NASA began investigating the intrusion of its servers on Oct. 23, according to a memo sent Tuesday by Assistant Administrator Bob Gibbs.
Employees who were hired, transferred or left the agency from July 2006 to October 2018 may have been affected by the hack, he said. The agency has about 17,400 civil service employees.
Gibbs's memo was first published online by Spaceref.com and NASA confirmed its authenticity.
The agency said it doesn't believe that any of its missions were affected.
NASA doesn't yet know how many employees are affected by the breach, spokeswoman Karen Northon told Bloomberg.
"Once discovered, NASA took immediate action to secure the impacted servers and has been working to perform a forensic analysis since then—this process will take time," she wrote in an email. "The ongoing investigation is a top NASA priority."
The agency said it would offer identity-protection services to those who were affected.
