CSP: form-action
CSP: form-action
The HTTP Content-Security-Policy
(CSP) form-action
directive restricts the URLs which can be used as the target of a form submissions from a given context.
Warning: Whether form-action
should block redirects after a form submission is debated and browser implementations of this aspect are inconsistent (e.g. Firefox 57 doesn't block the redirects whereas Chrome 63 does).
CSP version | 2 |
---|---|
Directive type | Navigation directive |
default-src fallback | No. Not setting this allows anything. |
Syntax
One or more sources can be set for the form-action
policy:
Content-Security-Policy: form-action <source>; Content-Security-Policy: form-action <source> <source>;
Sources
<source> can be one of the following:
<host-source>
-
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 thehttp:
URL scheme. -
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 usinghttps:
. -
*.example.com
: Matches all attempts to load from any subdomain of example.com using the current protocol.
-
<scheme-source>
-
A scheme such as
http:
orhttps:
. The colon is required. Unlike other values below, single quotes shouldn't be used. You can also specify data schemes (not recommended).-
data:
Allowsdata:
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:
Allowsmediastream:
URIs to be used as a content source. -
blob:
Allowsblob:
URIs to be used as a content source. -
filesystem:
Allowsfilesystem:
URIs to be used as a content source.
-
'self'
-
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
blob
andfilesystem
from source directives. Sites needing to allow these content types can specify them using the Data attribute. 'unsafe-eval'
-
Allows the use of
eval()
and similar methods for creating code from strings. You must include the single quotes. 'unsafe-hashes'
-
Allows enabling specific inline event handlers. If you only need to allow inline event handlers and not inline
<script>
elements orjavascript:
URLs, this is a safer method than using theunsafe-inline
expression. 'unsafe-inline'
-
Allows the use of inline resources, such as inline
<script>
elements,javascript:
URLs, inline event handlers, and inline<style>
elements. The single quotes are required. 'none'
-
Refers to the empty set; that is, no URLs match. The single quotes are required.
'nonce-<base64-value>'
-
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
nonce
source can only be applied to nonceable elements (e.g., as the<img>
element has nononce
attribute, there is no way to associate it with this CSP source). '<hash-algorithm>-<base64-value>'
-
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-src
for external scripts. 'strict-dynamic'
-
The
strict-dynamic
source 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'self'
or'unsafe-inline'
are ignored. See script-src for an example. 'report-sample'
-
Requires a sample of the violating code to be included in the violation report.
Examples
Meta tag configuration
<meta http-equiv="Content-Security-Policy" content="form-action 'none'">
Apache configuration
<IfModule mod_headers.c> Header set Content-Security-Policy "form-action 'none';" </IfModule>
Nginx configuration
add_header Content-Security-Policy "form-action 'none';"
Violation case
Using a <form>
element with an action set to inline JavaScript will result in a CSP violation.
<meta http-equiv="Content-Security-Policy" content="form-action 'none'"> <form action="javascript:alert('Foo')" id="form1" method="post"> <input type="text" name="fieldName" value="fieldValue"> <input type="submit" id="submit" value="submit"> </form> // Error: Refused to send form data because it violates the following // Content Security Policy directive: "form-action 'none'".
Specifications
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
form-action |
40 |
15 |
36 |
No |
27 |
10 |
Yes |
Yes |
36 |
? |
9.3 |
Yes |
See also
© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/form-action