Berkshire Hathaway’s portfolio added six new stocks, including the three new secret stocks. Warren Buffett was a net seller of stocks for the eleventh straight quarter.

The investment comes as UnitedHealth faces soaring medical costs, federal investigations, the fallout of the killing of a top executive and a cyberattack last year.

Shares of UnitedHealth Group are surging before the market open Friday as Warren Buffett's Berkshire Hathaway disclosed that it recently purchased shares of the beleaguered insurer.

The conglomerate sold Apple shares during the second quarter, though the iPhone maker remains Berkshire’s largest stockholding.

Given Buffett's concern over the stock market, it makes sense for investors to buy the most conservative stocks in the Berkshire Hathaway portfolio.

For decades, Berkshire Hathaway (NYSE:BRK-A)(NYSE:BRK-B) has been synonymous with Warren Buffett, the legendary investor whose Midas touch turned a modest textile company into a global powerhouse. However,