What qualification do you need to become a forensic auditor?

you need to be a CPA or have a professional qualification like ACCA, CA, CIMA..after your graduate program.

Regarding CPA exam:

Eligibility to sit for the Uniform CPA Exam is determined by individual State Boards of Accountancy. Typically the requirement is a U.S. bachelors degree which includes a minimum number of qualifying credit hours in accounting and business administration with an additional 1 year study. This requirement for 5 years study is known as the "150 hour rule" and has been adopted by the majority of state boards, although there are still some exceptions (e.g.California). This requirement mandating 150 hours of study has been adopted by 45 states.

you can see www.aicpa.org

and also www.nasba.org