It’s easy to forget these days, but former President George W. Bush’s illegal warrantless surveillance program was never halted by Congress, nor by the Obama administration; it was merely legalized in a 2008 law called the FISA Amendments Act. That means the surveillance of Americans’ international phone calls and Internet use—complete with secret rooms in AT&T data centers around the country—is likely still ongoing.

On Monday, a federal appeals court reinstated a key legal challenge to that surveillance: a lawsuit filed by the ACLU and others within hours of the FISA Amendments Act (PDF) being signed into law. The lawsuit attacks the constitutionality of the legislation, which allows the government to electronically eavesdrop on Americans without a probable-cause warrant, so long as one of the parties to the communication resides outside the US, and is suspected of a link to terrorism.

The decision by the 2nd US Circuit Court of Appeals means the ACLU, and other rights groups involved in the suit, might get their day in court. “This is a really big victory,” said ACLU spokeswoman Rachel Myers. “The ruling is that you don’t have to prove you’ve been spied on to challenge an unlawful spy act.”

A lower court had ruled the ACLU, Amnesty International, Global Fund for Women, Global Rights, Human Rights Watch, International Criminal Defence Attorneys Association, The Nation magazine, PEN American Center, Service Employees International Union, and other plaintiffs did not have standing to bring the case, because they could not demonstrate that they were subject to the eavesdropping.

The groups appealed, arguing that they often work with overseas dissidents who might be targets of the National Security Agency program. Instead of speaking with those people on the phone or via e-mails, the groups asserted that they have had to make expensive overseas trips in a bid to maintain attorney-client confidentiality. The plaintiffs, some of them journalists, also claim the 2008 legislation chills their speech, and violates their Fourth Amendment privacy rights.

Without ruling on the merits of the case, the appeals court on Monday agreed with the plaintiffs that they have ample reason to fear the surveillance program, and thus have legal standing to pursue their claim. From the ruling:

[The] plaintiffs have good reason to believe that their communications in particular, will fall within the scope of the broad surveillance that they can assume the government will conduct. The plaintiffs testify that in order to carry out their jobs they must regularly communicate by telephone and e-mail with precisely the sorts of individuals that the government will most likely seek to monitor—i.e., individuals “the US government believes or believed to be associated with terrorist organizations,” “political and human rights activists who oppose governments that are supported economically or militarily by the US government,” and “people located in geographical areas that are a special focus of the US government’s counterterrorism or diplomatic efforts.” The plaintiffs’ assessment that these individuals are likely targets of [FISA Amendments Act] surveillance is reasonable, and the government has not disputed that assertion.

The case will now return to the courtroom of US District Court Judge John G. Koeltl in New York, where, if past is prologue, the Obama administration will play its trump card: an assertion of the powerful State Secrets Privilege that lets the executive branch effectively kill lawsuits by claiming they threaten to expose national security secrets.

“State secrets could definitely come into it,” Myers said.

The courts tend to defer to such claims, but in a rare exception in 2008, a San Francisco federal judge refused to throw out a wiretapping lawsuit against AT&T under the State Secrets Privilege. The AT&T lawsuit was later killed anyway, because the same FISA Amendments Act also granted the phone companies retroactive legal immunity for their participation in the NSA program. That immunity does not apply to the government.

The FISA Amendments Act—which passed with the support of then-senator Obama—generally requires the Foreign Intelligence Surveillance Act Court to rubber stamp terror-related electronic surveillance requests. The government does not have to identify the target or facility to be monitored. It can begin surveillance a week before making the request, and the surveillance can continue during the appellate process in a rare instance of rejection by the secret FISA court.