There are many safeguards built into the process to prevent Scripbox (or any third party) from misusing your confidential information. Your authorisation to the bank is very specific and only allows for money to be transferred to the mutual fund companies. When you redeem a mutual fund it will only transfer the money to the registered bank account.
To ensure this, the bank must be provided with a cancelled cheque as proof of ownership of your account.
Moreover, you are entitled to monetary compensation under Section 43A of the (Indian) Information Technology Act, 2000 if there is any accidental disclosure of your sensitive personal information.