Surveillance reformers, fresh off a week of tenuous victories, have vowed to ensure there are further overhauls to the National Security Agency’s vast dragnets after a new report detailed another stretch of legal authority by the US government to stop malicious hackers.

Based on documents leaked by whistleblower Edward Snowden, the New York Times and ProPublica reported on Thursday that the Justice Department in 2012 permitted the NSA to use widespread surveillance authorities passed by Congress to stop terrorism and foreign espionage in order to find digital signatures associated with high-level cyber intrusions.

The FBI is also able to access the data – which, the reports noted, may contain information associated with Americans or data stolen from Americans outright. Searching for the threat signatures often involves accessing the stolen data.

The searches occur without warrants, evidently providing a novel avenue for intelligence and law enforcement agencies to circumvent longstanding legal requirements for individualized suspicion of wrongdoing.

Searches like those, occurring within the NSA’s vast troves of Americans’ international communications authorized by a 2008 revision of the Foreign Intelligence Surveillance Act – known as Section 702 – have been termed “back-door searches” by Senator Ron Wyden, an Oregon Democrat on the intelligence committee.

The Guardian revealed the legal loophole permitting the searches, thanks to Snowden, in August 2013; director of national intelligence James Clapper conceded the following April that the NSA has conducted such searches.

Surveillance reformers said the new evidence of migratory surveillance on hacking and associated warrantless searches provided a reminder of the need to ban back-door searches, as well as the insufficiency of the new USA Freedom Act, signed into law this week by Barack Obama.

“This report shows why our efforts to reform Section 702 of the FISA Amendments Act are so urgent, and is an example of how the USA Freedom Act did not end bulk collection of communications and data,” Representative Zoe Lofgren, a California Democrat, told the Guardian on Thursday.

“To add insult to injury, under this program victims of cybercrime are doubly harmed when their government collects and searches their private stolen communications and data.”

The USA Freedom Act does not stop the NSA or the FBI from making warrantless searches. A ban on back-door searches was stripped from the bill in 2014 amid pressure from the Obama administration, intelligence agencies and House GOP leadership.

In response, the House passed a separate bill in July 2014 banning back-door searches. Though a 293-121 margin signaled an appetite to drive surveillance rollbacks further than the USA Freedom Act permitted, the measure, which lacked Senate follow-up, never became law.

Now Lofgren and privacy-minded colleagues, both Republicans and Democrats, are vowing to revive the back-door-search ban. They intend it to be part of a raft of new surveillance restrictions, first reported by the Guardian on Tuesday, to be launched through 2017, when the law underlying the broad internet dragnets is up for renewal.

Those efforts advanced on Wednesday. Congress added amendments – by Lofgren and her GOP allies Thomas Massie of Kentucky and Ted Poe of Texas, preventing the NSA from undermining encryption standards and the FBI from compelling companies to permit the intelligence and law enforcement agency surreptitious access to user data – to a Justice Department funding bill. The House ultimately passed the bill by a 242-183 margin.

Clapper’s spokesman, Brian Hale, told the Times and ProPublica that it was unsurprising for the NSA and FBI to expand their counterterrorism authorities to search for data-intrusion signatures.

“Targeting overseas individuals engaging in hostile cyber activities on behalf of a foreign power is a lawful foreign intelligence purpose,” Hale said.

While the 2008 law requires the ostensible target of the international-communications dragnet to be a foreign-based agent of a foreign power or terrorist, the dragnets inevitably gather in large volumes of Americans’ communication. (The term the NSA prefers to describe such practices is “incidental collection”.)

A 2009 White House memo reported by the Times and ProPublica dismissed as quaint concerns over the migration of counterterrorism authorities for the purpose of cybersecurity: “Reliance on legal authorities that make theoretical distinctions between armed attacks, terrorism and criminal activity may prove impractical.”