WordPress Security Update: It’s an easy fix:
You can upgrade by overwriting your old 1.5 files or if you would like to apply the fix manually it is relatively simple:
Open the wp-includes/template-functions-category.php file in a text editor like Wordpad.
Go to around line 103 where it says get_the_category_by_ID.
Create a new line after that and paste in $cat_ID = (int) $cat_ID;