It is no secret that employees supplement their employers’ in-house information technology with external cloud computing services—but even chief information officers may not realize just how often. New data from Cisco Systems Inc. suggest that employees route around corporate networks to a startling extent, posing risks for security and data governance.

CIOs estimate their organizations use 91 cloud computing services, on average, Cisco said. But a more accurate average is 1,120, the Silicon Valley company said Wednesday....