Customizer: Allow arbitrary custom CSS #10667

sirreal · 2025-12-29T15:37:04Z

Allow arbitrary text in Customizer custom CSS.
Protect the CSS data from mangling by KSES HTML filters.

Under some circumstances KSES would run post content filters and change the resulting content like this:

 @property --animate {
-  syntax: "<custom-ident>";
+  syntax: "";
   inherits: true;
   initial-value: false;
 }

✅ Merged in [61418]. ~This depends on #10656 to ensure styles output is safely printed in the HTML (merged here).`

r61486 relaxed CSS content checks for Global styles with some additional restrictions that do not apply here.

Trac ticket: https://core.trac.wordpress.org/ticket/64418

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

…ing.

This reverts commit 6c6a72b.

…er-allow-arbitrary-custom-css

Arbitrary CSS is allowed and escaped when printed.

github-actions · 2025-12-29T15:37:14Z

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props jonsurrell, westonruter, peterwilsoncc, johnbillion.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

github-actions · 2025-12-29T15:53:20Z

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src/wp-includes/script-loader.php

src/wp-includes/theme.php

westonruter

Just a couple trivial suggestions, but otherwise this looks great.

Co-authored-by: Weston Ruter <westonruter@gmail.com>

…er-allow-arbitrary-custom-css

sirreal · 2026-01-08T11:03:34Z

#10641 includes some protection to ensure bad style tag contents are not saved (including </style> or ending in something that could become </style> after concatenation).

This change should be fine because of the work in #10656. Is it worth including that additional safety here?

On one hand, this change should be perfectly safe now.

On the other hand, there's not a very good reason to allow possible style tag closers inside of style tags. It will be safely escaped by the HTML API, but I wonder whether it's more helpful to prevent it from ever being stored.

westonruter · 2026-01-09T02:11:41Z

In other words, to restore the validate method but to change this:

if ( preg_match( '#</?\w+#', $css ) ) {

To be rather:

if ( false !== stripos( $css, '</style' ) ) {

Right? That seems fine to me. To be more conservative, and only make it even looser if we find this is actually needed.

johnbillion · 2026-01-15T17:24:38Z

Can we get test coverage added for the example CSS in the PR description?

peterwilsoncc

Nit pick inline.

Some users may have the customize capability but not the unfiltered_html capability.

For those users I think it would be good to run urls through the kses protocol check, see this section of code in the safe css portion of kses.

wordpress-develop/src/wp-includes/kses.php

Lines 2885 to 2908 in 6d60f73

    
           if ( $found && $url_attr ) { 
        
           	// Simplified: matches the sequence `url(*)`. 
        
           	preg_match_all( '/url\([^)]+\)/', $parts[1], $url_matches ); 
        
           	foreach ( $url_matches[0] as $url_match ) { 
        
           		// Clean up the URL from each of the matches above. 
        
           		preg_match( '/^url\(\s*([\'\"]?)(.*)(\g1)\s*\)$/', $url_match, $url_pieces ); 
        
           		if ( empty( $url_pieces[2] ) ) { 
        
           			$found = false; 
        
           			break; 
        
           		} 
        
           		$url = trim( $url_pieces[2] ); 
        
           		if ( empty( $url ) || wp_kses_bad_protocol( $url, $allowed_protocols ) !== $url ) { 
        
           			$found = false; 
        
           			break; 
        
           		} else { 
        
           			// Remove the whole `url(*)` bit that was matched above from the CSS. 
        
           			$css_test_string = str_replace( $url_match, '', $css_test_string ); 
        
           		} 
        
           	} 
        
           }

Apart from that I think it's fine to bypass kses, the allow list it currently uses is currently dated and represents a time that browsers where less secure than they are now.

src/wp-includes/theme.php

westonruter · 2026-01-15T22:24:12Z

Some users may have the customize capability but not the unfiltered_html capability.

This may not be entirely relevant here because the Custom CSS requires the edit_css capability:

wordpress-develop/src/wp-includes/class-wp-customize-manager.php

Line 5758 in 98e8753

'capability' => 'edit_css',

And this is treated the same as unfiltered_html:

wordpress-develop/src/wp-includes/capabilities.php

Lines 594 to 595 in 1fdc11a

    
           case 'edit_css': 
        
           case 'unfiltered_html':

sirreal · 2026-01-16T09:56:07Z

Some users may have the customize capability but not the unfiltered_html capability.

I believe this is covered by edit_css capability as Weston mentioned #10667 (comment).

For those users I think it would be good to run urls through the kses protocol check, see this section of code in the safe css portion of kses.

I'm reluctant to add this if avoidable. If there are specific concerns I'm happy to work on addressing them. Do you know whether this filter was already being run on the content?

