_wp_is_template_path_allowed( string $path )

function _wp_is_template_path_allowed( $path ) {
	// A file path that exists and does not contain `..` is allowed.
	if ( 0 === preg_match( '#(?:^|/)\.\.[. ]*(?:/|$)#', wp_normalize_path( $path ) ) ) {
		return true;
	}

	// Resolve the true location of the requested file for later comparison.
	$real_path = realpath( $path );

	if ( false === $real_path ) {
		return false;
	}

	$real_path = trailingslashit( wp_normalize_path( $real_path ) );

	$directories = array(
		STYLESHEETPATH,
		TEMPLATEPATH,
		ABSPATH . WPINC . '/theme-compat',
	);

	// If a theme is in a subdirectory, accept templates from its direct parent directory.
	if ( str_contains( get_stylesheet(), '/' ) ) {
		$directories[] = dirname( STYLESHEETPATH );
	}

	// If a parent theme is in a subdirectory, accept templates from its direct parent directory.
	if ( str_contains( get_template(), '/' ) ) {
		$directories[] = dirname( TEMPLATEPATH );
	}

	foreach ( $directories as $directory ) {
		$real_directory = realpath( $directory );

		if ( false === $real_directory ) {
			continue;
		}

		// The true location of the requested file must be inside one of the allowed directories.
		if ( str_starts_with( $real_path, trailingslashit( wp_normalize_path( $real_directory ) ) ) ) {
			return true;
		}
	}

	return false;
}