Bad PHP code sample: Remote Code Execution (RCE) vulnerability

bad-code-sample-remote-code-execution.php

<?php 

  if( isset( $_POST[ 'action' ] ) && $_POST[ 'action' ] == 'scupdates' ) {
          if( isset( $_POST[ 'wp_cache_location' ] ) && $_POST[ 'wp_cache_location' ] != '' ) {
                  $dir = realpath( trailingslashit( dirname( $_POST[ 'wp_cache_location' ] ) ) );
                  if ( $dir == false ) {
                          $dir = WP_CONTENT_DIR . '/cache/';
                  } else {
                          $dir = trailingslashit( $dir ) . trailingslashit(wpsc_deep_replace( array( '..', '\\' ), basename( $_POST[ 'wp_cache_location' ] ) ) );
                  }
                  $new_cache_path = $dir;
          } else {
                  $new_cache_path = WP_CONTENT_DIR . '/cache/';
          }
          if ( $new_cache_path != $cache_path ) {
                  if ( file_exists( $new_cache_path ) == false )
                          rename( $cache_path, $new_cache_path );
                  $cache_path = $new_cache_path;
                  wp_cache_replace_line('^ *\$cache_path', "\$cache_path = '" . $cache_path . "';", $wp_cache_config_file);
          }
  }