NEXT STEP IN CONGRESS

Snowden, a former NSA contractor who lives as a fugitive in Russia, in June 2013 exposed the agency's collection of "bulk telephony metadata." This data includes the existence and duration of calls made, but not the content of conversations.

US Attorney General Loretta Lynch said at a Senate budget hearing on Thursday that NSA data collection was a "vital tool in our national security arsenal," and that she was unaware of privacy violations under its existing program.

Snowden could not immediately be reached for comment.

The 2nd Circuit is the first federal appeals court to rule on the NSA program's legality. Federal appeals courts in Washington, D.C. and California are also weighing the matter.

While the government could appeal Thursday's decision, it will likely wait for Congress.

If Congress revamps the NSA program, then courts may need to review what it does. And if Congress reauthorizes Section 215, there could be further litigation that may ultimately require the Supreme Court's attention.

Scott Vernick, chair of the privacy and data security practice at Fox Rothschild in Philadelphia, said Congress may struggle to reach a consensus given how "the pendulum in this country is swinging toward privacy."

Ned Price, a spokesman for the White House's National Security Council, said President Barack Obama wants to end the NSA program, and is encouraged by the "good progress" on Capitol Hill to find an alternative that preserves its "essential capabilities."

Last week, the House Judiciary Committee voted 25-2 to end the bulk collection of telephone data through the USA Freedom Act. The bill is expected to pass the full House, and the White House has signaled support for it.

While a similar bipartisan bill is pending in the Senate, Majority Leader Mitch McConnell and Intelligence Committee chair Richard Burr, both Republicans, have proposed extending Section 215 and other parts of the Patriot Act through 2020.

Harry Reid, the Nevada Democrat and Senate minority leader, rejected that alternative, calling it "the height of irresponsibility to extend these illegal spying powers when we could pass bipartisan reform into law instead."

The existing NSA program has repeatedly been approved in secret by a national security court established under a 1978 law, the Foreign Intelligence Surveillance Act.

"FISA has been critically important in keeping us safe in America," McConnell said on Thursday.