The OPM confirmed the data breach, detected in April, and said it will notify the 4 million federal employees with potentially compromised information.

"Cybersecurity is everyone's concern and this is another example of how disruptive and dangerous it is.  Unfortunately these attacks are quickly becoming the new normal, whether from financial criminals or state-sponsored," NAFCU President/CEO Dan Berger said.

NAFCU member credit unions include some of the nation's largest that serve military and federal employee select employee groups.

The OPM is the human resources department for the federal government, and conducts background checks for security clearances. The OPM conducts more than 90% of federal background investigations, according to its website.

The agency said it is offering credit monitoring and identity theft insurance for 18 months to individuals potentially affected. The National Treasury Employees Union, which represents workers in 31 federal agencies, said it is encouraging members to sign up for the monitoring as soon as possible.

In November, a former DHS contractor disclosed another cyberattack that compromised the private files of more than 25,000 DHS workers and thousands of other federal employees.

"NTEU is very concerned about a breach by computer hackers of a range of federal-employee data held by the Office of Personnel Management. Data security, particularly in an era of rising incidence of identity theft, is a critically important matter," National Treasury Employees Union President Colleen M. Kelley said.

Kelley added, "NTEU will continue to urge OPM to share all available information with us as the situation develops. NTEU will encourage its members to sign up for the credit monitoring as soon as possible, follow all advice such as placing fraud alerts with credit bureaus and carefully monitoring activity for evidence of fraud and identity theft."

She stated it is vital to know as soon as possible the exposure of any personal information and encouraged employees to take all possible steps to protect themselves from financial or other risks.

Federal agencies are hastening the implementation of two-factor authentication with smart cards, a system designed defend against malicious access. But implementing that technology takes time.

"NTEU is very concerned about a breach by computer hackers of a range of federal-employee data held by the Office of Personnel Management (OPM). Data security, particularly in an era of rising incidence of identity theft, is a critically important matter," Kelley said.