The shorts are targeting one of Wall Street's best known short sellers.

Bearish investors have piled on bets against a reinsurer started by hedge fund manager David Einhorn as the company's losses climbed. Short interest on Greenlight Capital Re Ltd., or bets that the stock will drop, climbed to the highest since 2009, at 5.4 percent of shares outstanding, according to Dec. 15 data compiled by Bloomberg and Markit Ltd. Short sellers are also targeting Dan Loeb's Third Point Reinsurance Ltd., holding 3 percent of shares outstanding, compared with an average of 0.6 percent since the start of 2014.