Experts still trying to assess scope of months-long operation that slipped code into software update used on servers across the globe and in all 5 branches of US military

'There are some serious physical concerns about potential victims being tracked,' FireEye's head of intelligence John Hultquist tells cybersecurity conference in Tel Aviv

Cybersecurity firms see surge in attempts to gain access to American government agencies, oil and gas sectors, including by impersonating email from president's office