Basically these planes have been found to have a WiFi passenger network that uses the same network as the avionics systems in the plane. This potentially allows hackers to hijack the plane’s navigation system, or in worse cases, commandeer the plane through its in-plane network all the while tossing back some peanuts and OJ.
According to the report released by the US Government Accountability Office, the hacker would be required to first bypass the firewall the separates the WiFi system from the avionics system. This should provide sufficient security assuming the firewalls are configured properly, although security experts have suggested that separating the networks physically would be a better idea.
That being said, Boeing has stated that their planes have a pilot manual override system that would prevent a hacker from commandeering the plane remotely. Airbus has also released a statement in which the company claims it “constantly assesses and revisits the system architecture of our products, with an eye to establishing and maintaining the highest standards of safety and security.”