Companies including utilities, banks and phone carriers would have to spend almost nine times more on cybersecurity to prevent a digital Pearl Harbor from plunging millions into darkness, paralyzing the financial system or cutting communications, a Bloomberg Government study found.

Spies, criminals and hacker-activists are stepping up assaults on U.S. government and corporate systems, spurring efforts by Congress and President Barack Obama to shield infrastructure essential to U.S. national and economic security, such as power grids and water-treatment plants.

Hardening those systems would require a significant investment given the increasing stealth and sophistication of hackers, according to Lawrence Ponemon, chairman of the Ponemon Institute LLC, a research firm that collaborated with Bloomberg on the study released today in Washington.