Details of 2.2 million suspected terrorists have been leaked and published on the internet.
The World-Check database, administered by information firm Thomson Reuters, is used by more than 300 government and intelligence agencies, as well as 4,500 other institutions around the world, including 49 of the world's 50 biggest banks.
The database is intended as "an early warning system for hidden risk", rather than the definitive guide to global terrorism. It is intended to help organisations, such as banks, know which people to keep an especially close eye on.
Thomson Reuters charges up to $1m in annual subs for the list. All subscribers are vetted to make sure that the data doesn't get into the hands of miscreants, which makes the leak highly embarrassing.
Security researcher Chris Vickery said in a Reddit post that he has obtained a 2014 copy of the database, but won't reveal how he did so until "a later time".
Vickery claimed that the insecurities that enabled him to download the entire database without paying a $1m subscription are still there, although Thomson Reuters is working to secure the site.
Vickery is still mulling over whether to publish the full database. "No hacking was involved in my acquisition of this data. I would call it more of a leak than anything, although not directly from Thomson Reuters. The exact details behind that can be shared at a later time," he wrote on Reddit, before asking: "Should I release this database to the world?"
According to Vickery, Thomson Reuters looked him up after reading his post on Reddit and gave him a friendly call.
"They were very nice. I have passed along details of where the leak is and, to the best of my knowledge, they are are working to get it secured," he said.
One of the names on the list is Maajid Nawaz, a Liberal Democrat politician and founder of the counter-extremism Quilliam think tank.
Nawaz is a former member of radical Islamist group Hizb ut-Tahrir, but apparently gave all that up in 2007 following a spell in chokey in Egypt during which he saw the light by reading Amnesty International literature.
Nawaz, however, wasn't impressed. "I have consulted to every UK prime minister from Tony Blair onwards on how our society can best address Islamist extremism," he told The Stack.
"If true, compiling lists of ‘undesirable individuals' by name, especially if such lists are not open to public scrutiny or regular updating, is a terrible move to adopt by any organisation."
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