Reuters on the Polish national airline hit by a cyber attack highlighting how all carriers are at risk (emphasis added):

He said the problem was most likely caused by what is known as a Distributed Denial of Service (DDoS) attack — when a hacker deluges an organization’s system with so many communication requests that it overloads the server, and it can no longer carry out its normal functions.

“This was a capacity attack, which overloaded our network,” said the spokesman, Adrian Kubicki.

Ruben Santamarta, a researcher on airline’s cyber-security, said there were not enough details on the LOT attack to properly assess what happened. But he said it highlighted the vulnerability of passenger jets when they are on the tarmac preparing to fly.

“There are multiple systems at ground level that provide critical services for airlines and aircraft, in terms of operations, maintenance, safety and logistics,” said Santamarta, who is principal security consultant for Seattle-based security research firm IOActive.

Santamarta last year said he had figured out how to hack into the satellite communications equipment on passenger jets through their WiFi and inflight entertainment systems.