One concern I have is that these functions are working with HTML entities, indicating they're not designed to operate on CSS or the RAWTEXT content of STYLE tags. This type of content-type processing mismatch is why this change is necessary.

wordpress-develop/src/wp-includes/kses.php

Lines 1995 to 2007 in bee3356

    
            * This function searches for URL protocols at the beginning of the string, while 
        
            * handling whitespace and HTML entities. 
        
            * 
        
            * @since 1.0.0 
        
            * 
        
            * @param string   $content           Content to check for bad protocols. 
        
            * @param string[] $allowed_protocols Array of allowed URL protocols. 
        
            * @param int      $count             Depth of call recursion to this function. 
        
            * @return string Sanitized content. 
        
            */ 
        
           function wp_kses_bad_protocol_once( $content, $allowed_protocols, $count = 1 ) { 
        
           	$content  = preg_replace( '/(&#0*58(?![;0-9])|&#x0*3a(?![;a-f0-9]))/i', '$1;', $content ); 
        
           	$content2 = preg_split( '/:|&#0*58;|&#x0*3a;|&colon;/i', $content, 2 );

wordpress-develop/src/wp-includes/kses.php

Lines 2030 to 2043 in bee3356

    
            * This function processes URL protocols, checks to see if they're in the 
        
            * list of allowed protocols or not, and returns different data depending 
        
            * on the answer. 
        
            * 
        
            * @access private 
        
            * @ignore 
        
            * @since 1.0.0 
        
            * 
        
            * @param string   $scheme            URI scheme to check against the list of allowed protocols. 
        
            * @param string[] $allowed_protocols Array of allowed URL protocols. 
        
            * @return string Sanitized content. 
        
            */ 
        
           function wp_kses_bad_protocol_once2( $scheme, $allowed_protocols ) { 
        
           	$scheme = wp_kses_decode_entities( $scheme );

…sired

sirreal · 2026-01-16T10:15:12Z

Thanks everyone! I believe all feedback is addressed.

…dation

sirreal · 2026-01-19T16:38:45Z

After further discussion, it seemed prudent to restore the validation method and reject CSS that contains a closing style tag </style> or some prefix thereof in the same way that r61486 validates CSS content for global styles.

I've added that implementation and corresponding tests.

peterwilsoncc · 2026-01-19T21:53:53Z

This may not be entirely relevant here because the Custom CSS requires the edit_css capability:

Sorry, wrong cap but it's still possible someone can edit css without using unfiltered HTML with something like:

// Untested so there may be a syntax error.
add_filter( 'map_meta_cap', function ( $cap, $caps ) {
	if ( 'unfiltered_html' === $cap ) {
		$caps[] = 'do_not_allow';
	}
	return $caps;
}, 10, 2 );

The above would place URL protocol restrictions on all users so I think it's needed for this too.

westonruter · 2026-01-21T00:22:08Z

@sirreal Are such protocol restrictions being enforced in the Site Editor?

sirreal · 2026-01-22T13:09:08Z

I spent a lot of time testing the behavior here. I set up a multisite install for testing testing network admins and site admins and performed the same tests on a single site install.

Multisite site admins do not have sufficient capabilities and their additional CSS edits are rejected. The additional CSS panel does not appear in customize for them and AJAX requests are rejected:

{
  "custom_css[twentyten]": {
    "unauthorized": {
      "message": "Unauthorized to modify setting due to capability.",
      "data": null
    }
  }
}

Multisite network (super) admins have access, just like regular admins on single site installs.

I added debugging inside the related filters and I never saw them run when updating this setting. I believe that's because either the user has unfilitered_html/edit_css and the filters do not apply, or the user does not have the capability and the content is rejected before filters would be applied.

I've removed the filter changes from this PR because they don't seem to be necessary. I worked on this simultaneously with #10641, and I may have changed the filters here after confusing test results on the two PRs.

All tests are passing on this PR without any changes to filters. This supports the idea that any user with sufficient capabilities to modify custom CSS will not have the CSS content filtered by KSES.

Discussion related to content filtering should no longer be relevant for this change.

Copilot

Pull request overview

Updates Customizer custom CSS handling to permit arbitrary CSS text (including </> used by modern CSS constructs) while preventing CSS from breaking out of the <style> element, and adds regression tests for the behavior described in Trac #64418.

