In Avengers: Age of Ultron movie,
Ultron infected tech of Avengers Tower just after his birth. It means he is capable of infecting tech. Also, if this wasn't the case, he could never be able to create his drones.
Just after its birth,
He escaped using the Internet and infected digital devices across the entire planet. Again, awesome capability (think Skynet).
We also saw that
Iron Man's armor was connected to the Internet. He googled for the keyword "Hulk" during flight just before he deployed Veronica.
Then, why did Ultron let Iron Man win the fight? Why didn't he simply hack Iron Man's armor to use him as own drone?