Source

File: wp-admin/includes/class-custom-image-header.php

public function step() {
		if ( ! isset( $_GET['step'] ) ) {
			return 1;
		}

		$step = (int) $_GET['step'];
		if ( $step < 1 || 3 < $step ||
			( 2 == $step && ! wp_verify_nonce( $_REQUEST['_wpnonce-custom-header-upload'], 'custom-header-upload' ) ) ||
			( 3 == $step && ! wp_verify_nonce( $_REQUEST['_wpnonce'], 'custom-header-crop-image' ) )
		) {
			return 1;
		}

		return $step;
	}