To Wordpress Users,

There is a security vulnerability in WordPress that may be triggered if you’re running the default template.

If you would like to apply the fix manually it is relatively simple:

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;

or

You can upgrade by overwriting only one of your old Wordpress 1.5 files. I’ve added the above codes into my template-functions-category.php. Feel free to download it and use it in your Wordpress 1.5.

template-functions-category.php (Updated)

[Source]