Qualys sold security as a web service years before the phrase cloud computing caught on. Its scanners lived on its own servers and probed customers' networks from the outside, looking for weak spots an attacker might use. Customers logged in through a browser and read the results. When Qualys went public in September 2012, it had over 5,800 customers in more than 100 countries. Cloud software had finally become a category investors understood.

No Software to Install

Qualys was incorporated at the end of 1999. Its main product, QualysGuard, checked computers and websites for known flaws and missing patches. Because the service ran in Qualys's own data centers, it was always up to date, and customers did not need to buy hardware or hire experts to run it. Philippe Courtot led the company as chairman and chief executive for most of its first two decades.

Twelve Dollars and Still Going

Qualys priced its IPO at $12 a share on September 28, 2012, and listed on Nasdaq as QLYS. It kept adding tools on the same cloud platform, from asset tracking to compliance checks, and sold them to the same customers. The company has been profitable for years, which is unusual among security software makers. Qualys is still public and is based in Foster City, California.

Cover: The Sentinel, 3270 feet, Yosemite Valley, Mariposa County, Cal., No. 6, Carleton Watkins. Art Institute of Chicago, public domain. Chosen for a sentinel rock (always watching).