But in some cases, Mr. Moore discovered he could leap from one open system into its address book and dial into the conference rooms of other companies, even those companies that put their system behind the firewall.

That was the case with Goldman Sachs. The bank’s boardroom did not show up in Mr. Moore’s initial scan but an entry labeled “Goldman Sachs Board Room” popped up in the directory of a law firm that Goldman Sachs videoconferences with. Mr. Moore did not disclose the name of the law firm and said that because he was afraid of “crossing a line,” he did not dial into Goldman Sachs.

Said Mr. Tuchen, “Any reasonably computer literate 6-year-old can try this at home.”