What are the cyber vulnerabilities most often cited?

As FRONTLINE reports in "Cyber War!" a weak link in America's vital infrastructures are digital control systems, such as SCADA systems. These digital Supervisory Control and Data Acquisition systems manage critical infrastructures such as gas and propane lines, water, chemical manufacturing systems, power grids, etc. They can be remotely accessed and, because of software vulnerabilities in older systems, an attacker could penetrate the systems and manipulate them without being discovered, potentially inflicting physical damage on the critical infrastructure.

FRONTLINE focused on one particular part of this threat, the electric power grid, because the grid is tied into so many other critical infrastructures and because, if the power grid were taken down, it would have a serious pyschological impact on the population.

http://www.pbs.org/wgbh/pages/frontline/shows/cyberwar/etc/faqs.html