A surge in malware disguised as online advertisements aimed at unsuspecting web users has hit major U.S. military contractors in the past few weeks, marking a dangerous twist on a decade-old scourge for advertisers, security researchers said.

Researchers from Fairfax, Virginia-based security software company Invincea said they had documented new uses of so-called "malvertising" to carry out highly-targeted cyber espionage campaigns against three firms in the military-industrial arena.