To solve this question, the user needs to have knowledge on securing web applications against authenticated users.

Option A: Client-side data validation is not sufficient to secure web applications against authenticated users. It can be easily bypassed by attackers, and therefore this option is not recommended.

Option B: Filtering data with a default deny regular expression can help prevent malicious input from being accepted, which is a good security practice. Therefore, this option is recommended.

Option C: Running the application under least privileges necessary is a recommended security practice because it limits the damage that can be caused by a successful attack. Therefore, this option is recommended.

Option D: Using parameterized queries to access a database can help prevent SQL injection attacks, which is a good security practice. Therefore, this option is recommended.

Therefore, the option that is NOT recommended for securing web applications against authenticated users is option A: Client-side data validation.

The Answer is: A