A contractor who leaked a top-secret document to The Intercept was identified because the NSA was able to track who had printed out the document, using the yellow dot pattern that many printers place on documents, giving the time and date when the document was printed.
The Intercept ran its story on Monday and posted the document online; the arrest and charging of Reality Leigh Winner, 25, was announced by the Department of Justice the same day.
A detailed explanation of how she was tracked down was provided by Errata Security which pointed out that the document posted by The Intercept was a PDF containing the original printed version that had been scanned in.
"The problem is that most new printers print nearly invisibly yellow dots that track down exactly when and where documents, any document, is printed," the Errata researchers wrote.
{loadposition sam08}"Because the NSA logs all printing jobs on its printers, it can use this to match up precisely who printed the document."
Reports say that the document was printed out six times and that the NSA zeroed in on Winner because she had been in touch with The Intercept.
Errata posted a method of revealing the yellow dot pattern on the document and, using a tool developed by the Electronic Frontier Foundation, found that Winner had printed the document on a printer with model number 54, serial number 29535218. The document was printed on 9 May 2017 at 6:20.
"The NSA almost certainly has a record of who used the printer at that time," Errata said.
The Intercept's Glenn Greenwald, who broke the original Edward Snowden stories, has detailed in his book how he was taught about security measures by the ex-NSA contractor.
Winner case is 2nd time Matt Cole was involved in a story where the source ended up prosecuted for Espionage
— unR̶A̶D̶A̶C̶K̶ted (@JesselynRadack) 6 June 2017
1st was my client @JohnKiriakou
However, in this case, it looks like he and the rest of his staff made a bad slip-up which could lead to other potential whistle-blowers being reluctant to trust that their identities would not be compromised by the publication.
Fingers are being pointed at Matt Cole, one of the reporters of the Winner story, as this is the second time that a source in a story he was involved in ended up being prosecuted for espionage. The firsts was CIA anti-torture whistleblower John Kiriakou.
Graphics: courtesy Errata Security.