Prevent stored XSS in the block editor.

Prevent escaped unicode characters become unescaped in unsafe HTML during JSON decoding.

Props: aduth, epiqueras, 


git-svn-id: https://develop.svn.wordpress.org/trunk@46896 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/formatting.php

@@ -4904,6 +4904,31 @@ function wp_pre_kses_less_than_callback( $matches ) {
 	return $matches[0];
 }
 
+/**
+ * Remove non-allowable HTML from parsed block attribute values when filtering
+ * in the post context.
+ *
+ * @since 5.3.1
+ *
+ * @param string         $string            Content to be run through KSES.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return string Filtered text to run through KSES.
+ */
+function wp_pre_kses_block_attributes( $string, $allowed_html, $allowed_protocols ) {
+	/*
+	 * `filter_block_content` is expected to call `wp_kses`. Temporarily remove
+	 * the filter to avoid recursion.
+	 */
+	remove_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10 );
+	$string = filter_block_content( $string, $allowed_html, $allowed_protocols );
+	add_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10, 3 );
+
+	return $string;
+}
+
 /**
  * WordPress implementation of PHP sprintf() with filters.
  *