Check out which companies are making headlines before the bell:
Herbalife — Hedge Fund manager Bill Ackman told CNBC he was contacted by Jefferies Group to buy a stake in Herbalife. Ackman made his comments after the Wall Street Journal reported that Carl Icahn, another well-known hedge fund manager, was likely to sell his stake in the company to a group that included Ackman.
Apple — The tech firm issued a security update to the iPhone after researchers discovered that an Arab activist's device was targeted with a previously unknown device.
St. Jude Medical — Shares of the pacemaker manufacturer are the target of short-selling firm muddy Waters, which says St. Jude's devices were vulnerable to cyber attacks.
Viacom — The media giant's stock was downgraded to "market perform" from "outperform" at Telsey Advisory Group, as investors start focusing again on Viacom's "high debt leverage and lackluster operations." Telsey also cut its price target on Viacom's stock to $47 from $51.
Signet Jewelers — Citi downgraded Signet's stock to "neutral" from "buy" and slashed its price target to $83 from $125, citing weakening fundamentals. "The sales/merch margin equation has turned negative and weakness at Zale over the past two quarters causes us to question if that business is likely to close the sales productivity and profit gap (vs Kay) we once thought could be closed," Citi said in a note to clients.
Dollar Tree — Analysts at MKM Partners downgraded the discount retailer's stock to "neutral" from "buy," citing Dollar Tree's disappointing second-quarter results. "Same-store sales at Dollar Tree slowed sequentially and were the weakest they've been since 4Q13, while Family Dollar, which enters the reported comp base in 4Q16, turned negative after three positive quarters," MKM said in a note to clients.
Splunk — Stifel Nicolaus downgraded Splunk shares to "hold" from "buy" despite the firm reporting stronger-than-expected quarterly results.
Ulta Salon — The beauty product retailer posted its seventh straight quarter of double-digit sales growth and raised its full-year guidance. That said, Ulta's forecast came in below estimates.
GameStop — The video-game retailer reported mixed quarterly results, with earnings per share meeting expectations while sales missed. GameStop cited weak sales of new gaming software and hardware.
Pure Storage — The data storage firm's stock surged in the premarket after posting a narrower-than-expected quarterly loss and revenues that came above expectations.
— Reuters contributed to this report.