In a letter to authorities, the American Federation of Government Employees said it believes hackers - who others have linked to China - obtained confidential identification numbers that each American needs to get a job, a driving license or a bank account.
"We believe that Social Security numbers were not encrypted, a cyber-security failure that is absolutely indefensible and outrageous," union president David Cox said in a letter to the government's Office of Personnel Management.
"The hackers are now in possession of all personnel data for every federal employee, every federal retiree, and up to one million former federal employees."
The union said that data concerning an individual's address, date of birth, salary and military record where also compromised.
The US government last week admitted that data linked to at least four million current and former federal employees was hacked. It added additional exposures "may come to light."
The Washington Post and other US media cited government officials as saying that Chinese hackers were behind the breach. Officials refused to assign attribution or motive, but pointed affected parties to measures that could prevent fraud and identify theft.
The government will, through a third party, offer $1 million in identity theft protection services at no cost.
"Protecting our federal employee data from malicious cyber incidents is of the highest priority," Office of Personnel Management director Katherine Archuleta had said.
The employee records attack was just the latest in a series of major breaches that have shown the vulnerability of the federal government. Last year Russian hackers are believed to have accessed unclassified computer systems at the White House and State Department.
Obama has ranked China and Russia's cyber-attack capabilities as "very good", Iran's as "good," and North Korea's as not "particularly good."
Written with inputs from AFP
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