UBS analysts also left its $25 a share price target and "buy" rating unchanged, however, they cautioned that Fortinet's shares are likely to come under pressure, given the timing of the surprise announcement.

"We believe FTNT stock will be under considerable pressure given the abrupt resignation, but we do not think the company's outlook has changed at this point. We think it is prudent to do more homework to understand the transition before declaring the coast is clear," UBS said.

Fortinet shares were falling nearly 5% in pre-market trading to $18.66. Shares have fallen over 6% year-to-date, significantly under-performing the S&P 500.

--Written by Antoine Gara in New York

Follow @antoineGara

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