Before Tenable was a company, it was Nessus, a free security scanner that a young French programmer, Renaud Deraison, released in 1998. System administrators everywhere used it to probe their own networks for holes. In 2002 Deraison joined Ron Gula and Jack Huffard in Maryland to build a business around it. By its 2018 public offering, Tenable sold to governments and large companies and pitched a bigger idea it called Cyber Exposure.

Open Source, Then a Business

Nessus checked computers for known weaknesses: unpatched software, open ports, default passwords. Because it was free and open, it spread across the security world. Tenable sold tools on top of it for large networks, then in 2005 made new versions of Nessus closed source so it could fund the research behind its checks. The company grew into a full platform for vulnerability management, selling to banks, governments and thousands of mid-sized firms near its home in Columbia, Maryland.

Thirty-Two Percent on Day One

Tenable priced its shares at $23, above its expected range of $20 to $22, and began trading on Nasdaq on July 26, 2018, under TENB. The stock closed its first day up nearly 32 percent. The company sold 12,535,000 shares once the underwriters took their full option. Tenable has since expanded into cloud security and attack-surface tools through acquisitions, and it remains an independent public company on Nasdaq.

Cover: Hercules and Nessus, from The Labors of Hercules, Hans Sebald Beham. Art Institute of Chicago, public domain. Chosen for Hercules and the centaur Nessus (the Nessus scanner).