"CLARIFICATION: This story was updated Feb. 1 with additional information about Kathy Sandy's Work Number disclosure report.
The Equifax credit reporting agency, with the aid of thousands of human resource departments around the country, has assembled what may be the most powerful and thorough private database of Americans' personal information ever created, containing 190 million employment and salary records covering more than one-third of U.S. adults.
Some of the information in the little-known database, created through an Equifax-owned company called The Work Number, is sold to debt collectors, financial service companies and other entities.
"It's the biggest privacy breach in our time, and it's legal and no one knows it's going on," said Robert Mather, who runs a small employment background company named Pre-Employ.com. "It's like a secret CIA."
Despite all the information Americans now share on social media and websites, and all the data we know companies collect on us, one piece of information is still sacred to most people: their salaries. After all, who would post their salary as a status update on Facebook or in a tweet?
But salary information is also for sale by Equifax through The Work Number. Its database is so detailed that it contains week-by-week paystub information dating back years for many individuals, as well as other kinds of human resources-related information, such as health care provider, whether someone has dental insurance and if they've ever filed an unemployment claim. In 2009, Equifax said the data covered 30 percent of the U.S. working population, and it now says The Work Number is adding 12 million records annually.
How does Equifax obtain this sensitive and secret information? With the willing aid of thousands of U.S. businesses, including many of the Fortune 500. Government agencies -- representing 85 percent of the federal civilian population, including workers at the Department of Defense, according to Equifax -- and schools also work with The Work Number. Many of them let Equifax tap directly into their data so the credit bureau can always have the latest employment information. In fact, these organizations actually pay Equifax for the privilege of giving away their employees' personal information.
Equifax turns around and sells some of this data to third parties, including debt collectors and other financial services companies.
Equifax declined to be interviewed, but in an emailed statement to NBCNews.com, it confirmed that it shares "employment data" with debt collectors and others, and said it does so in compliance with Fair Credit Reporting Act guidelines.
"In all cases, these entities must have a permissible purpose to request employment information," Equifax spokesman Timothy Klein said.
He also said consumers give these third parties the right to access the data "at the time of application" for credit.
"A consumer grants verifiers (creditors) and their assigned debt collectors the right to verify employment should the consumer default on their account," he said. "