** Cloud security firm Zscaler's ZS.O shares fall 1.1% to $248.80 premarket

** Bernstein downgrades stock to "market perform" from "outperform"

** Brokerage says co's 2026 guidance for annualized recurring revenue fed into worries about increased competition

** "We don’t, yet, see the company expanding beyond their core cloud-centric network cybersecurity segment" - Bernstein

** Average rating of 38 analysts is "buy," their median PT is $330.45 - data compiled by LSEG

** Up to last close, stock has risen 39.4% this year

(Reporting by Anhata Rooprai in Bengaluru)

((Anhata.Rooprai@thomsonreuters.com))