WP_REST_Global_Styles_Controller::validate_custom_css( string $css ): true|WP_Error

Source

protected function validate_custom_css( $css ) {
	$length = strlen( $css );
	for (
		$at = strcspn( $css, '<' );
		$at < $length;
		$at += strcspn( $css, '<', ++$at )
	) {
		$remaining_strlen = $length - $at;
		/**
		 * Custom CSS text is expected to render inside an HTML STYLE element.
		 * A STYLE closing tag must not appear within the CSS text because it
		 * would close the element prematurely.
		 *
		 * The text must also *not* end with a partial closing tag (e.g., `<`,
		 * `</`, … `</style`) because subsequent styles which are concatenated
		 * could complete it, forming a valid `</style>` tag.
		 *
		 * Example:
		 *
		 *     $style_a = 'p { font-weight: bold; </sty';
		 *     $style_b = 'le> gotcha!';
		 *     $combined = "{$style_a}{$style_b}";
		 *
		 *     $style_a = 'p { font-weight: bold; </style';
		 *     $style_b = 'p > b { color: red; }';
		 *     $combined = "{$style_a}\n{$style_b}";
		 *
		 * 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 styles combined to form content
		 * that would have broken out of the containing STYLE element, thus
		 * corrupting the page and potentially introducing security issues.
		 *
		 * @link https://html.spec.whatwg.org/multipage/parsing.html#rawtext-end-tag-name-state
		 */
		$possible_style_close_tag = 0 === substr_compare(
			$css,
			'</style',
			$at,
			min( 7, $remaining_strlen ),
			true
		);
		if ( $possible_style_close_tag ) {
			if ( $remaining_strlen < 8 ) {
				return new WP_Error(
					'rest_custom_css_illegal_markup',
					sprintf(
						/* translators: %s is the CSS that was provided. */
						__( 'The CSS must not end in "%s".' ),
						esc_html( substr( $css, $at ) )
					),
					array( 'status' => 400 )
				);
			}

			if ( 1 === strspn( $css, " \t\f\r\n/>", $at + 7, 1 ) ) {
				return new WP_Error(
					'rest_custom_css_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 )
				);
			}
		}
	}

	return true;
}

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