UPI applications are highly encrypted and have the bandwidth to be able to facilitate numerous transactions at the same time. Currently, they handle near around Rs. 8000 crores worth of transactions on a daily basis. Furthermore, each transaction, just like the credit cards and debit cards are secured with an OTP (One time password) which is send you your registered mobile number. Only once you add this yourself, can the transaction be approved. This is by far one of the most secure ways to make financial transactions.