A quick update has been released by WordPress team. This version fix the security bug on the previous version. You can simply download the new version and overwrite the old files or manually modify your WordPress files like below (taken from WP Dev Blog):

1. Open the wp-includes/template-functions-category.php file in a text editor like Wordpad.
2. Go to around line 103 where it says get_the_category_by_ID.
3. Create a new line after that and paste in $cat_ID = (int) $cat_ID;

If you manually edit the file above, you can also modify WP version if you like :p

1. Open the wp-includes/version.php file in a text editor like Wordpad.
2. Look at line 5 and change ‘1.5.1.1‘ to ‘1.5.1.2‘ or anything you like :d