Icahn is the target of the poison pill Hertz put into place Monday, the sources said. They also noted that Icahn's position is not entirely in common stock and includes some derivatives reflecting shares in the company.
Were it all common stock, it would be nearly enough to make Icahn the company's largest shareholder.
Hertz shares rose 2.4 percent at one point. (What's the stock doing now? Click here)
Sources have also told CNBC that Icahn protege Keith Meister of Corvex Capital has added to his holdings in Hertz. Meister met with Hertz CEO Mark P. Frissora within the past month in what sources called a "constructive" encounter.
—Reporting by CNBC's David Faber. Follow him on Twitter