Iranian-backed hackers have stolen vast amounts of data from a major software company that handles sensitive computer projects for the White House communications agency, the U.S. military, the F.B.I. and many American corporations, a cybersecurity firm told NBC News.

Citrix Systems Inc. came under attack twice, once in December and again on Monday of this week, according to Resecurity, which notified the firm and law enforcement authorities.

Employing brute force attacks that guess passwords, the assault was carried out by the Iranian-linked hacking group known as Iridium, which was also behind recent cyber hits against numerous government agencies, oil-and-gas companies and other targets, said Charles Yoo, Resecurity's president.

The hackers extracted at least six terabytes of data and possibly up to 10 terabytes in the assault on Citrix, Yoo said. The attackers gained access to Citrix through several compromised employee accounts, he said.