comes from a certain whitelist of origins, for example, preventing developers from including third-party scripts or content in violation of organizational standards and practices. In this approach, the combination algorithm forms the combined policy by removing disallowed hosts from the policies supplied by upstream resource owners.

Interactions between the default-src and other directives SHOULD be given special consideration when combining policies. If none of the policies contains a default-src directive, adding new src directives results in a more restrictive policy. However, if one or more of the input policies contain a default-src directive, adding new src directives might result in a less restrictive policy, for example, if the more specific directive contains a more permissive set of allowed origins.

Using a more restrictive policy than the input policy authored by the resource owner might prevent the resource from rendering or operating as intended.

Note also that migration to HTTPS from HTTP may require updates to the policy in order to keep things running as before. Source expressions like http://example.com do not match HTTPS resources. For example, administrators SHOULD carefully examine existing policies before rolling out HTTP Strict Transport Security headers for an application.

IANA Considerations

The permanent message header field registry (see [RFC3864]) should be updated with the following registrations: