NEW YORK (TheStreet) - Fortinet (FTNT_) shares tanked almost 10% in pre-market trading on Friday following the surprise resignation of the company's CFO Ahmed Rubaie.<br /><br />In a statement released after market close on Nov.27, Fortinet said that Rubaie is leaving the company "for personal reasons". Rubaie, who has been with the security specialist for just seven months, will remain employed with Fortinet until Dec.5. He will also be available "on a reasonable basis" through March 5, 2014, to assist with the CFO transition. Nancy Bush, Fortinet's vice president and worldwide corporate controller, will serve as interim CFO.<br /><br />Fortinet shares plunged 9.62% to $17.75 before market open. 