They can monitor what you type.This time it involves so-called keylogging, unauthorized persons can monitor and record what you type on the phone.
The technique is well known from before, and certainly nothing new. It has also been previously noted that such keyloggers can be installed on phones that have been tampered with, so-called jailbreak.
What is new is that such keylogger apps can also be installed on a standard, non-jailbroken phone.
There are security company Fireeye which has revealed hole. The company says that they are now working closely with Apple to resolve the issue.