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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 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 any one of the values listed in CSP Source Values.

Note that this same set of values can be used in all fetch directives (and a number of other directives).

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

Specification
Content Security Policy Level 3
# directive-form-action

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
No
9.3
Yes

See also

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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