Key Highlights

● Palo Alto Networks fell on October 29 despite having no news of its own.

● Fortinet cut its full-year service Revenue forecast to $5.15 billion to $5.17 billion.

● ServiceNow suffered its worst day on record after weak Backlog guidance.

● Tenable beat estimates and raised guidance, showing the group was not uniformly weak.

Palo Alto Networks (NASDAQ:PANW) had no news of its own, but a service revenue cut at Fortinet (NASDAQ:FTNT) and a backlog scare at ServiceNow (NYSE:NOW) pulled the whole security group lower.

Caught in a sector sell-off

Palo Alto Networks did not report Earnings on Thursday. It did not cut guidance or change its outlook. It fell anyway. Shares dropped on October 29, sinking sharply early in the session before trimming some of the decline, according to market data.

The cause was next door. Two big software reports released Wednesday evening, from Fortinet and ServiceNow, set off a broad sell-off in Cybersecurity and enterprise software, and Palo Alto was swept up in it.

The Fortinet trigger

Fortinet is one of Palo Alto's most direct competitors in network security, so its results are read closely across the industry. Its third quarter beat headline estimates, but service revenue of $1.313 billion grew more slowly than analysts expected, and the company cut its full-year service revenue forecast to $5.15 billion to $5.17 billion. Product revenue growth also slowed sharply. A brokerage downgraded Fortinet to Neutral, and the stock plunged to $139.10.

When a close rival signals softer subscription and service demand, investors tend to ask whether the same forces are at work elsewhere. That question hit Palo Alto, CrowdStrike (NASDAQ:CRWD) and Zscaler (NASDAQ:ZS), all of which fell hard early in the session, according to market data.

ServiceNow adds fuel

ServiceNow made things worse. The workflow software company suffered its worst day on record after narrowly missing earnings estimates and guiding growth in its contracted subscription backlog below expectations. The IGV software ETF sank sharply, and Salesforce (NYSE:CRM) slid.

Market commentary also pointed to worries about AI disruption weighing on software and security names. The fear is that rapid advances in artificial intelligence could reshape how companies buy and use software, putting pressure on established vendors.

Not every security name stumbled

The picture was not uniformly grim. Tenable (NASDAQ:TENB) beat on revenue, Operating Income and free Cash Flow and raised its full-year guidance, according to market data. Zscaler, after opening well below its prior close, recovered much of its early loss during the session, according to one report.

That divergence suggests investors were selling the group first and sorting out the details later, a common pattern when a big peer stumbles.

The broader market

The security sell-off took place inside a sharply divided market. The Nasdaq Composite lost 365.13 points to 26,184.72, while the S&P 500 slipped 23.59 points to 7,720.39 as Alphabet (NASDAQ:GOOGL) suffered its worst day since its 2004 IPO. Information technology was one of only three S&P 500 sectors to fall.

Elsewhere, the mood was much brighter. More than 400 S&P 500 members rose, health care led the way, and the Russell 2000 gained 28.71 points to 2,859.22. One market strategist said, "Money is rotating within the market rather than leaving it."

What to watch

Palo Alto's own next report will be the real test, and investors will want to hear whether Demand for security subscriptions is holding up better than Fortinet's guidance implied. Until then, the stock may continue to trade alongside its peers and the software group.

In the nearer term, Friday's Employment Cost index and the Federal Reserve's December 8–9 meeting, where futures lean toward a hike, matter for highly valued software stocks. After-hours declines in Amazon (NASDAQ:AMZN) and Intel (NASDAQ:INTC) also pushed Nasdaq-100 futures lower on Thursday evening, which could weigh on Friday's open for technology broadly.