The latest reveleation by former United States National Security Agency (NSA) contractor Edward Snowden shows that the spy agency helped its British counterpart, the Government Communications Headquarters (GCHQ) capture images from millions of Yahoo users' webcams.
Documents seen by the Guardian show that the GCHQ's Optic Nerve bulk surveillance program snapped still images every five minutes from random Yahoo users' video chats and stored them in a database.
GCHQ tapped into optical-fibre cables carrying internet data for the surveillance program for Optic Nerve, and fed the information into NSA's systems such as the XKeyscore search tool.
Users picked for the program were "unselected" of no particular interest to intelligence agencies. Vast amounts of their private communications over Yahoo video chat was captured by the GCHQ as part of an experiment in automated facial recognition to monitor existing targets and to discover new ones of interest.
Many of the video chats were sexually explicit. The documents note that the spy agency's pornography detection software was inadequate at correctly filtering out nudity and some of it may have been viewed by analysts.
According to the documents, the program started in 2008. In the first six months of that year alone, video chat images from more than 1.8 million Yahoo accounts globally were captured.
It is not known how many more accounts the GCHQ has been surveilling without warrants. Optic Nerve was still active in 2012, the documents reveal, but was closed down some time after. Whether or not the GCHQ still has the webcam imagery is not known.
Yahoo is reported to be furious about the interception of its users' video chats. It denied that it knew about Optic Nerve and slammed the spy agencies for the gross intrusion..
"This report, if true, represents a whole new level of violation of our users' privacy that is completely unacceptable, and we strongly call on the world's governments to reform surveillance law consistent with the principles we outlined in December," a Yahoo spokesperson said in a statement.
GCHQ was aware of privacy issues surrounding the mass capture of user video discussions in the leaked documents show, but nevertheless went ahead with gathering large amounts of what it termed was "undesirable nudity", insisting in a statement to the Guardian it is lawful for it do so.
The NSA denied to the Guardian that it asks its Five-Eyes partners such as GCHQ to collect intelligence it is legally barred from gathering, such as information on US citizens.