Changes:

Relax WP_Customize_Custom_CSS_Setting::validate() to only reject sequences that can prematurely close a <style> element (or partial end-tags that could become one when concatenated).
Add PHPUnit coverage for safely printing custom CSS and for accepting @property at-rules without KSES mangling.
Add a cross-reference in REST global styles CSS validation docs to the Customizer validator.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/phpunit/tests/customize/custom-css-setting.php	Adds tests for safe `<style>` output and updated validation behavior (including `@property`).
src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php	Adds `@see` linking REST CSS validation docs to the Customizer validator.
src/wp-includes/customize/class-wp-customize-custom-css-setting.php	Replaces broad markup detection with targeted `</style` / partial end-tag detection while preserving parent CSS validation.

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Copilot · 2026-01-23T17:08:00Z

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

+			 * Note how in the second example, both of the style contents are benign
+			 * when analyzed on their own. The first style was likely the result of
+			 * improper truncation, while the second is perfectly sound. It was only
+			 * through concatenation that these two scripts combined to form content


In this explanatory block comment, it says "these two scripts combined" but the examples and surrounding text are about concatenating styles (CSS), not scripts. Please update the wording to avoid confusion in this security-relevant rationale.

Suggested change

* through concatenation that these two scripts combined to form content

* through concatenation that these two styles combined to form content

Copilot · 2026-01-23T17:08:00Z

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

+		$length = strlen( $css );
+		for (
+			$at = strcspn( $css, '<' );
+			$at < $length;
+			$at += strcspn( $css, '<', ++$at )
+		) {
+			$remaining_strlen = $length - $at;


This STYLE end-tag scanning logic is now duplicated here and in WP_REST_Global_Styles_Controller::validate_custom_css() (see src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php:682+). To reduce the chance of the two validators drifting, consider extracting the shared check into a small internal helper function and reusing it from both call sites (mapping to different error codes as needed).

westonruter

Just some nits, but looks good.

westonruter · 2026-01-23T16:56:35Z

tests/phpunit/tests/customize/custom-css-setting.php

+		$css = <<<'CSS'
+@property --animate {
+	syntax: "<custom-ident>";
+	inherits: true;
+	initial-value: false;
+}
+CSS;


This can all be indented now for readability, yes?

Suggested change

$css = <<<'CSS'

@property --animate {

syntax: "<custom-ident>";

inherits: true;

initial-value: false;

}

CSS;

$css = <<<'CSS'

@property --animate {

syntax: "<custom-ident>";

inherits: true;

initial-value: false;

}

CSS;

westonruter · 2026-01-23T16:56:52Z

tests/phpunit/tests/customize/custom-css-setting.php

+		$css = <<<'CSS'
+@property --animate {
+	syntax: "<custom-ident>";
+	inherits: true;
+	initial-value: false;
+}
+CSS;


Suggested change

$css = <<<'CSS'

@property --animate {

syntax: "<custom-ident>";

inherits: true;

initial-value: false;

}

CSS;

$css = <<<'CSS'

@property --animate {

syntax: "<custom-ident>";

inherits: true;

initial-value: false;

}

CSS;

westonruter · 2026-01-23T16:57:08Z

tests/phpunit/tests/customize/custom-css-setting.php

+		$expected = <<<'HTML'
+<style id="wp-custom-css">
+*::before { content: "\3c\2fstyle><script>alert(1)</script>"; }
+</style>
+HTML;


Suggested change

$expected = <<<'HTML'

<style id="wp-custom-css">

*::before { content: "\3c\2fstyle><script>alert(1)</script>"; }

</style>

HTML;

$expected = <<<'HTML'

<style id="wp-custom-css">

*::before { content: "\3c\2fstyle><script>alert(1)</script>"; }

</style>

HTML;

westonruter · 2026-01-23T17:08:28Z

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

 		// Restores the more descriptive, specific name for use within this method.
 		$css = $value;

-		$validity = new WP_Error();


Not critical, this could be restored with the suggestion below.

westonruter · 2026-01-23T17:09:06Z

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

+					return new WP_Error(
+						'illegal_markup',
+						sprintf(
+							/* translators: %s is the CSS that was provided. */
+							__( 'The CSS must not contain "%s".' ),
+							esc_html( substr( $css, $at, 8 ) )
+						),
+						array( 'status' => 400 )
+					);


To retain the prior structure, since in theory multiple errors could be added for an invalid setting (although this doesn't currently happen here):

Suggested change

return new WP_Error(

'illegal_markup',

sprintf(

/* translators: %s is the CSS that was provided. */

__( 'The CSS must not contain "%s".' ),

esc_html( substr( $css, $at, 8 ) )

),

array( 'status' => 400 )

);

$validity->add(

'illegal_markup',

sprintf(

/* translators: %s is the CSS that was provided. */

__( 'The CSS must not contain "%s".' ),

esc_html( substr( $css, $at, 8 ) )

)

);

break;

This also removes the 400 status code, which isn't relevant in this context.

westonruter · 2026-01-23T17:09:50Z

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

-		if ( ! $validity->has_errors() ) {
-			$validity = parent::validate( $css );
-		}
-		return $validity;
+		return parent::validate( $css );


This change could be reverted with the above suggestions.

sirreal added 14 commits December 23, 2025 13:22

Update wp_custom_css_cb to rely on HTML API for safe SCRIPT tag print…

c124eb5

…ing.

