The NSA is forbidden to 'target' American citizens, green-card holders or companies for surveillance without an individual warrant from a judge.

Experts said public companies worry about the loss of customer confidence and the legal liability to shareholders or security vendors when they report flaws.

When the 'New York Times' revealed the warrantless surveillance of voice calls, in December 2005, the telephone companies got nervous.

Companies that receive government information demands have to obey the law, but they often have room for maneuver. They scarcely ever use it.