The strategy, adopted by the board of directors this week, will be in effect for one year. It was triggered by Icahn's moving in fast on the company, the sources said.
The activist investor has purchased 30 million to 40 million Hertz shares and staked out a larger position with derivatives. Had his entire investment been in the form of common shares, Icahn would be Hertz' largest shareholder, CNBC News reported.
The board put in place a poison pill strategy Monday that allows it to intervene if any one shareholder builds up a stake of 10 percent or more in the company.
A poison pill intervention often includes issuing a large number of new shares to existing shareholders, which dilutes the current share price. In a regulatory filing, Hertz said "unusual and substantial [shares] activity" prompted it to put the poison pill strategy in place.
The company said it was attempting to "enhance shareholder value," the Wall Street Journal reported Saturday.
Don Bilson, an analyst at Gordon Haskett Research Advisors, told the Journal Hertz was "ready-made for an activist investor."