Wrap customizer CSS test in newlines

e055156

Use HTML API for style tags in script-loader

33f9616

Use HTML Tag Processor to produce WP_Styles style tags

606539e

Use HTML Tag Processor for STYLE tags in theme.php

c938d4c

Build font style tags with HTML API

dd919f1

PICKME: Update font tests to use semantic HTML comparison

d29900a

Use HTML API for hide header text

6c6a72b

Revert "Use HTML API for hide header text"

aad4744

This reverts commit 6c6a72b.

Merge branch 'trunk' into styles/use-html-api-for-style-tags

c3ae9a9

Fix lint

4e88745

Merge branch 'trunk' into styles/use-html-api-for-style-tags

d296d6c

Merge branch 'styles/use-html-api-for-style-tags' into 64418/customiz…

d8a6f02

…er-allow-arbitrary-custom-css

Allow arbitrary customizer custom CSS

67500e0

Arbitrary CSS is allowed and escaped when printed.

Fix lints

01b6fb8

westonruter reviewed Dec 29, 2025

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Styles: Use HTML API to generate <style> tags #10656

Closed

sirreal and others added 6 commits December 29, 2025 18:08

Restore STYLE tag trailing newline

0141653

Co-authored-by: Weston Ruter <westonruter@gmail.com>

Restore STYLE tag trailing newlines in theme.php

6585099

Co-authored-by: Weston Ruter <westonruter@gmail.com>

Merge branch 'styles/use-html-api-for-style-tags' into 64418/customiz…

b0020d8

…er-allow-arbitrary-custom-css

Move trailing newline out of Tag Processor

407d43f

Merge branch 'styles/use-html-api-for-style-tags' into 64418/customiz…

ffd5b45

…er-allow-arbitrary-custom-css

Merge branch 'trunk' into 64418/customizer-allow-arbitrary-custom-css

8268865

peterwilsoncc reviewed Jan 15, 2026

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sirreal added 4 commits January 16, 2026 09:51

Merge branch 'trunk' into 64418/customizer-allow-arbitrary-custom-css

f515c02

Add covers annotations to new test

cbf7e21

Use verbose variable name for clarity

b13930f

Expand explanatory comment

5a414b6

sirreal added 2 commits January 16, 2026 10:58

Move new test to end of file

2407549

Add test that at-property rule passes validation and is printed as de…

ae66f56

…sired

sirreal added 3 commits January 19, 2026 17:26

Restore custom CSS validation, add check like global styles rest vali…

def70e2

…dation

Merge branch 'trunk' into 64418/customizer-allow-arbitrary-custom-css

91adeb5

Add validation tests

b8f92d2

lint

cd7712e

sirreal added 2 commits January 22, 2026 12:47

Merge branch 'trunk' into 64418/customizer-allow-arbitrary-custom-css

10971f6

Try restoring filter

96615aa

sirreal added 2 commits January 22, 2026 14:09

Remove empty line

a774a01

Restore previous validate test

363650d

sirreal requested review from peterwilsoncc and westonruter January 22, 2026 16:53

Merge branch 'trunk' into 64418/customizer-allow-arbitrary-custom-css

848c53b

westonruter requested a review from Copilot January 23, 2026 16:58

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	if ( $found && $url_attr ) {
	// Simplified: matches the sequence `url(*)`.
	preg_match_all( '/url\([^)]+\)/', $parts[1], $url_matches );

	foreach ( $url_matches[0] as $url_match ) {
	// Clean up the URL from each of the matches above.
	preg_match( '/^url\(\s([\'\"]?)(.)(\g1)\s*\)$/', $url_match, $url_pieces );

	if ( empty( $url_pieces[2] ) ) {
	$found = false;
	break;
	}

	$url = trim( $url_pieces[2] );

	if ( empty( $url ) \|\| wp_kses_bad_protocol( $url, $allowed_protocols ) !== $url ) {
	$found = false;
	break;
	} else {
	// Remove the whole `url(*)` bit that was matched above from the CSS.
	$css_test_string = str_replace( $url_match, '', $css_test_string );
	}
	}
	}

	* through concatenation that these two scripts combined to form content
	* through concatenation that these two styles combined to form content

Customizer: Allow arbitrary custom CSS #10667

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Customizer: Allow arbitrary custom CSS #10667

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