The canon novel Tarkin mentions a cyber attack conducted by the Separatists during the Clone Wars:
It had been just nine months after the Battle of Geonosis that Count Dooku’s scientists had succeeded in slicing into the Republic HoloNet by seeding the spaceways with hyperwave transceiver nodes of a novel design. The Separatists could have kept quiet about the infiltration and tasked the nodes to gather intelligence about Republic military operations. Instead, Dooku—as if suddenly intent on winning hearts and minds rather than defeating the Republic with his droid armies—began using the HoloNet to broadcast propaganda Shadowfeeds, providing Separatist accounts of battle wins and disinformation about Republic war crimes, and in the end spreading apprehension among the populations of the Core Worlds that a Separatist victory was imminent.
It's not mentioned how the Separatists hacked ("computer slicing" in Star Wars terminology) into the HoloNet, but the injection of propaganda would certainly constitute "cyber attack".
The same novel also involves an incident in which rebels launched a cyber attack in tandem with a physical attack against a base involved in the construction of the Death Star. As described by Wilhuff Tarkin, the commander of that base:
A base under my command was recently attacked by unknown parties. The attack followed the successful sabotaging of a HoloNet relay station and the insertion of both prerecorded and real-time holovids, in an attempt to mislead us into dispatching reinforcements to a secondary base.
It's not clear how the HoloNet relay station was sabotaged, but again this is an example of cyberwarfare.