The US National Security Agency and its British counterpart are tapping into smartphone applications such as Angry Birds to scoop up personal data, the New York Times has reported.
They are the latest revelations from former NSA contractor Edward Snowden.
Classified documents show the NSA and Britain's GCHQ are working to snatch location and other sensitive information by exploiting "leaky" smartphone apps, such as Google Maps or mobile versions of Facebook, Twitter and other services.
Previous media leaks have shown that the NSA sifts through phone records, text messages, online games and other internet traffic in its surveillance of terror suspects and other intelligence "targets".
Each time someone opens an application, the eavesdropping agencies can vacuum up data that reveals the user's location, age, personal address list and other information, according to the Times.
In one top secret NSA slide from 2010 published by the Times, an agency analyst is clearly excited at the espionage opportunity offered by the smartphone applications.
The slide is titled "Golden Nugget!" and describes a "perfect scenario" in which a "target" uploads a photo to a social media site using a mobile phone.
The scope of the spying on apps is unclear, as well as many details of the espionage, the newspaper said.
When asked about the report, the NSA did not deny the account but reiterated, as it has for months, that the agency is not out to spy on ordinary people and is respectful of privacy rights.
"The communications of people who are not valid foreign intelligence targets are not of interest to the National Security Agency," spokeswoman Vanee Vines said in a statement.
"Any implication that NSA's foreign intelligence collection is focused on the smartphone or social media communications of everyday Americans is not true.
"We collect only those communications that we are authorised by law to collect for valid foreign intelligence and counter-intelligence purposes - regardless of the technical means used by the targets."
The NSA's funding for the programme has quadrupled since 2007, from $204m to $767m, according to a secret Canadian analysis, the Times said.
In a speech this month, US President Barack Obama announced changes to NSA surveillance that he said would better protect privacy, including an end to the government's storage of Americans' phone records.
But Mr Obama did not touch on the reams of data gathered from apps and other smartphone functions.