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Fix problem with encoding of css entities when post with existing block level custom css is edited by user without unfiltered_html #11104

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Fix problem with encoding of css entities when post with existing block level custom css is edited by user without unfiltered_html #11104

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292f17d
Fix problem with encoding of css entities when post with block level …
glendaviesnz Mar 1, 2026
51f3c12
Fix WP version
glendaviesnz Mar 1, 2026
35eab15
Sanitise CSS separate to other attribs
glendaviesnz Mar 8, 2026
a007350
Revert "Sanitise CSS separate to other attribs"
glendaviesnz Mar 9, 2026
19537ea
switch to encoded string to stop kses mangling
glendaviesnz Mar 10, 2026
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5 changes: 0 additions & 5 deletions src/wp-includes/block-supports/custom-css.php
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Original file line number Diff line number Diff line change
Expand Up @@ -26,11 +26,6 @@ function wp_render_custom_css_support_styles( $parsed_block ) {
return $parsed_block;
}

// Validate CSS doesn't contain HTML markup (same validation as global styles REST API).
if ( preg_match( '#</?\w+#', $custom_css ) ) {
return $parsed_block;
}

// Generate a unique class name for this block instance.
$class_name = wp_unique_id_from_values( $parsed_block, 'wp-custom-css-' );
$updated_class_name = isset( $parsed_block['attrs']['className'] )
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23 changes: 23 additions & 0 deletions src/wp-includes/blocks.php
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Original file line number Diff line number Diff line change
Expand Up @@ -2075,8 +2075,31 @@ function _filter_block_content_callback( $matches ) {
* @return array The filtered and sanitized block object result.
*/
function filter_block_kses( $block, $allowed_html, $allowed_protocols = array() ) {
/*
* Per-block custom CSS (attrs.style.css) may contain characters like <, >,
* and & that are valid in CSS but would be mangled by wp_kses(), which treats
* all values as HTML. Encode these characters as JSON unicode escapes before
* KSES runs, then decode afterwards. This is the same approach used for
* Global Styles custom CSS (see r61486).
*/
$has_block_css = isset( $block['attrs']['style']['css'] );
if ( $has_block_css ) {
// wp_json_encode wraps the string in quotes: "encoded content".
// Trim them to get the raw escaped content as a PHP string.
$block['attrs']['style']['css'] = trim(
wp_json_encode( $block['attrs']['style']['css'], JSON_HEX_TAG | JSON_HEX_AMP ),
'"'
);
}

$block['attrs'] = filter_block_kses_value( $block['attrs'], $allowed_html, $allowed_protocols, $block );

@ramonjd ramonjd Mar 2, 2026 •
edited
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Thanks for getting up a fix!

I was wondering: what is the important sanitization step for block CSS attributes?

wp_kses() treats the CSS string as HTML. But should it?

I'm wondering if an alternative solution would be to target the css attribute and run it through wp_strip_all_tags rather than wp_kses.

Or running through something similar (or reuse this same validation in a helper) that @sirreal and @dmsnell worked on in WP_REST_Global_Styles_Controller::validate_custom_css() for https://core.trac.wordpress.org/ticket/64418

There, for users without unfiltered_html, & and > in block custom CSS were being double-encoded by KSES + JSON, so the CSS broke.

(Sorry Jon and Dennis - you've become my default go-to brains trust for this stuff 😄 )

if ( $has_block_css && isset( $block['attrs']['style']['css'] ) ) {
$block['attrs']['style']['css'] = json_decode(
'"' . $block['attrs']['style']['css'] . '"'
);
}

if ( is_array( $block['innerBlocks'] ) ) {
foreach ( $block['innerBlocks'] as $i => $inner_block ) {
$block['innerBlocks'][ $i ] = filter_block_kses( $inner_block, $allowed_html, $allowed_protocols );
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