prefetch-src directive specifies valid resources that may
be prefetched or prerendered.
|Directive type||Fetch directive|
Yes. If this directive is absent, the user agent will look for the
One or more sources can be allowed for the
Content-Security-Policy: prefetch-src <source>; Content-Security-Policy: prefetch-src <source> <source>;
<source> can be one of the following:
Internet hosts by name or IP address, as well as an optional URL scheme and/or port number. The site's address may include an optional leading wildcard (the asterisk character,
'*'), and you may use a wildcard (again,
'*') as the port number, indicating that all legal ports are valid for the source. Examples:
http://*.example.com: Matches all attempts to load from any subdomain of example.com using the
mail.example.com:443: Matches all attempts to access port 443 on mail.example.com.
https://store.example.com: Matches all attempts to access store.example.com using
*.example.com: Matches all attempts to load from any subdomain of example.com using the current protocol.
A scheme such as
https:. The colon is required. Unlike other values below, single quotes shouldn't be used. You can also specify data schemes (not recommended).
data:URIs to be used as a content source. This is insecure; an attacker can also inject arbitrary data: URIs. Use this sparingly and definitely not for scripts.
mediastream:URIs to be used as a content source.
blob:URIs to be used as a content source.
filesystem:URIs to be used as a content source.
Refers to the origin from which the protected document is being served, including the same URL scheme and port number. You must include the single quotes. Some browsers specifically exclude
filesystemfrom source directives. Sites needing to allow these content types can specify them using the Data attribute.
Allows the use of
eval()and similar methods for creating code from strings. You must include the single quotes.
Allows enabling specific inline event handlers. If you only need to allow inline event handlers and not inline
Refers to the empty set; that is, no URLs match. The single quotes are required.
An allow-list for specific inline scripts using a cryptographic nonce (number used once). The server must generate a unique nonce value each time it transmits a policy. It is critical to provide an unguessable nonce, as bypassing a resource's policy is otherwise trivial. See unsafe inline script for an example. Specifying nonce makes a modern browser ignore
'unsafe-inline'which could still be set for older browsers without nonce support.
Note: The CSP
noncesource can only be applied to nonceable elements (e.g., as the
<img>element has no
nonceattribute, there is no way to associate it with this CSP source).
A sha256, sha384 or sha512 hash of scripts or styles. The use of this source consists of two portions separated by a dash: the encryption algorithm used to create the hash and the base64-encoded hash of the script or style. When generating the hash, don't include the <script> or <style> tags and note that capitalization and whitespace matter, including leading or trailing whitespace. See unsafe inline script for an example. In CSP 2.0, this is applied only to inline scripts. CSP 3.0 allows it in the case of
script-srcfor external scripts.
strict-dynamicsource expression specifies that the trust explicitly given to a script present in the markup, by accompanying it with a nonce or a hash, shall be propagated to all the scripts loaded by that root script. At the same time, any allow-list or source expressions such as
'unsafe-inline'are ignored. See script-src for an example.
Requires a sample of the violating code to be included in the violation report.
Given a page with the following Content Security Policy:
Content-Security-Policy: prefetch-src https://example.com/
Fetches for the following code will return network errors, as the URLs provided do not
prefetch-src's source list:
<link rel="prefetch" src="https://example.org/"></link> <link rel="prerender" src="https://example.org/"></link>
|Content Security Policy Level 3 (Content Security Policy 3)|
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