Names, email addresses and passwords were among the sensitive details of defence personnel stolen after website Defence News was hacked earlier this month.
Affected users would include military personnel and "defense decision-makers", according to the website's description of its reader base.
Gannett Government Media alerted readers to the breach yesterday after it noticed the attack on 7 June.
"We also discovered that the attacker gained unauthorised access to files containing information of some of our users. The information in those files included first and last name, userID, password, email address, the internal number we assigned to the account, and, if provided, ZIP code, duty status, paygrade, and branch of service," the organisation wrote on its web site.
It asked readers to change passwords across its sister sites, Military Times, and Federal Times, and for any external accounts that reuse the access credentials.
The company said no financial information was compromised and noted it has hired external contractors to help investigate the cause of the incident.
"We deeply regret any inconvenience that this may cause and appreciate your understanding."