The company may be under criminal investigation for alleged irregularities in its Medicare business—but it's been touched by the Warren Buffett effect.

Shares of UnitedHealth Group Inc. (NYSE: UNH) climbed 2.81% in Monday's pre-market trading session, extending Friday's gain following Michael Burry and Warren Buffett's stake acquisition in the insurance behemoth.

UnitedHealth Group (UNH) stock is still down 40% YTD but has surged more than +20% this month following reports that Warren Buffett, and other billionaire hedge fund managers have taken a stake in the company.