The stock closed at $135.84, down from $174.08 the previous session.

A quarter that beat, and a guidance line that didn't

Third-quarter revenue grew 20% year over year to $2.08 billion, Fortinet said in its earnings release, with product revenue up 37% to $765 million and total billings up 29% to $2.34 billion. GAAP operating margin reached 34% and non-GAAP operating margin 38%. Diluted GAAP earnings per share came to $0.86 and non-GAAP earnings per share to $0.91. Operating cash flow was $946 million and free cash flow $865 million. The company also increased its share-repurchase authorisation by $1.2 billion, leaving $2 billion still available.

None of those figures disappointed.

A move far larger than a typical trading day

Thursday's decline was 7.1x Fortinet's typical daily move over the past year of 3.1%, on volume 3.3x its 30-day average. It was the stock's largest move in this direction since 7 August 2025. The reaction looked specific to Fortinet rather than sector-wide: other covered systems-software stocks fell a median 2.5% that same session, and the S&P 500 fell 0.6%, both far smaller than Fortinet's drop.

Does the sell-off change what Fortinet is worth?

Fortinet's own guidance implies slower growth in a segment that makes up the majority of its revenue, and growth is the input our discounted cash-flow model is most sensitive to. The model's base case assumes stage-one growth of 20.0%, built from trailing revenue growth. If growth runs five percentage points below that assumption, modelled value per share falls 23.2%; five points above it, value rises 29.6%. By comparison, a one-percentage-point increase in the discount rate lowers modelled value by −15.6%, and each one-point decline in free cash flow margin reduces it by −2.2%. The growth assumption moves modelled value far more than either of those inputs here. Separately, Fundamentalytics' base-case model currently puts value per share at $120.00, a figure built from fundamentals rather than from Thursday's share-price move; it changes only if the inputs above change. These are model values under stated assumptions, not a comparison with the share price.

What would confirm or undo this

The reading rests on Fortinet's own forward numbers, not yet on actual results. The company guided to fourth-quarter revenue of $2.155 billion to $2.245 billion and full-year service revenue of $5.150 billion to $5.170 billion.