Rosneft is now suing private firm Sistema for 170 billions rubles (2.55 billion euros, $2.88 billion) over the state giant's acquisition of oil company Bashneft. Rosneft has alleged that certain assets were removed from Bashneft, while Sistema has rejected these claims as groundless.
It also said that it hopes the attack is not related to, "current legal proceedings", with Russian conglomerate Sistema.
A court on Tuesday announced a halt in the case between Rosneft and Sistema - owned by tycoon Vladimir Yevtushenkov - until July 12.
Michigan health chief charged with involuntary manslaughter in Flint water probe
Lyon and his co-defendants are accused of covering up evidence revealing the health hazards of Flint's water . Legionnaires' disease is a form of pneumonia contracted by drinking contaminated warm water with bacteria .
The dispute between Sistema and Rosneft has rekindled investor fears from 2014, when the Russian government seized Sistema's stake in oil firm Bashneft, saying Bashneft's privatisation had been illegal.
"The company's servers underwent a powerful hacking attack", the company said on Twitter.
According to a court order, an arrest was imposed on a 31.76 percent stake in MTS, 100 percent of its Medsi chain of medical clinics, and 90.47 percent of Bashkirian Power Grid company owned by Sistema and its unit Sistema-Invest.
"It would not hurt Sistema's ability to service its debt or fund its operational activity, but could restrict its freedom when it comes to mergers and acquisitions", Raiffeisenbank analyst Sergey Libin said.